Merge pull request #1607 from rtfeldman/editor-let-value

file loading/saving/running, headers, top level defs

.gitignore

@@ -26,6 +26,9 @@ editor/benches/resources/25000000_lines.roc
 editor/benches/resources/50000_lines.roc
 editor/benches/resources/500_lines.roc
 
+# file editor creates when no arg is passed
+new-roc-project
+
 # rust cache (sccache folder)
 sccache_dir
 

Cargo.lock

@@ -1215,6 +1215,12 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "00b0228411908ca8685dba7fc2cdd70ec9990a6e753e89b6ac91a84c40fbaf4b"
 
+[[package]]
+name = "fs_extra"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2022715d62ab30faffd124d40b76f4134a550a87792276512b18d63272333394"
+
 [[package]]
 name = "fuchsia-cprng"
 version = "0.1.1"
@@ -3567,6 +3573,7 @@ dependencies = [
  "confy",
  "copypasta",
  "env_logger 0.8.4",
+ "fs_extra",
  "futures",
  "glyph_brush",
  "im 15.0.0",
@@ -3584,9 +3591,11 @@ dependencies = [
  "quickcheck 1.0.3",
  "quickcheck_macros 1.0.0",
  "rand 0.8.4",
+ "roc_builtins",
  "roc_can",
  "roc_collections",
  "roc_fmt",
+ "roc_load",
  "roc_module",
  "roc_parse",
  "roc_problem",
@@ -3598,6 +3607,8 @@ dependencies = [
  "ropey",
  "serde",
  "snafu",
+ "tempfile",
+ "uuid",
  "ven_graph",
  "wgpu",
  "wgpu_glyph",
@@ -4758,6 +4769,15 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "936e4b492acfd135421d8dca4b1aa80a7bfc26e702ef3af710e0752684df5372"
 
+[[package]]
+name = "uuid"
+version = "0.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc5cf98d8186244414c848017f0e2676b3fcb46807f6668a97dfe67359a3c4b7"
+dependencies = [
+ "getrandom 0.2.3",
+]
+
 [[package]]
 name = "vec_map"
 version = "0.8.2"

Earthfile

@@ -65,7 +65,7 @@ check-rustfmt:
     RUN cargo fmt --all -- --check
 
 check-typos:
-    RUN cargo install --version 1.0.11 typos-cli
+    RUN cargo install typos-cli --version 1.0.11 # version set to prevent confusion if the version is updated automatically
     COPY --dir .github ci cli compiler docs editor examples linker nightly_benches packages roc_std www *.md LEGAL_DETAILS shell.nix ./
     RUN typos
 
@@ -97,7 +97,6 @@ test-all:
     BUILD +test-zig
     BUILD +check-rustfmt
     BUILD +check-clippy
-    BUILD +check-typos
     BUILD +test-rust
     BUILD +verify-no-git-changes
 

cli/src/main.rs

@@ -34,7 +34,7 @@ fn main() -> io::Result<()> {
                 }
 
                 None => {
-                    launch_editor(&[])?;
+                    launch_editor(None)?;
 
                     Ok(0)
                 }
@@ -91,16 +91,13 @@ If you're building the compiler from source you'll want to do `cargo run [FILE]`
                 .subcommand_matches(CMD_EDIT)
                 .unwrap()
                 .values_of_os(DIRECTORY_OR_FILES)
+                .map(|mut values| values.next())
             {
-                None => {
-                    launch_editor(&[])?;
+                Some(Some(os_str)) => {
+                    launch_editor(Some(Path::new(os_str)))?;
                 }
-                Some(values) => {
-                    let paths = values
-                        .map(|os_str| Path::new(os_str))
-                        .collect::<Vec<&Path>>();
-
-                    launch_editor(&paths)?;
+                _ => {
+                    launch_editor(None)?;
                 }
             }
 
@@ -187,8 +184,8 @@ fn roc_files_recursive<P: AsRef<Path>>(
 }
 
 #[cfg(feature = "editor")]
-fn launch_editor(filepaths: &[&Path]) -> io::Result<()> {
-    roc_editor::launch(filepaths)
+fn launch_editor(project_dir_path: Option<&Path>) -> io::Result<()> {
+    roc_editor::launch(project_dir_path)
 }
 
 #[cfg(not(feature = "editor"))]

compiler/load/src/file.rs

@@ -1967,7 +1967,7 @@ fn update<'a>(
                 );
             }
 
-            if module_id == state.root_id && state.goal_phase == Phase::SolveTypes {
+            if is_host_exposed && state.goal_phase == Phase::SolveTypes {
                 debug_assert!(work.is_empty());
                 debug_assert!(state.dependencies.solved_all());
 

compiler/module/src/symbol.rs

@@ -551,7 +551,53 @@ impl IdentIds {
         }
     }
 
-    /// Generates a unique, new name that's just a stringified integer
+    // necessary when the name of a value is changed in the editor
+    pub fn update_key(
+        &mut self,
+        old_ident_name: &str,
+        new_ident_name: &str,
+    ) -> Result<IdentId, String> {
+        let old_ident: Ident = old_ident_name.into();
+
+        let ident_id_ref_opt = self.by_ident.get(&old_ident);
+
+        match ident_id_ref_opt {
+            Some(ident_id_ref) => {
+                let ident_id = *ident_id_ref;
+
+                self.by_ident.remove(&old_ident);
+                self.by_ident.insert(new_ident_name.into(), ident_id);
+
+                let by_id = &mut self.by_id;
+                let key_index_opt = by_id.iter().position(|x| *x == old_ident);
+
+                if let Some(key_index) = key_index_opt {
+                    if let Some(vec_elt) = by_id.get_mut(key_index) {
+                        *vec_elt = new_ident_name.into();
+                    } else {
+                        // we get the index from by_id
+                        unreachable!()
+                    }
+
+                    Ok(ident_id)
+                } else {
+                    Err(
+                        format!(
+                            "Tried to find position of key {:?} in IdentIds.by_id but I could not find the key. IdentIds.by_id: {:?}",
+                            old_ident_name,
+                            self.by_id
+                        )
+                    )
+                }
+            }
+            None => Err(format!(
+                "Tried to update key in IdentIds ({:?}) but I could not find the key ({}).",
+                self.by_ident, old_ident_name
+            )),
+        }
+    }
+
+    /// Generates a unique, new name that's just a strigified integer
     /// (e.g. "1" or "5"), using an internal counter. Since valid Roc variable
     /// names cannot begin with a number, this has no chance of colliding
     /// with actual user-defined variables.

compiler/parse/src/module.rs

@@ -32,7 +32,7 @@ fn end_of_file<'a>() -> impl Parser<'a, (), SyntaxError<'a>> {
 
 #[inline(always)]
 pub fn module_defs<'a>() -> impl Parser<'a, Vec<'a, Located<Def<'a>>>, SyntaxError<'a>> {
-    // force that we pare until the end of the input
+    // force that we parse until the end of the input
     let min_indent = 0;
     skip_second!(
         specialize(

compiler/parse/src/test_helpers.rs

@@ -1,6 +1,9 @@
 use crate::ast;
+use crate::module::module_defs;
 // use crate::module::module_defs;
+use crate::parser::Parser;
 use crate::parser::{State, SyntaxError};
+use bumpalo::collections::Vec as BumpVec;
 use bumpalo::Bump;
 use roc_region::all::Located;
 
@@ -23,3 +26,15 @@ pub fn parse_loc_with<'a>(
         Err(fail) => Err(SyntaxError::Expr(fail)),
     }
 }
+
+pub fn parse_defs_with<'a>(
+    arena: &'a Bump,
+    input: &'a str,
+) -> Result<BumpVec<'a, Located<ast::Def<'a>>>, SyntaxError<'a>> {
+    let state = State::new(input.trim().as_bytes());
+
+    match module_defs().parse(arena, state) {
+        Ok(tuple) => Ok(tuple.1),
+        Err(tuple) => Err(tuple.1),
+    }
+}

editor/Cargo.toml

@@ -9,6 +9,8 @@ exclude = ["src/shaders/*.spv"]
 
 [dependencies]
 roc_collections = { path = "../compiler/collections" }
+roc_load = { path = "../compiler/load" }
+roc_builtins = { path = "../compiler/builtins" }
 roc_can = { path = "../compiler/can" }
 roc_parse = { path = "../compiler/parse" }
 roc_region = { path = "../compiler/region" }
@@ -49,6 +51,9 @@ confy = { git = 'https://github.com/rust-cli/confy', features = [
 ], default-features = false }
 serde = { version = "1.0.123", features = ["derive"] }
 nonempty = "0.6.0"
+tempfile = "3.2.0"
+uuid = { version = "0.8", features = ["v4"] }
+fs_extra = "1.2.0"
 
 [dependencies.bytemuck]
 version = "1.4"

editor/src/editor/code_lines.rs

@@ -1,10 +1,10 @@
 use crate::ui::text::lines::Lines;
 use crate::ui::text::selection::Selection;
-use crate::ui::ui_error::UIResult;
+use crate::ui::text::text_pos::TextPos;
+use crate::ui::ui_error::{LineInsertionFailed, OutOfBounds, UIResult};
 use crate::ui::util::slice_get;
 use crate::ui::util::slice_get_mut;
-use bumpalo::collections::String as BumpString;
-use bumpalo::Bump;
+use std::cmp::Ordering;
 use std::fmt;
 
 #[derive(Debug)]
@@ -14,31 +14,102 @@ pub struct CodeLines {
 }
 
 impl CodeLines {
-    pub fn from_str(code_str: &str) -> CodeLines {
-        CodeLines {
-            lines: code_str
-                .split_inclusive('\n')
-                .map(|s| s.to_owned())
-                .collect(),
-            nr_of_chars: code_str.len(),
-        }
-    }
-
     pub fn insert_between_line(
         &mut self,
         line_nr: usize,
         index: usize,
         new_str: &str,
     ) -> UIResult<()> {
+        let nr_of_lines = self.lines.len();
+
+        if line_nr < nr_of_lines {
             let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
 
             line_ref.insert_str(index, new_str);
+        } else if line_nr >= self.lines.len() {
+            for _ in 0..((line_nr - nr_of_lines) + 1) {
+                self.push_empty_line();
+            }
+
+            self.insert_between_line(line_nr, index, new_str)?;
+        } else {
+            LineInsertionFailed {
+                line_nr,
+                nr_of_lines,
+            }
+            .fail()?;
+        }
 
         self.nr_of_chars += new_str.len();
 
         Ok(())
     }
 
+    pub fn insert_empty_line(&mut self, line_nr: usize) -> UIResult<()> {
+        if line_nr <= self.lines.len() {
+            self.lines.insert(line_nr, String::new());
+
+            Ok(())
+        } else {
+            OutOfBounds {
+                index: line_nr,
+                collection_name: "code_lines.lines".to_owned(),
+                len: self.lines.len(),
+            }
+            .fail()
+        }
+    }
+
+    pub fn push_empty_line(&mut self) {
+        self.lines.push(String::new())
+    }
+
+    pub fn break_line(&mut self, line_nr: usize, col_nr: usize) -> UIResult<()> {
+        // clippy prefers this over if-else
+        match line_nr.cmp(&self.lines.len()) {
+            Ordering::Less => {
+                self.insert_empty_line(line_nr + 1)?;
+
+                let line_ref = self.lines.get_mut(line_nr).unwrap(); // safe because we checked line_nr
+
+                if col_nr < line_ref.len() {
+                    let next_line_str: String = line_ref.drain(col_nr..).collect();
+
+                    let next_line_ref = self.lines.get_mut(line_nr + 1).unwrap(); // safe because we just added the line
+
+                    *next_line_ref = next_line_str;
+                }
+
+                Ok(())
+            }
+            Ordering::Equal => self.insert_empty_line(line_nr + 1),
+            Ordering::Greater => OutOfBounds {
+                index: line_nr,
+                collection_name: "code_lines.lines".to_owned(),
+                len: self.lines.len(),
+            }
+            .fail(),
+        }
+    }
+
+    pub fn clear_line(&mut self, line_nr: usize) -> UIResult<()> {
+        let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
+
+        *line_ref = String::new();
+
+        Ok(())
+    }
+
+    pub fn del_line(&mut self, line_nr: usize) -> UIResult<()> {
+        let line_len = self.line_len(line_nr)?;
+
+        self.lines.remove(line_nr);
+
+        self.nr_of_chars -= line_len;
+
+        Ok(())
+    }
+
     pub fn del_at_line(&mut self, line_nr: usize, index: usize) -> UIResult<()> {
         let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
 
@@ -49,6 +120,18 @@ impl CodeLines {
         Ok(())
     }
 
+    pub fn del_range_at_line(
+        &mut self,
+        line_nr: usize,
+        col_range: std::ops::Range<usize>,
+    ) -> UIResult<()> {
+        let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
+
+        line_ref.drain(col_range);
+
+        Ok(())
+    }
+
     pub fn del_selection(&mut self, selection: Selection) -> UIResult<()> {
         if selection.is_on_same_line() {
             let line_ref = slice_get_mut(selection.start_pos.line, &mut self.lines)?;
@@ -60,17 +143,36 @@ impl CodeLines {
 
         Ok(())
     }
+
+    // last column of last line
+    pub fn end_txt_pos(&self) -> TextPos {
+        let last_line_nr = self.nr_of_lines() - 1;
+
+        TextPos {
+            line: last_line_nr,
+            column: self.line_len(last_line_nr).unwrap(), // safe because we just calculated last_line
+        }
+    }
+}
+
+impl Default for CodeLines {
+    fn default() -> Self {
+        CodeLines {
+            lines: Vec::new(),
+            nr_of_chars: 0,
+        }
+    }
 }
 
 impl Lines for CodeLines {
-    fn get_line(&self, line_nr: usize) -> UIResult<&str> {
+    fn get_line_ref(&self, line_nr: usize) -> UIResult<&str> {
         let line_string = slice_get(line_nr, &self.lines)?;
 
         Ok(line_string)
     }
 
     fn line_len(&self, line_nr: usize) -> UIResult<usize> {
-        self.get_line(line_nr).map(|line| line.len())
+        self.get_line_ref(line_nr).map(|line| line.len())
     }
 
     fn nr_of_lines(&self) -> usize {
@@ -81,14 +183,8 @@ impl Lines for CodeLines {
         self.nr_of_chars
     }
 
-    fn all_lines<'a>(&self, arena: &'a Bump) -> BumpString<'a> {
-        let mut lines = BumpString::with_capacity_in(self.nr_of_chars(), arena);
-
-        for line in &self.lines {
-            lines.push_str(line);
-        }
-
-        lines
+    fn all_lines_as_string(&self) -> String {
+        self.lines.join("\n")
     }
 
     fn is_last_line(&self, line_nr: usize) -> bool {
@@ -96,7 +192,7 @@ impl Lines for CodeLines {
     }
 
     fn last_char(&self, line_nr: usize) -> UIResult<Option<char>> {
-        Ok(self.get_line(line_nr)?.chars().last())
+        Ok(self.get_line_ref(line_nr)?.chars().last())
     }
 }
 
@@ -105,14 +201,16 @@ impl fmt::Display for CodeLines {
         for row in &self.lines {
             let row_str = row
                 .chars()
-                .map(|code_char| format!("'{}'", code_char))
+                .map(|code_char| format!("{}", code_char))
                 .collect::<Vec<String>>()
-                .join(", ");
+                .join(" ");
 
-            write!(f, "\n{}", row_str)?;
+            let escaped_row_str = row_str.replace("\n", "\\n");
+
+            write!(f, "\n{}", escaped_row_str)?;
         }
 
-        write!(f, "      (code_lines)")?;
+        writeln!(f, "      (code_lines, {:?} lines)", self.lines.len())?;
 
         Ok(())
     }

editor/src/editor/config.rs

@@ -5,6 +5,7 @@ use crate::editor::theme::EdTheme;
 #[derive(Serialize, Deserialize)]
 pub struct Config {
     pub code_font_size: f32,
+    pub debug_font_size: f32,
     pub ed_theme: EdTheme,
 }
 
@@ -12,6 +13,7 @@ impl Default for Config {
     fn default() -> Self {
         Self {
             code_font_size: 30.0,
+            debug_font_size: 20.0,
             ed_theme: EdTheme::default(),
         }
     }

editor/src/editor/ed_error.rs

@@ -1,3 +1,4 @@
+use crate::lang::parse::ASTNodeId;
 use crate::ui::ui_error::UIResult;
 use crate::{editor::slow_pool::MarkNodeId, ui::text::text_pos::TextPos};
 use colored::*;
@@ -11,6 +12,24 @@ use snafu::{Backtrace, ErrorCompat, NoneError, ResultExt, Snafu};
 #[derive(Debug, Snafu)]
 #[snafu(visibility(pub))]
 pub enum EdError {
+    #[snafu(display(
+        "ASTNodeIdWithoutDefId: The expr_id_opt in ASTNode({:?}) was `None` but I was expexting `Some(DefId)` .",
+        ast_node_id
+    ))]
+    ASTNodeIdWithoutDefId {
+        ast_node_id: ASTNodeId,
+        backtrace: Backtrace,
+    },
+
+    #[snafu(display(
+        "ASTNodeIdWithoutExprId: The expr_id_opt in ASTNode({:?}) was `None` but I was expexting `Some(ExprId)` .",
+        ast_node_id
+    ))]
+    ASTNodeIdWithoutExprId {
+        ast_node_id: ASTNodeId,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display(
         "CaretNotFound: No carets were found in the expected node with id {}",
         node_id
@@ -43,6 +62,17 @@ pub enum EdError {
         backtrace: Backtrace,
     },
 
+    #[snafu(display(
+        "EmptyCodeString: I need to have a code string (code_str) that contains either an app, interface or Package-Config header. The code string was empty.",
+    ))]
+    EmptyCodeString { backtrace: Backtrace },
+
+    #[snafu(display("FailedToUpdateIdentIdName: {}", err_str))]
+    FailedToUpdateIdentIdName {
+        err_str: String,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display("GetContentOnNestedNode: tried to get string content from Nested MarkupNode. Can only get content from Text or Blank nodes."))]
     GetContentOnNestedNode { backtrace: Backtrace },
 
@@ -99,6 +129,12 @@ pub enum EdError {
         backtrace: Backtrace,
     },
 
+    #[snafu(display("NoDefMarkNodeBeforeLineNr: I could not find a MarkupNode whose root parent points to a DefId located before the given line number: {}.", line_nr))]
+    NoDefMarkNodeBeforeLineNr {
+        line_nr: usize,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display("NodeWithoutAttributes: expected to have a node with attributes. This is a Nested MarkupNode, only Text and Blank nodes have attributes."))]
     NodeWithoutAttributes { backtrace: Backtrace },
 
@@ -131,6 +167,17 @@ pub enum EdError {
         backtrace: Backtrace,
     },
 
+    #[snafu(display(
+        "UnexpectedPattern2Variant: required a {} at this position, Pattern2 was a {}.",
+        required_pattern2,
+        encountered_pattern2,
+    ))]
+    UnexpectedPattern2Variant {
+        required_pattern2: String,
+        encountered_pattern2: String,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display(
         "UnexpectedEmptyPoolVec: expected PoolVec {} to have at least one element.",
         descriptive_vec_name
@@ -154,7 +201,10 @@ pub enum EdError {
     },
 
     #[snafu(display("ParseError: Failed to parse AST: SyntaxError: {}.", syntax_err))]
-    ParseError { syntax_err: String },
+    SrcParseError {
+        syntax_err: String,
+        backtrace: Backtrace,
+    },
 
     #[snafu(display("RecordWithoutFields: expected record to have at least one field because it is not an EmptyRecord."))]
     RecordWithoutFields { backtrace: Backtrace },
@@ -176,14 +226,6 @@ pub fn print_err(err: &EdError) {
     }
 }
 
-pub fn print_ui_err(err: &UIError) {
-    eprintln!("{}", format!("{}", err).truecolor(255, 0, 0));
-
-    if let Some(backtrace) = ErrorCompat::backtrace(err) {
-        eprintln!("{}", color_backtrace(backtrace));
-    }
-}
-
 fn color_backtrace(backtrace: &snafu::Backtrace) -> String {
     let backtrace_str = format!("{}", backtrace);
     let backtrace_split = backtrace_str.split('\n');

editor/src/editor/grid_node_map.rs

@@ -1,40 +1,28 @@
 use crate::editor::ed_error::EdResult;
 use crate::editor::ed_error::NestedNodeWithoutChildren;
-use crate::editor::ed_error::NodeIdNotInGridNodeMap;
+use crate::editor::ed_error::{NoDefMarkNodeBeforeLineNr, NodeIdNotInGridNodeMap};
 use crate::editor::mvc::ed_model::EdModel;
 use crate::editor::slow_pool::MarkNodeId;
 use crate::editor::slow_pool::SlowPool;
 use crate::editor::util::first_last_index_of;
 use crate::editor::util::index_of;
-use crate::lang::ast::ExprId;
+use crate::lang::parse::ASTNodeId;
 use crate::ui::text::selection::Selection;
 use crate::ui::text::text_pos::TextPos;
-use crate::ui::ui_error::UIResult;
+use crate::ui::ui_error::{LineInsertionFailed, OutOfBounds, UIResult};
 use crate::ui::util::{slice_get, slice_get_mut};
 use snafu::OptionExt;
+use std::cmp::Ordering;
 use std::fmt;
 
+use super::markup::nodes::get_root_mark_node_id;
+
 #[derive(Debug)]
 pub struct GridNodeMap {
     pub lines: Vec<Vec<MarkNodeId>>,
 }
 
 impl GridNodeMap {
-    pub fn new() -> GridNodeMap {
-        GridNodeMap {
-            lines: vec![vec![]],
-        }
-    }
-
-    pub fn add_to_line(&mut self, line_nr: usize, len: usize, node_id: MarkNodeId) -> UIResult<()> {
-        let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
-        let mut new_cols_vec: Vec<MarkNodeId> = std::iter::repeat(node_id).take(len).collect();
-
-        line_ref.append(&mut new_cols_vec);
-
-        Ok(())
-    }
-
     pub fn insert_between_line(
         &mut self,
         line_nr: usize,
@@ -42,18 +30,105 @@ impl GridNodeMap {
         len: usize,
         node_id: MarkNodeId,
     ) -> UIResult<()> {
+        let nr_of_lines = self.lines.len();
+
+        if line_nr < nr_of_lines {
             let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
             let new_cols_vec: Vec<MarkNodeId> = std::iter::repeat(node_id).take(len).collect();
 
             line_ref.splice(index..index, new_cols_vec);
+        } else if line_nr >= nr_of_lines {
+            for _ in 0..((line_nr - nr_of_lines) + 1) {
+                self.push_empty_line();
+            }
+
+            self.insert_between_line(line_nr, index, len, node_id)?;
+        } else {
+            LineInsertionFailed {
+                line_nr,
+                nr_of_lines,
+            }
+            .fail()?;
+        }
 
         Ok(())
     }
 
-    pub fn del_at_line(&mut self, line_nr: usize, index: usize) -> UIResult<()> {
+    pub fn insert_empty_line(&mut self, line_nr: usize) -> UIResult<()> {
+        if line_nr <= self.lines.len() {
+            self.lines.insert(line_nr, Vec::new());
+
+            Ok(())
+        } else {
+            OutOfBounds {
+                index: line_nr,
+                collection_name: "code_lines.lines".to_owned(),
+                len: self.lines.len(),
+            }
+            .fail()
+        }
+    }
+
+    pub fn push_empty_line(&mut self) {
+        self.lines.push(vec![]);
+    }
+
+    pub fn break_line(&mut self, line_nr: usize, col_nr: usize) -> UIResult<()> {
+        // clippy prefers this over if-else
+        match line_nr.cmp(&self.lines.len()) {
+            Ordering::Less => {
+                self.insert_empty_line(line_nr + 1)?;
+
+                let line_ref = self.lines.get_mut(line_nr).unwrap(); // safe because we checked line_nr
+
+                if col_nr < line_ref.len() {
+                    let next_line_str: Vec<MarkNodeId> = line_ref.drain(col_nr..).collect();
+
+                    let next_line_ref = self.lines.get_mut(line_nr + 1).unwrap(); // safe because we just added the line
+
+                    *next_line_ref = next_line_str;
+                }
+
+                Ok(())
+            }
+            Ordering::Equal => self.insert_empty_line(line_nr + 1),
+            Ordering::Greater => OutOfBounds {
+                index: line_nr,
+                collection_name: "grid_node_map.lines".to_owned(),
+                len: self.lines.len(),
+            }
+            .fail(),
+        }
+    }
+
+    pub fn clear_line(&mut self, line_nr: usize) -> UIResult<()> {
         let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
 
-        line_ref.remove(index);
+        *line_ref = vec![];
+
+        Ok(())
+    }
+
+    pub fn del_line(&mut self, line_nr: usize) {
+        self.lines.remove(line_nr);
+    }
+
+    pub fn del_at_line(&mut self, line_nr: usize, column: usize) -> UIResult<()> {
+        let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
+
+        line_ref.remove(column);
+
+        Ok(())
+    }
+
+    pub fn del_range_at_line(
+        &mut self,
+        line_nr: usize,
+        col_range: std::ops::Range<usize>,
+    ) -> UIResult<()> {
+        let line_ref = slice_get_mut(line_nr, &mut self.lines)?;
+
+        line_ref.drain(col_range);
 
         Ok(())
     }
@@ -64,16 +139,12 @@ impl GridNodeMap {
 
             line_ref.drain(selection.start_pos.column..selection.end_pos.column);
         } else {
-            // TODO support multiline
+            unimplemented!("TODO support deleting multiline selection")
         }
 
         Ok(())
     }
 
-    /*pub fn new_line(&mut self) {
-        self.lines.push(vec![])
-    }*/
-
     pub fn get_id_at_row_col(&self, caret_pos: TextPos) -> UIResult<MarkNodeId> {
         let line = slice_get(caret_pos.line, &self.lines)?;
         let node_id = slice_get(caret_pos.column, line)?;
@@ -133,15 +204,15 @@ impl GridNodeMap {
         }
     }
 
-    // returns start and end pos of Expr2, relevant AST node and MarkNodeId of the corresponding MarkupNode
-    pub fn get_expr_start_end_pos(
+    // returns start and end pos of Expr2/Def2, relevant AST node and MarkNodeId of the corresponding MarkupNode
+    pub fn get_block_start_end_pos(
         &self,
         caret_pos: TextPos,
         ed_model: &EdModel,
-    ) -> EdResult<(TextPos, TextPos, ExprId, MarkNodeId)> {
+    ) -> EdResult<(TextPos, TextPos, ASTNodeId, MarkNodeId)> {
         let line = slice_get(caret_pos.line, &self.lines)?;
         let node_id = slice_get(caret_pos.column, line)?;
-        let node = ed_model.markup_node_pool.get(*node_id);
+        let node = ed_model.mark_node_pool.get(*node_id);
 
         if node.is_nested() {
             let (start_pos, end_pos) = self.get_nested_start_end_pos(*node_id, ed_model)?;
@@ -151,10 +222,7 @@ impl GridNodeMap {
             let (first_node_index, last_node_index) = first_last_index_of(*node_id, line)?;
 
             let curr_node_id = slice_get(first_node_index, line)?;
-            let curr_ast_node_id = ed_model
-                .markup_node_pool
-                .get(*curr_node_id)
-                .get_ast_node_id();
+            let curr_ast_node_id = ed_model.mark_node_pool.get(*curr_node_id).get_ast_node_id();
 
             let mut expr_start_index = first_node_index;
             let mut expr_end_index = last_node_index;
@@ -165,7 +233,7 @@ impl GridNodeMap {
             for i in (0..first_node_index).rev() {
                 let prev_pos_node_id = slice_get(i, line)?;
                 let prev_ast_node_id = ed_model
-                    .markup_node_pool
+                    .mark_node_pool
                     .get(*prev_pos_node_id)
                     .get_ast_node_id();
 
@@ -187,7 +255,7 @@ impl GridNodeMap {
             for i in last_node_index..line.len() {
                 let next_pos_node_id = slice_get(i, line)?;
                 let next_ast_node_id = ed_model
-                    .markup_node_pool
+                    .mark_node_pool
                     .get(*next_pos_node_id)
                     .get_ast_node_id();
 
@@ -204,7 +272,7 @@ impl GridNodeMap {
             }
 
             let correct_mark_node_id =
-                GridNodeMap::get_top_node_with_expr_id(*curr_node_id, &ed_model.markup_node_pool);
+                GridNodeMap::get_top_node_with_expr_id(*curr_node_id, &ed_model.mark_node_pool);
 
             Ok((
                 TextPos {
@@ -225,12 +293,12 @@ impl GridNodeMap {
     // `{` is not the entire Expr2
     fn get_top_node_with_expr_id(
         curr_node_id: MarkNodeId,
-        markup_node_pool: &SlowPool,
+        mark_node_pool: &SlowPool,
     ) -> MarkNodeId {
-        let curr_node = markup_node_pool.get(curr_node_id);
+        let curr_node = mark_node_pool.get(curr_node_id);
 
         if let Some(parent_id) = curr_node.get_parent_id_opt() {
-            let parent = markup_node_pool.get(parent_id);
+            let parent = mark_node_pool.get(parent_id);
 
             if parent.get_ast_node_id() == curr_node.get_ast_node_id() {
                 parent_id
@@ -247,29 +315,108 @@ impl GridNodeMap {
         nested_node_id: MarkNodeId,
         ed_model: &EdModel,
     ) -> EdResult<(TextPos, TextPos)> {
-        let parent_mark_node = ed_model.markup_node_pool.get(nested_node_id);
+        let left_most_leaf = self.get_leftmost_leaf(nested_node_id, ed_model)?;
 
-        let all_child_ids = parent_mark_node.get_children_ids();
-        let first_child_id = all_child_ids
+        let right_most_leaf = self.get_rightmost_leaf(nested_node_id, ed_model)?;
+
+        let expr_start_pos = ed_model
+            .grid_node_map
+            .get_node_position(left_most_leaf, true)?;
+        let expr_end_pos = ed_model
+            .grid_node_map
+            .get_node_position(right_most_leaf, false)?
+            .increment_col();
+
+        Ok((expr_start_pos, expr_end_pos))
+    }
+
+    fn get_leftmost_leaf(
+        &self,
+        nested_node_id: MarkNodeId,
+        ed_model: &EdModel,
+    ) -> EdResult<MarkNodeId> {
+        let mut children_ids = ed_model
+            .mark_node_pool
+            .get(nested_node_id)
+            .get_children_ids();
+        let mut first_child_id = 0;
+
+        while !children_ids.is_empty() {
+            first_child_id = *children_ids
                 .first()
                 .with_context(|| NestedNodeWithoutChildren {
                     node_id: nested_node_id,
                 })?;
-        let last_child_id = all_child_ids
+
+            children_ids = ed_model
+                .mark_node_pool
+                .get(first_child_id)
+                .get_children_ids();
+        }
+
+        Ok(first_child_id)
+    }
+
+    fn get_rightmost_leaf(
+        &self,
+        nested_node_id: MarkNodeId,
+        ed_model: &EdModel,
+    ) -> EdResult<MarkNodeId> {
+        let mut children_ids = ed_model
+            .mark_node_pool
+            .get(nested_node_id)
+            .get_children_ids();
+        let mut last_child_id = 0;
+
+        while !children_ids.is_empty() {
+            last_child_id = *children_ids
                 .last()
                 .with_context(|| NestedNodeWithoutChildren {
                     node_id: nested_node_id,
                 })?;
 
-        let expr_start_pos = ed_model
-            .grid_node_map
-            .get_node_position(*first_child_id, true)?;
-        let expr_end_pos = ed_model
-            .grid_node_map
-            .get_node_position(*last_child_id, false)?
-            .increment_col();
+            children_ids = ed_model
+                .mark_node_pool
+                .get(last_child_id)
+                .get_children_ids();
+        }
 
-        Ok((expr_start_pos, expr_end_pos))
+        Ok(last_child_id)
+    }
+
+    // get id of root mark_node whose ast_node_id points to a DefId
+    pub fn get_def_mark_node_id_before_line(
+        &self,
+        line_nr: usize,
+        mark_node_pool: &SlowPool,
+    ) -> EdResult<MarkNodeId> {
+        for curr_line_nr in (0..line_nr).rev() {
+            let first_col_pos = TextPos {
+                line: curr_line_nr,
+                column: 0,
+            };
+
+            if self.node_exists_at_pos(first_col_pos) {
+                let mark_node_id = self.get_id_at_row_col(first_col_pos)?;
+                let root_mark_node_id = get_root_mark_node_id(mark_node_id, mark_node_pool);
+
+                let ast_node_id = mark_node_pool.get(root_mark_node_id).get_ast_node_id();
+
+                if let ASTNodeId::ADefId(_) = ast_node_id {
+                    return Ok(root_mark_node_id);
+                }
+            }
+        }
+
+        NoDefMarkNodeBeforeLineNr { line_nr }.fail()
+    }
+}
+
+impl Default for GridNodeMap {
+    fn default() -> Self {
+        GridNodeMap {
+            lines: vec![Vec::new()],
+        }
     }
 }
 
@@ -282,10 +429,10 @@ impl fmt::Display for GridNodeMap {
                 .collect::<Vec<String>>()
                 .join(", ");
 
-            write!(f, "{}", row_str)?;
+            writeln!(f, "{}", row_str)?;
         }
 
-        write!(f, "      (grid_node_map)")?;
+        writeln!(f, "(grid_node_map, {:?} lines)", self.lines.len())?;
 
         Ok(())
     }

editor/src/editor/keyboard_input.rs

@@ -43,7 +43,7 @@ pub fn handle_keydown(
             }
         }
 
-        A | Home | End => pass_keydown_to_focused(&modifiers, virtual_keycode, app_model)?,
+        A | S | R | Home | End => pass_keydown_to_focused(&modifiers, virtual_keycode, app_model)?,
 
         F11 => pass_keydown_to_focused(&modifiers, virtual_keycode, app_model)?,
 

editor/src/editor/main.rs

@@ -1,9 +1,8 @@
 use super::keyboard_input;
 use super::style::CODE_TXT_XY;
-use crate::editor::ed_error::print_ui_err;
 use crate::editor::mvc::ed_view;
 use crate::editor::mvc::ed_view::RenderedWgpu;
-use crate::editor::resources::strings::NOTHING_OPENED;
+use crate::editor::resources::strings::{HELLO_WORLD, NOTHING_OPENED};
 use crate::editor::{
     config::Config,
     ed_error::print_err,
@@ -20,17 +19,23 @@ use crate::graphics::{
 };
 use crate::lang::expr::Env;
 use crate::lang::pool::Pool;
-use crate::ui::ui_error::UIError::FileOpenFailed;
-use crate::ui::util::slice_get;
-use bumpalo::collections::String as BumpString;
+use crate::ui::text::caret_w_select::CaretPos;
+use crate::ui::util::path_to_string;
 use bumpalo::Bump;
 use cgmath::Vector2;
+use fs_extra::dir::{copy, ls, CopyOptions, DirEntryAttr, DirEntryValue};
 use pipelines::RectResources;
-use roc_module::symbol::Interns;
-use roc_module::symbol::{IdentIds, ModuleIds};
+use roc_can::builtins::builtin_defs_map;
+use roc_collections::all::MutMap;
+use roc_load;
+use roc_load::file::LoadedModule;
+use roc_module::symbol::IdentIds;
 use roc_types::subs::VarStore;
+use std::collections::HashSet;
+use std::fs::{self, File};
+use std::io::Write;
 use std::{error::Error, io, path::Path};
-use wgpu::{CommandEncoder, RenderPass, TextureView};
+use wgpu::{CommandEncoder, LoadOp, RenderPass, TextureView};
 use wgpu_glyph::GlyphBrush;
 use winit::{
     dpi::PhysicalSize,
@@ -48,26 +53,13 @@ use winit::{
 
 /// The editor is actually launched from the CLI if you pass it zero arguments,
 /// or if you provide it 1 or more files or directories to open on launch.
-pub fn launch(filepaths: &[&Path]) -> io::Result<()> {
-    //TODO support using multiple filepaths
-    let first_path_opt = if !filepaths.is_empty() {
-        match slice_get(0, filepaths) {
-            Ok(path_ref_ref) => Some(*path_ref_ref),
-            Err(e) => {
-                eprintln!("{}", e);
-                None
-            }
-        }
-    } else {
-        None
-    };
-
-    run_event_loop(first_path_opt).expect("Error running event loop");
+pub fn launch(project_dir_path_opt: Option<&Path>) -> io::Result<()> {
+    run_event_loop(project_dir_path_opt).expect("Error running event loop");
 
     Ok(())
 }
 
-fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
+fn run_event_loop(project_dir_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
     env_logger::init();
 
     // Open window and create a surface
@@ -134,51 +126,37 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
     let env_arena = Bump::new();
     let code_arena = Bump::new();
 
+    let (file_path_str, code_str) = read_main_roc_file(project_dir_path_opt);
+    println!("Loading file {}...", file_path_str);
+
+    let file_path = Path::new(&file_path_str);
+
+    let loaded_module = load_module(file_path);
+
     let mut var_store = VarStore::default();
     let dep_idents = IdentIds::exposed_builtins(8);
-
     let exposed_ident_ids = IdentIds::default();
-    let mut module_ids = ModuleIds::default();
-    let mod_id = module_ids.get_or_insert(&"ModId123".into());
-
-    let interns = Interns {
-        module_ids,
-        all_ident_ids: IdentIds::exposed_builtins(8),
-    };
+    let module_ids = loaded_module.interns.module_ids.clone();
 
     let env = Env::new(
-        mod_id,
+        loaded_module.module_id,
         &env_arena,
         &mut env_pool,
         &mut var_store,
         dep_idents,
-        &interns.module_ids,
+        &module_ids,
         exposed_ident_ids,
     );
 
-    let mut code_str = BumpString::from_str_in("", &code_arena);
-
-    let file_path = if let Some(file_path) = file_path_opt {
-        match std::fs::read_to_string(file_path) {
-            Ok(file_as_str) => {
-                code_str = BumpString::from_str_in(&file_as_str, &code_arena);
-                file_path
-            }
-
-            Err(e) => {
-                print_ui_err(&FileOpenFailed {
-                    path_str: file_path.to_string_lossy().to_string(),
-                    err_msg: e.to_string(),
-                });
-                Path::new("")
-            }
-        }
-    } else {
-        Path::new("")
-    };
-
     let ed_model_opt = {
-        let ed_model_res = ed_model::init_model(&code_str, file_path, env, &interns, &code_arena);
+        let ed_model_res = ed_model::init_model(
+            &code_str,
+            file_path,
+            env,
+            loaded_module,
+            &code_arena,
+            CaretPos::End,
+        );
 
         match ed_model_res {
             Ok(mut ed_model) => {
@@ -251,7 +229,8 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
                 event: event::WindowEvent::ReceivedCharacter(ch),
                 ..
             } => {
-                let input_outcome_res = app_update::handle_new_char(&ch, &mut app_model);
+                let input_outcome_res =
+                    app_update::handle_new_char(&ch, &mut app_model, keyboard_modifiers);
                 if let Err(e) = input_outcome_res {
                     print_err(&e)
                 } else if let Ok(InputOutcome::Ignored) = input_outcome_res {
@@ -314,23 +293,55 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
                     }
 
                     if let Some(ref rendered_wgpu) = rendered_wgpu_opt {
-                        for text_section in &rendered_wgpu.text_sections {
+                        draw_rects(
+                            &rendered_wgpu.rects_behind,
+                            &mut encoder,
+                            &frame.view,
+                            &gpu_device,
+                            &rect_resources,
+                            wgpu::LoadOp::Clear(to_wgpu_color(ed_theme.background)),
+                        );
+
+                        for text_section in &rendered_wgpu.text_sections_behind {
                             let borrowed_text = text_section.to_borrowed();
 
                             glyph_brush.queue(borrowed_text);
                         }
 
-                        draw_all_rects(
-                            &rendered_wgpu.rects,
+                        // draw first layer of text
+                        glyph_brush
+                            .draw_queued(
+                                &gpu_device,
+                                &mut staging_belt,
+                                &mut encoder,
+                                &frame.view,
+                                size.width,
+                                size.height,
+                            )
+                            .expect("Failed to draw first layer of text.");
+
+                        // draw rects on top of first text layer
+                        draw_rects(
+                            &rendered_wgpu.rects_front,
                             &mut encoder,
                             &frame.view,
                             &gpu_device,
                             &rect_resources,
-                            &ed_theme,
-                        )
+                            wgpu::LoadOp::Load,
+                        );
+
+                        for text_section in &rendered_wgpu.text_sections_front {
+                            let borrowed_text = text_section.to_borrowed();
+
+                            glyph_brush.queue(borrowed_text);
+                        }
                     }
                 } else {
-                    begin_render_pass(&mut encoder, &frame.view, &ed_theme);
+                    begin_render_pass(
+                        &mut encoder,
+                        &frame.view,
+                        wgpu::LoadOp::Clear(to_wgpu_color(ed_theme.background)),
+                    );
 
                     queue_no_file_text(
                         &size,
@@ -341,7 +352,7 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
                     );
                 }
 
-                // draw all text
+                // draw text
                 glyph_brush
                     .draw_queued(
                         &gpu_device,
@@ -351,7 +362,7 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
                         size.width,
                         size.height,
                     )
-                    .expect("Draw queued");
+                    .expect("Failed to draw queued text.");
 
                 staging_belt.finish();
                 cmd_queue.submit(Some(encoder.finish()));
@@ -374,17 +385,17 @@ fn run_event_loop(file_path_opt: Option<&Path>) -> Result<(), Box<dyn Error>> {
     Ok(())
 }
 
-fn draw_all_rects(
+fn draw_rects(
     all_rects: &[Rect],
     encoder: &mut CommandEncoder,
     texture_view: &TextureView,
     gpu_device: &wgpu::Device,
     rect_resources: &RectResources,
-    ed_theme: &EdTheme,
+    load_op: LoadOp<wgpu::Color>,
 ) {
     let rect_buffers = create_rect_buffers(gpu_device, encoder, all_rects);
 
-    let mut render_pass = begin_render_pass(encoder, texture_view, ed_theme);
+    let mut render_pass = begin_render_pass(encoder, texture_view, load_op);
 
     render_pass.set_pipeline(&rect_resources.pipeline);
     render_pass.set_bind_group(0, &rect_resources.ortho.bind_group, &[]);
@@ -399,16 +410,14 @@ fn draw_all_rects(
 fn begin_render_pass<'a>(
     encoder: &'a mut CommandEncoder,
     texture_view: &'a TextureView,
-    ed_theme: &EdTheme,
+    load_op: LoadOp<wgpu::Color>,
 ) -> RenderPass<'a> {
-    let bg_color = to_wgpu_color(ed_theme.background);
-
     encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
         color_attachments: &[wgpu::RenderPassColorAttachment {
             view: texture_view,
             resolve_target: None,
             ops: wgpu::Operations {
-                load: wgpu::LoadOp::Clear(bg_color),
+                load: load_op,
                 store: true,
             },
         }],
@@ -417,6 +426,166 @@ fn begin_render_pass<'a>(
     })
 }
 
+type PathStr = String;
+
+fn read_main_roc_file(project_dir_path_opt: Option<&Path>) -> (PathStr, String) {
+    if let Some(project_dir_path) = project_dir_path_opt {
+        let mut ls_config = HashSet::new();
+        ls_config.insert(DirEntryAttr::FullName);
+
+        let dir_items = ls(project_dir_path, &ls_config)
+            .unwrap_or_else(|err| panic!("Failed to list items in project directory: {:?}", err))
+            .items;
+
+        let file_names = dir_items
+            .iter()
+            .map(|info_hash_map| {
+                info_hash_map
+                    .values()
+                    .map(|dir_entry_value| {
+                        if let DirEntryValue::String(file_name) = dir_entry_value {
+                            Some(file_name)
+                        } else {
+                            None
+                        }
+                    })
+                    .flatten() // remove None
+                    .collect::<Vec<&String>>()
+            })
+            .flatten();
+
+        let roc_file_names: Vec<&String> = file_names
+            .filter(|file_name| file_name.contains(".roc"))
+            .collect();
+
+        let project_dir_path_str = path_to_string(project_dir_path);
+
+        if let Some(&roc_file_name) = roc_file_names.first() {
+            let full_roc_file_path_str = vec![
+                project_dir_path_str.clone(),
+                "/".to_owned(),
+                roc_file_name.clone(),
+            ]
+            .join("");
+            let file_as_str = std::fs::read_to_string(&Path::new(&full_roc_file_path_str))
+                .unwrap_or_else(|err| panic!("In the provided project {:?}, I found the roc file {}, but I failed to read it: {}", &project_dir_path_str, &full_roc_file_path_str, err));
+
+            (full_roc_file_path_str, file_as_str)
+        } else {
+            init_new_roc_project(&project_dir_path_str)
+        }
+    } else {
+        init_new_roc_project("new-roc-project")
+    }
+}
+
+// returns path and content of app file
+fn init_new_roc_project(project_dir_path_str: &str) -> (PathStr, String) {
+    let orig_platform_path = Path::new("./examples/hello-world/platform");
+
+    let project_dir_path = Path::new(project_dir_path_str);
+
+    let roc_file_path_str = vec![project_dir_path_str, "/UntitledApp.roc"].join("");
+    let roc_file_path = Path::new("./new-roc-project/UntitledApp.roc");
+
+    let project_platform_path_str = vec![project_dir_path_str, "/platform"].join("");
+    let project_platform_path = Path::new(&project_platform_path_str);
+
+    if !project_dir_path.exists() {
+        fs::create_dir(project_dir_path).expect("Failed to create dir for roc project.");
+    }
+
+    copy_roc_platform_if_not_exists(orig_platform_path, project_platform_path, project_dir_path);
+
+    let code_str = create_roc_file_if_not_exists(project_dir_path, roc_file_path);
+
+    (roc_file_path_str, code_str)
+}
+
+// returns contents of file
+fn create_roc_file_if_not_exists(project_dir_path: &Path, roc_file_path: &Path) -> String {
+    if !roc_file_path.exists() {
+        let mut roc_file = File::create(roc_file_path).unwrap_or_else(|err| {
+            panic!("No roc file path was passed to the editor, so I wanted to create a new roc project with the file {:?}, but it failed: {}", roc_file_path, err)
+        });
+
+        write!(roc_file, "{}", HELLO_WORLD).unwrap_or_else(|err| {
+            panic!(
+                r#"No roc file path was passed to the editor, so I created a new roc project with the file {:?}
+                I wanted to write roc hello world to that file, but it failed: {:?}"#,
+                roc_file_path,
+                err
+            )
+        });
+
+        HELLO_WORLD.to_string()
+    } else {
+        std::fs::read_to_string(roc_file_path).unwrap_or_else(|err| {
+            panic!(
+                "I detected an existing {:?} inside {:?}, but I failed to read from it: {}",
+                roc_file_path, project_dir_path, err
+            )
+        })
+    }
+}
+
+fn copy_roc_platform_if_not_exists(
+    orig_platform_path: &Path,
+    project_platform_path: &Path,
+    project_dir_path: &Path,
+) {
+    if !orig_platform_path.exists() && !project_platform_path.exists() {
+        panic!(
+            r#"No roc file path was passed to the editor, I wanted to create a new roc project but I could not find the platform at {:?}.
+            Are you at the root of the roc repository?"#,
+            orig_platform_path
+        );
+    } else if !project_platform_path.exists() {
+        copy(orig_platform_path, project_dir_path, &CopyOptions::new()).unwrap_or_else(|err|{
+            panic!(r#"No roc file path was passed to the editor, so I wanted to create a new roc project and roc projects require a platform,
+            I tried to copy the platform at {:?} to {:?} but it failed: {}"#,
+            orig_platform_path,
+            project_platform_path,
+            err
+        )
+        });
+    }
+}
+
+pub fn load_module(src_file: &Path) -> LoadedModule {
+    let subs_by_module = MutMap::default();
+
+    let arena = Bump::new();
+    let loaded = roc_load::file::load_and_typecheck(
+        &arena,
+        src_file.to_path_buf(),
+        arena.alloc(roc_builtins::std::standard_stdlib()),
+        src_file.parent().unwrap_or_else(|| {
+            panic!(
+                "src_file {:?} did not have a parent directory but I need to have one.",
+                src_file
+            )
+        }),
+        subs_by_module,
+        8,
+        builtin_defs_map,
+    );
+
+    match loaded {
+        Ok(x) => x,
+        Err(roc_load::file::LoadingProblem::FormattedReport(report)) => {
+            panic!(
+                "Failed to load module from src_file {:?}. Report: {:?}",
+                src_file, report
+            );
+        }
+        Err(e) => panic!(
+            "Failed to load module from src_file {:?}: {:?}",
+            src_file, e
+        ),
+    }
+}
+
 fn queue_no_file_text(
     size: &PhysicalSize<u32>,
     text: &str,

editor/src/editor/markup/common_nodes.rs

@@ -0,0 +1,107 @@
+use crate::{
+    editor::{slow_pool::MarkNodeId, syntax_highlight::HighlightStyle},
+    lang::{ast::ExprId, parse::ASTNodeId},
+};
+
+use super::{attribute::Attributes, nodes, nodes::MarkupNode};
+
+pub fn new_equals_mn(ast_node_id: ASTNodeId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::EQUALS.to_owned(),
+        ast_node_id,
+        syn_high_style: HighlightStyle::Operator,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_comma_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::COMMA.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Blank,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_blank_mn(ast_node_id: ASTNodeId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Blank {
+        ast_node_id,
+        syn_high_style: HighlightStyle::Blank,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_blank_mn_w_nls(
+    ast_node_id: ASTNodeId,
+    parent_id_opt: Option<MarkNodeId>,
+    nr_of_newlines: usize,
+) -> MarkupNode {
+    MarkupNode::Blank {
+        ast_node_id,
+        syn_high_style: HighlightStyle::Blank,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: nr_of_newlines,
+    }
+}
+
+pub fn new_colon_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::COLON.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Operator,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_left_accolade_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::LEFT_ACCOLADE.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_right_accolade_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::RIGHT_ACCOLADE.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_left_square_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::LEFT_SQUARE_BR.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}
+
+pub fn new_right_square_mn(expr_id: ExprId, parent_id_opt: Option<MarkNodeId>) -> MarkupNode {
+    MarkupNode::Text {
+        content: nodes::RIGHT_SQUARE_BR.to_owned(),
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::Bracket,
+        attributes: Attributes::new(),
+        parent_id_opt,
+        newlines_at_end: 0,
+    }
+}

editor/src/editor/markup/mod.rs

@@ -1,2 +1,3 @@
 pub mod attribute;
+pub mod common_nodes;
 pub mod nodes;

editor/src/editor/markup/nodes.rs

@@ -1,42 +1,62 @@
 use super::attribute::Attributes;
 use crate::editor::ed_error::EdResult;
 use crate::editor::ed_error::ExpectedTextNode;
-use crate::editor::ed_error::GetContentOnNestedNode;
 use crate::editor::ed_error::{NestedNodeMissingChild, NestedNodeRequired};
+use crate::editor::markup::common_nodes::new_blank_mn;
+use crate::editor::markup::common_nodes::new_blank_mn_w_nls;
+use crate::editor::markup::common_nodes::new_colon_mn;
+use crate::editor::markup::common_nodes::new_comma_mn;
+use crate::editor::markup::common_nodes::new_equals_mn;
+use crate::editor::markup::common_nodes::new_left_accolade_mn;
+use crate::editor::markup::common_nodes::new_left_square_mn;
+use crate::editor::markup::common_nodes::new_right_accolade_mn;
+use crate::editor::markup::common_nodes::new_right_square_mn;
+use crate::editor::mvc::tld_value_update::tld_mark_node;
 use crate::editor::slow_pool::MarkNodeId;
 use crate::editor::slow_pool::SlowPool;
 use crate::editor::syntax_highlight::HighlightStyle;
 use crate::editor::util::index_of;
-use crate::lang::ast::{Expr2, ExprId, RecordField};
-use crate::lang::{expr::Env, pool::PoolStr};
+use crate::lang::ast::Def2;
+use crate::lang::ast::DefId;
+use crate::lang::ast::ExprId;
+use crate::lang::ast::RecordField;
+use crate::lang::ast::ValueDef;
+use crate::lang::parse::ASTNodeId;
+use crate::lang::parse::{AppHeader, AST};
+use crate::lang::pattern::get_identifier_string;
+use crate::lang::{ast::Expr2, expr::Env, pool::PoolStr};
 use crate::ui::util::slice_get;
 use bumpalo::Bump;
+use roc_module::symbol::Interns;
 use std::fmt;
 
 #[derive(Debug)]
 pub enum MarkupNode {
     Nested {
-        ast_node_id: ExprId,
+        ast_node_id: ASTNodeId,
         children_ids: Vec<MarkNodeId>,
         parent_id_opt: Option<MarkNodeId>,
+        newlines_at_end: usize,
     },
     Text {
         content: String,
-        ast_node_id: ExprId,
+        ast_node_id: ASTNodeId,
         syn_high_style: HighlightStyle,
         attributes: Attributes,
         parent_id_opt: Option<MarkNodeId>,
+        newlines_at_end: usize,
     },
     Blank {
-        ast_node_id: ExprId,
+        ast_node_id: ASTNodeId,
         attributes: Attributes,
         syn_high_style: HighlightStyle, // TODO remove HighlightStyle, this is always HighlightStyle::Blank
         parent_id_opt: Option<MarkNodeId>,
+        newlines_at_end: usize,
     },
 }
 
 impl MarkupNode {
-    pub fn get_ast_node_id(&self) -> ExprId {
+    pub fn get_ast_node_id(&self) -> ASTNodeId {
         match self {
             MarkupNode::Nested { ast_node_id, .. } => *ast_node_id,
             MarkupNode::Text { ast_node_id, .. } => *ast_node_id,
@@ -60,9 +80,9 @@ impl MarkupNode {
         }
     }
 
-    pub fn get_sibling_ids(&self, markup_node_pool: &SlowPool) -> Vec<MarkNodeId> {
+    pub fn get_sibling_ids(&self, mark_node_pool: &SlowPool) -> Vec<MarkNodeId> {
         if let Some(parent_id) = self.get_parent_id_opt() {
-            let parent_node = markup_node_pool.get(parent_id);
+            let parent_node = mark_node_pool.get(parent_id);
 
             parent_node.get_children_ids()
         } else {
@@ -74,7 +94,7 @@ impl MarkupNode {
     pub fn get_child_indices(
         &self,
         child_id: MarkNodeId,
-        markup_node_pool: &SlowPool,
+        mark_node_pool: &SlowPool,
     ) -> EdResult<(usize, usize)> {
         match self {
             MarkupNode::Nested { children_ids, .. } => {
@@ -87,7 +107,7 @@ impl MarkupNode {
                         mark_child_index_opt = Some(indx);
                     }
 
-                    let child_mark_node = markup_node_pool.get(mark_child_id);
+                    let child_mark_node = mark_node_pool.get(mark_child_id);
                     // a node that points to the same ast_node as the parent is a ',', '[', ']'
                     // those are not "real" ast children
                     if child_mark_node.get_ast_node_id() != self_ast_id {
@@ -147,15 +167,25 @@ impl MarkupNode {
         }
     }
 
-    // can't be &str, this creates borrowing issues
-    pub fn get_content(&self) -> EdResult<String> {
+    pub fn get_content(&self) -> String {
         match self {
-            MarkupNode::Nested { .. } => GetContentOnNestedNode {}.fail(),
-            MarkupNode::Text { content, .. } => Ok(content.clone()),
-            MarkupNode::Blank { .. } => Ok(BLANK_PLACEHOLDER.to_owned()),
+            MarkupNode::Nested { .. } => "".to_owned(),
+            MarkupNode::Text { content, .. } => content.clone(),
+            MarkupNode::Blank { .. } => BLANK_PLACEHOLDER.to_owned(),
         }
     }
 
+    // gets content and adds newline from newline_at_end
+    pub fn get_full_content(&self) -> String {
+        let mut full_content = self.get_content();
+
+        for _ in 0..self.get_newlines_at_end() {
+            full_content.push('\n')
+        }
+
+        full_content
+    }
+
     pub fn get_content_mut(&mut self) -> EdResult<&mut String> {
         match self {
             MarkupNode::Nested { .. } => ExpectedTextNode {
@@ -172,11 +202,10 @@ impl MarkupNode {
         }
     }
 
-    pub fn is_all_alphanumeric(&self) -> EdResult<bool> {
-        Ok(self
-            .get_content()?
+    pub fn is_all_alphanumeric(&self) -> bool {
+        self.get_content()
             .chars()
-            .all(|chr| chr.is_ascii_alphanumeric()))
+            .all(|chr| chr.is_ascii_alphanumeric())
     }
 
     pub fn add_child_at_index(&mut self, index: usize, child_id: MarkNodeId) -> EdResult<()> {
@@ -209,6 +238,34 @@ impl MarkupNode {
     pub fn is_nested(&self) -> bool {
         matches!(self, MarkupNode::Nested { .. })
     }
+
+    pub fn get_newlines_at_end(&self) -> usize {
+        match self {
+            MarkupNode::Nested {
+                newlines_at_end, ..
+            } => *newlines_at_end,
+            MarkupNode::Text {
+                newlines_at_end, ..
+            } => *newlines_at_end,
+            MarkupNode::Blank {
+                newlines_at_end, ..
+            } => *newlines_at_end,
+        }
+    }
+
+    pub fn add_newline_at_end(&mut self) {
+        match self {
+            MarkupNode::Nested {
+                newlines_at_end, ..
+            } => *newlines_at_end += 1,
+            MarkupNode::Text {
+                newlines_at_end, ..
+            } => *newlines_at_end += 1,
+            MarkupNode::Blank {
+                newlines_at_end, ..
+            } => *newlines_at_end += 1,
+        }
+    }
 }
 
 fn get_string<'a>(env: &Env<'a>, pool_str: &PoolStr) -> String {
@@ -223,12 +280,13 @@ pub const RIGHT_SQUARE_BR: &str = " ]";
 pub const COLON: &str = ": ";
 pub const COMMA: &str = ", ";
 pub const STRING_QUOTES: &str = "\"\"";
+pub const EQUALS: &str = " = ";
 
 fn new_markup_node(
     text: String,
-    node_id: ExprId,
+    node_id: ASTNodeId,
     highlight_style: HighlightStyle,
-    markup_node_pool: &mut SlowPool,
+    mark_node_pool: &mut SlowPool,
 ) -> MarkNodeId {
     let node = MarkupNode::Text {
         content: text,
@@ -236,9 +294,51 @@ fn new_markup_node(
         syn_high_style: highlight_style,
         attributes: Attributes::new(),
         parent_id_opt: None,
+        newlines_at_end: 0,
     };
 
-    markup_node_pool.add(node)
+    mark_node_pool.add(node)
+}
+
+pub fn def2_to_markup<'a, 'b>(
+    arena: &'a Bump,
+    env: &mut Env<'b>,
+    def2: &Def2,
+    def2_node_id: DefId,
+    mark_node_pool: &mut SlowPool,
+    interns: &Interns,
+) -> EdResult<MarkNodeId> {
+    let ast_node_id = ASTNodeId::ADefId(def2_node_id);
+
+    let mark_node_id = match def2 {
+        Def2::ValueDef {
+            identifier_id,
+            expr_id,
+        } => {
+            let expr_mn_id = expr2_to_markup(
+                arena,
+                env,
+                env.pool.get(*expr_id),
+                *expr_id,
+                mark_node_pool,
+                interns,
+            )?;
+
+            let tld_mn = tld_mark_node(
+                *identifier_id,
+                expr_mn_id,
+                ast_node_id,
+                mark_node_pool,
+                env,
+                interns,
+            )?;
+
+            mark_node_pool.add(tld_mn)
+        }
+        Def2::Blank => mark_node_pool.add(new_blank_mn_w_nls(ast_node_id, None, 2)),
+    };
+
+    Ok(mark_node_id)
 }
 
 // make Markup Nodes: generate String representation, assign Highlighting Style
@@ -247,55 +347,44 @@ pub fn expr2_to_markup<'a, 'b>(
     env: &mut Env<'b>,
     expr2: &Expr2,
     expr2_node_id: ExprId,
-    markup_node_pool: &mut SlowPool,
-) -> MarkNodeId {
-    match expr2 {
+    mark_node_pool: &mut SlowPool,
+    interns: &Interns,
+) -> EdResult<MarkNodeId> {
+    let ast_node_id = ASTNodeId::AExprId(expr2_node_id);
+
+    let mark_node_id = match expr2 {
         Expr2::SmallInt { text, .. }
         | Expr2::I128 { text, .. }
         | Expr2::U128 { text, .. }
         | Expr2::Float { text, .. } => {
             let num_str = get_string(env, text);
 
-            new_markup_node(
-                num_str,
-                expr2_node_id,
-                HighlightStyle::Number,
-                markup_node_pool,
-            )
+            new_markup_node(num_str, ast_node_id, HighlightStyle::Number, mark_node_pool)
         }
         Expr2::Str(text) => new_markup_node(
             "\"".to_owned() + text.as_str(env.pool) + "\"",
-            expr2_node_id,
+            ast_node_id,
             HighlightStyle::String,
-            markup_node_pool,
+            mark_node_pool,
         ),
         Expr2::GlobalTag { name, .. } => new_markup_node(
             get_string(env, name),
-            expr2_node_id,
+            ast_node_id,
             HighlightStyle::Type,
-            markup_node_pool,
+            mark_node_pool,
         ),
         Expr2::Call { expr: expr_id, .. } => {
             let expr = env.pool.get(*expr_id);
-            expr2_to_markup(arena, env, expr, *expr_id, markup_node_pool)
+            expr2_to_markup(arena, env, expr, *expr_id, mark_node_pool, interns)?
         }
         Expr2::Var(symbol) => {
             //TODO make bump_format with arena
             let text = format!("{:?}", symbol);
-            new_markup_node(
-                text,
-                expr2_node_id,
-                HighlightStyle::Variable,
-                markup_node_pool,
-            )
+            new_markup_node(text, ast_node_id, HighlightStyle::Variable, mark_node_pool)
         }
         Expr2::List { elems, .. } => {
-            let mut children_ids = vec![new_markup_node(
-                LEFT_SQUARE_BR.to_string(),
-                expr2_node_id,
-                HighlightStyle::Bracket,
-                markup_node_pool,
-            )];
+            let mut children_ids =
+                vec![mark_node_pool.add(new_left_square_mn(expr2_node_id, None))];
 
             let indexed_node_ids: Vec<(usize, ExprId)> =
                 elems.iter(env.pool).copied().enumerate().collect();
@@ -308,64 +397,43 @@ pub fn expr2_to_markup<'a, 'b>(
                     env,
                     sub_expr2,
                     *node_id,
-                    markup_node_pool,
-                ));
+                    mark_node_pool,
+                    interns,
+                )?);
 
                 if idx + 1 < elems.len() {
-                    children_ids.push(new_markup_node(
-                        ", ".to_string(),
-                        expr2_node_id,
-                        HighlightStyle::Operator,
-                        markup_node_pool,
-                    ));
+                    children_ids.push(mark_node_pool.add(new_comma_mn(expr2_node_id, None)));
                 }
             }
-            children_ids.push(new_markup_node(
-                RIGHT_SQUARE_BR.to_string(),
-                expr2_node_id,
-                HighlightStyle::Bracket,
-                markup_node_pool,
-            ));
+            children_ids.push(mark_node_pool.add(new_right_square_mn(expr2_node_id, None)));
 
             let list_node = MarkupNode::Nested {
-                ast_node_id: expr2_node_id,
+                ast_node_id,
                 children_ids,
                 parent_id_opt: None,
+                newlines_at_end: 0,
             };
 
-            markup_node_pool.add(list_node)
+            mark_node_pool.add(list_node)
         }
         Expr2::EmptyRecord => {
             let children_ids = vec![
-                new_markup_node(
-                    LEFT_ACCOLADE.to_string(),
-                    expr2_node_id,
-                    HighlightStyle::Bracket,
-                    markup_node_pool,
-                ),
-                new_markup_node(
-                    RIGHT_ACCOLADE.to_string(),
-                    expr2_node_id,
-                    HighlightStyle::Bracket,
-                    markup_node_pool,
-                ),
+                mark_node_pool.add(new_left_accolade_mn(expr2_node_id, None)),
+                mark_node_pool.add(new_right_accolade_mn(expr2_node_id, None)),
             ];
 
             let record_node = MarkupNode::Nested {
-                ast_node_id: expr2_node_id,
+                ast_node_id,
                 children_ids,
                 parent_id_opt: None,
+                newlines_at_end: 0,
             };
 
-            markup_node_pool.add(record_node)
+            mark_node_pool.add(record_node)
         }
         Expr2::Record { fields, .. } => {
-            let mut children_ids = vec![new_markup_node(
-                LEFT_ACCOLADE.to_string(),
-                expr2_node_id,
-                HighlightStyle::Bracket,
-                markup_node_pool,
-            )];
+            let mut children_ids =
+                vec![mark_node_pool.add(new_left_accolade_mn(expr2_node_id, None))];
 
             for (idx, field_node_id) in fields.iter_node_ids().enumerate() {
                 let record_field = env.pool.get(field_node_id);
@@ -374,21 +442,16 @@ pub fn expr2_to_markup<'a, 'b>(
 
                 children_ids.push(new_markup_node(
                     field_name.as_str(env.pool).to_owned(),
-                    expr2_node_id,
+                    ast_node_id,
                     HighlightStyle::RecordField,
-                    markup_node_pool,
+                    mark_node_pool,
                 ));
 
                 match record_field {
                     RecordField::InvalidLabelOnly(_, _) => (),
                     RecordField::LabelOnly(_, _, _) => (),
                     RecordField::LabeledValue(_, _, sub_expr2_node_id) => {
-                        children_ids.push(new_markup_node(
-                            COLON.to_string(),
-                            expr2_node_id,
-                            HighlightStyle::Operator,
-                            markup_node_pool,
-                        ));
+                        children_ids.push(mark_node_pool.add(new_colon_mn(expr2_node_id, None)));
 
                         let sub_expr2 = env.pool.get(*sub_expr2_node_id);
                         children_ids.push(expr2_to_markup(
@@ -396,66 +459,117 @@ pub fn expr2_to_markup<'a, 'b>(
                             env,
                             sub_expr2,
                             *sub_expr2_node_id,
-                            markup_node_pool,
-                        ));
+                            mark_node_pool,
+                            interns,
+                        )?);
                     }
                 }
 
                 if idx + 1 < fields.len() {
-                    children_ids.push(new_markup_node(
-                        ", ".to_string(),
-                        expr2_node_id,
-                        HighlightStyle::Operator,
-                        markup_node_pool,
-                    ));
+                    children_ids.push(mark_node_pool.add(new_comma_mn(expr2_node_id, None)));
                 }
             }
 
-            children_ids.push(new_markup_node(
-                RIGHT_ACCOLADE.to_string(),
-                expr2_node_id,
-                HighlightStyle::Bracket,
-                markup_node_pool,
-            ));
+            children_ids.push(mark_node_pool.add(new_right_accolade_mn(expr2_node_id, None)));
 
             let record_node = MarkupNode::Nested {
-                ast_node_id: expr2_node_id,
+                ast_node_id,
                 children_ids,
                 parent_id_opt: None,
+                newlines_at_end: 0,
             };
 
-            markup_node_pool.add(record_node)
+            mark_node_pool.add(record_node)
         }
-        Expr2::Blank => markup_node_pool.add(MarkupNode::Blank {
-            ast_node_id: expr2_node_id,
+        Expr2::Blank => mark_node_pool.add(new_blank_mn(ast_node_id, None)),
+        Expr2::LetValue {
+            def_id,
+            body_id: _,
+            body_var: _,
+        } => {
+            let pattern_id = env.pool.get(*def_id).get_pattern_id();
+
+            let pattern2 = env.pool.get(pattern_id);
+
+            let val_name = get_identifier_string(pattern2, interns)?;
+
+            let val_name_mn = MarkupNode::Text {
+                content: val_name,
+                ast_node_id,
+                syn_high_style: HighlightStyle::Variable,
                 attributes: Attributes::new(),
-            syn_high_style: HighlightStyle::Blank,
                 parent_id_opt: None,
-        }),
+                newlines_at_end: 0,
+            };
+
+            let val_name_mn_id = mark_node_pool.add(val_name_mn);
+
+            let equals_mn_id = mark_node_pool.add(new_equals_mn(ast_node_id, None));
+
+            let value_def = env.pool.get(*def_id);
+
+            match value_def {
+                ValueDef::NoAnnotation {
+                    pattern_id: _,
+                    expr_id,
+                    expr_var: _,
+                } => {
+                    let body_mn_id = expr2_to_markup(
+                        arena,
+                        env,
+                        env.pool.get(*expr_id),
+                        *expr_id,
+                        mark_node_pool,
+                        interns,
+                    )?;
+
+                    let body_mn = mark_node_pool.get_mut(body_mn_id);
+                    body_mn.add_newline_at_end();
+
+                    let full_let_node = MarkupNode::Nested {
+                        ast_node_id,
+                        children_ids: vec![val_name_mn_id, equals_mn_id, body_mn_id],
+                        parent_id_opt: None,
+                        newlines_at_end: 1,
+                    };
+
+                    mark_node_pool.add(full_let_node)
+                }
+                other => {
+                    unimplemented!(
+                        "I don't know how to convert {:?} into a MarkupNode yet.",
+                        other
+                    )
+                }
+            }
+        }
         Expr2::RuntimeError() => new_markup_node(
             "RunTimeError".to_string(),
-            expr2_node_id,
+            ast_node_id,
             HighlightStyle::Blank,
-            markup_node_pool,
+            mark_node_pool,
         ),
         rest => todo!("implement expr2_to_markup for {:?}", rest),
-    }
+    };
+
+    Ok(mark_node_id)
 }
 
-pub fn set_parent_for_all(markup_node_id: MarkNodeId, markup_node_pool: &mut SlowPool) {
-    let node = markup_node_pool.get(markup_node_id);
+pub fn set_parent_for_all(markup_node_id: MarkNodeId, mark_node_pool: &mut SlowPool) {
+    let node = mark_node_pool.get(markup_node_id);
 
     if let MarkupNode::Nested {
         ast_node_id: _,
         children_ids,
         parent_id_opt: _,
+        newlines_at_end: _,
     } = node
     {
         // need to clone because of borrowing issues
         let children_ids_clone = children_ids.clone();
 
         for child_id in children_ids_clone {
-            set_parent_for_all_helper(child_id, markup_node_id, markup_node_pool);
+            set_parent_for_all_helper(child_id, markup_node_id, mark_node_pool);
         }
     }
 }
@@ -463,9 +577,9 @@ pub fn set_parent_for_all(markup_node_id: MarkNodeId, markup_node_pool: &mut Slo
 pub fn set_parent_for_all_helper(
     markup_node_id: MarkNodeId,
     parent_node_id: MarkNodeId,
-    markup_node_pool: &mut SlowPool,
+    mark_node_pool: &mut SlowPool,
 ) {
-    let node = markup_node_pool.get_mut(markup_node_id);
+    let node = mark_node_pool.get_mut(markup_node_id);
 
     match node {
         MarkupNode::Nested {
@@ -479,7 +593,7 @@ pub fn set_parent_for_all_helper(
             let children_ids_clone = children_ids.clone();
 
             for child_id in children_ids_clone {
-                set_parent_for_all_helper(child_id, markup_node_id, markup_node_pool);
+                set_parent_for_all_helper(child_id, markup_node_id, mark_node_pool);
             }
         }
         MarkupNode::Text { parent_id_opt, .. } => *parent_id_opt = Some(parent_node_id),
@@ -487,23 +601,237 @@ pub fn set_parent_for_all_helper(
     }
 }
 
+fn header_mn(content: String, expr_id: ExprId, mark_node_pool: &mut SlowPool) -> MarkNodeId {
+    let mark_node = MarkupNode::Text {
+        content,
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: HighlightStyle::PackageRelated,
+        attributes: Attributes::new(),
+        parent_id_opt: None,
+        newlines_at_end: 0,
+    };
+
+    mark_node_pool.add(mark_node)
+}
+
+fn header_val_mn(
+    content: String,
+    expr_id: ExprId,
+    highlight_style: HighlightStyle,
+    mark_node_pool: &mut SlowPool,
+) -> MarkNodeId {
+    let mark_node = MarkupNode::Text {
+        content,
+        ast_node_id: ASTNodeId::AExprId(expr_id),
+        syn_high_style: highlight_style,
+        attributes: Attributes::new(),
+        parent_id_opt: None,
+        newlines_at_end: 0,
+    };
+
+    mark_node_pool.add(mark_node)
+}
+
+pub fn header_to_markup(app_header: &AppHeader, mark_node_pool: &mut SlowPool) -> MarkNodeId {
+    let expr_id = app_header.ast_node_id;
+    let ast_node_id = ASTNodeId::AExprId(expr_id);
+
+    let app_node_id = header_mn("app ".to_owned(), expr_id, mark_node_pool);
+
+    let app_name_node_id = header_val_mn(
+        app_header.app_name.clone(),
+        expr_id,
+        HighlightStyle::String,
+        mark_node_pool,
+    );
+
+    let full_app_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![app_node_id, app_name_node_id],
+        parent_id_opt: None,
+        newlines_at_end: 1,
+    };
+
+    let packages_node_id = header_mn("    packages ".to_owned(), expr_id, mark_node_pool);
+
+    let pack_left_acc_node_id = mark_node_pool.add(new_left_accolade_mn(expr_id, None));
+
+    let pack_base_node_id = header_val_mn(
+        "base: ".to_owned(),
+        expr_id,
+        HighlightStyle::RecordField,
+        mark_node_pool,
+    );
+
+    let pack_val_node_id = header_val_mn(
+        app_header.packages_base.clone(),
+        expr_id,
+        HighlightStyle::String,
+        mark_node_pool,
+    );
+
+    let pack_right_acc_node_id = mark_node_pool.add(new_right_accolade_mn(expr_id, None));
+
+    let full_packages_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![
+            packages_node_id,
+            pack_left_acc_node_id,
+            pack_base_node_id,
+            pack_val_node_id,
+            pack_right_acc_node_id,
+        ],
+        parent_id_opt: None,
+        newlines_at_end: 1,
+    };
+
+    let imports_node_id = header_mn("    imports ".to_owned(), expr_id, mark_node_pool);
+
+    let imports_left_square_node_id = mark_node_pool.add(new_left_square_mn(expr_id, None));
+
+    let mut import_child_ids: Vec<MarkNodeId> = add_header_mn_list(
+        &app_header.imports,
+        expr_id,
+        HighlightStyle::Import,
+        mark_node_pool,
+    );
+
+    let imports_right_square_node_id = mark_node_pool.add(new_right_square_mn(expr_id, None));
+
+    let mut full_import_children = vec![imports_node_id, imports_left_square_node_id];
+
+    full_import_children.append(&mut import_child_ids);
+    full_import_children.push(imports_right_square_node_id);
+
+    let full_import_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: full_import_children,
+        parent_id_opt: None,
+        newlines_at_end: 1,
+    };
+
+    let provides_node_id = header_mn("    provides ".to_owned(), expr_id, mark_node_pool);
+
+    let provides_left_square_node_id = mark_node_pool.add(new_left_square_mn(expr_id, None));
+
+    let mut provides_val_node_ids: Vec<MarkNodeId> = add_header_mn_list(
+        &app_header.provides,
+        expr_id,
+        HighlightStyle::Provides,
+        mark_node_pool,
+    );
+
+    let provides_right_square_node_id = mark_node_pool.add(new_right_square_mn(expr_id, None));
+
+    let provides_end_node_id = header_mn(" to base".to_owned(), expr_id, mark_node_pool);
+
+    let mut full_provides_children = vec![provides_node_id, provides_left_square_node_id];
+
+    full_provides_children.append(&mut provides_val_node_ids);
+    full_provides_children.push(provides_right_square_node_id);
+    full_provides_children.push(provides_end_node_id);
+
+    let full_provides_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: full_provides_children,
+        parent_id_opt: None,
+        newlines_at_end: 1,
+    };
+
+    let full_app_node_id = mark_node_pool.add(full_app_node);
+    let full_packages_node = mark_node_pool.add(full_packages_node);
+    let full_import_node_id = mark_node_pool.add(full_import_node);
+    let full_provides_node_id = mark_node_pool.add(full_provides_node);
+
+    let header_mark_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![
+            full_app_node_id,
+            full_packages_node,
+            full_import_node_id,
+            full_provides_node_id,
+        ],
+        parent_id_opt: None,
+        newlines_at_end: 1,
+    };
+
+    let header_mn_id = mark_node_pool.add(header_mark_node);
+
+    set_parent_for_all(header_mn_id, mark_node_pool);
+
+    header_mn_id
+}
+
+// Used for provides and imports
+fn add_header_mn_list(
+    str_vec: &[String],
+    expr_id: ExprId,
+    highlight_style: HighlightStyle,
+    mark_node_pool: &mut SlowPool,
+) -> Vec<MarkNodeId> {
+    let nr_of_elts = str_vec.len();
+
+    str_vec
+        .iter()
+        .enumerate()
+        .map(|(indx, provide_str)| {
+            let provide_str = header_val_mn(
+                provide_str.to_owned(),
+                expr_id,
+                highlight_style,
+                mark_node_pool,
+            );
+
+            if indx != nr_of_elts - 1 {
+                vec![provide_str, mark_node_pool.add(new_comma_mn(expr_id, None))]
+            } else {
+                vec![provide_str]
+            }
+        })
+        .flatten()
+        .collect()
+}
+
+pub fn ast_to_mark_nodes<'a, 'b>(
+    arena: &'a Bump,
+    env: &mut Env<'b>,
+    ast: &AST,
+    mark_node_pool: &mut SlowPool,
+    interns: &Interns,
+) -> EdResult<Vec<MarkNodeId>> {
+    let mut all_mark_node_ids = vec![header_to_markup(&ast.header, mark_node_pool)];
+
+    for &def_id in ast.def_ids.iter() {
+        let def2 = env.pool.get(def_id);
+
+        let expr2_markup_id = def2_to_markup(arena, env, def2, def_id, mark_node_pool, interns)?;
+
+        set_parent_for_all(expr2_markup_id, mark_node_pool);
+
+        all_mark_node_ids.push(expr2_markup_id);
+    }
+
+    Ok(all_mark_node_ids)
+}
+
 impl fmt::Display for MarkupNode {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        writeln!(
+        write!(
             f,
-            "{} ({})",
+            "{} ({}, {})",
             self.node_type_as_string(),
-            self.get_content().unwrap_or_else(|_| "".to_string())
+            self.get_content(),
+            self.get_newlines_at_end()
         )
     }
 }
 
 pub fn tree_as_string(root_node_id: MarkNodeId, mark_node_pool: &SlowPool) -> String {
-    let mut full_string = "\n\n(mark_node_tree)\n".to_owned();
+    let mut full_string = "\n(mark_node_tree)\n".to_owned();
 
     let node = mark_node_pool.get(root_node_id);
 
-    full_string.push_str(&format!("{}", node));
+    full_string.push_str(&format!("{} mn_id {}\n", node, root_node_id));
 
     tree_as_string_helper(node, 1, &mut full_string, mark_node_pool);
 
@@ -524,11 +852,25 @@ fn tree_as_string_helper(
             .to_owned();
 
         let child = mark_node_pool.get(child_id);
+        let child_str = format!("{}", mark_node_pool.get(child_id)).replace("\n", "\\n");
 
-        full_str.push_str(&format!("{}", child));
+        full_str.push_str(&format!("{} mn_id {}\n", child_str, child_id));
 
         tree_string.push_str(&full_str);
 
         tree_as_string_helper(child, level + 1, tree_string, mark_node_pool);
     }
 }
+
+// return to the the root parent_id of the current node
+pub fn get_root_mark_node_id(mark_node_id: MarkNodeId, mark_node_pool: &SlowPool) -> MarkNodeId {
+    let mut curr_mark_node_id = mark_node_id;
+    let mut curr_parent_id_opt = mark_node_pool.get(curr_mark_node_id).get_parent_id_opt();
+
+    while let Some(curr_parent_id) = curr_parent_id_opt {
+        curr_mark_node_id = curr_parent_id;
+        curr_parent_id_opt = mark_node_pool.get(curr_mark_node_id).get_parent_id_opt();
+    }
+
+    curr_mark_node_id
+}

editor/src/editor/mod.rs

@@ -1,6 +1,6 @@
 mod code_lines;
 mod config;
-mod ed_error;
+pub mod ed_error;
 mod grid_node_map;
 mod keyboard_input;
 pub mod main;

editor/src/editor/mvc/app_update.rs

@@ -1,8 +1,8 @@
 use super::app_model::AppModel;
 use super::ed_update;
-use crate::editor::ed_error::EdResult;
 use crate::window::keyboard_input::Modifiers;
-use winit::event::VirtualKeyCode;
+use crate::{editor::ed_error::EdResult, window::keyboard_input::from_winit};
+use winit::event::{ModifiersState, VirtualKeyCode};
 
 pub fn handle_copy(app_model: &mut AppModel) -> EdResult<()> {
     if let Some(ref mut ed_model) = app_model.ed_model_opt {
@@ -51,16 +51,26 @@ pub fn pass_keydown_to_focused(
 pub enum InputOutcome {
     Accepted,
     Ignored,
+    SilentIgnored,
 }
 
-pub fn handle_new_char(received_char: &char, app_model: &mut AppModel) -> EdResult<InputOutcome> {
+pub fn handle_new_char(
+    received_char: &char,
+    app_model: &mut AppModel,
+    modifiers_winit: ModifiersState,
+) -> EdResult<InputOutcome> {
     if let Some(ref mut ed_model) = app_model.ed_model_opt {
         if ed_model.has_focus {
+            let modifiers = from_winit(&modifiers_winit);
+
+            if modifiers.new_char_modifiers() {
+                // shortcuts with modifiers are handled by ed_handle_key_down
                 return ed_update::handle_new_char(received_char, ed_model);
             }
         }
+    }
 
-    Ok(InputOutcome::Ignored)
+    Ok(InputOutcome::SilentIgnored)
 }
 
 /*

editor/src/editor/mvc/break_line.rs

@@ -0,0 +1,80 @@
+use crate::editor::ed_error::EdResult;
+use crate::editor::markup::common_nodes::new_blank_mn_w_nls;
+use crate::editor::mvc::app_update::InputOutcome;
+use crate::editor::mvc::ed_model::EdModel;
+use crate::editor::util::index_of;
+use crate::lang::ast::Def2;
+use crate::lang::parse::ASTNodeId;
+use crate::ui::text::text_pos::TextPos;
+
+// put everything after caret on new line, create a Def2::Blank if there was nothing after the caret.
+pub fn break_line(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
+    let carets = ed_model.get_carets();
+
+    for caret_pos in carets.iter() {
+        let caret_line_nr = caret_pos.line;
+
+        // don't allow adding new lines on empty line
+        if caret_pos.column > 0
+            && ed_model.grid_node_map.node_exists_at_pos(TextPos {
+                line: caret_line_nr,
+                column: caret_pos.column - 1,
+            })
+        {
+            // one blank line between top level definitions
+            EdModel::insert_empty_line(
+                caret_line_nr + 1,
+                &mut ed_model.code_lines,
+                &mut ed_model.grid_node_map,
+            )?;
+            EdModel::insert_empty_line(
+                caret_line_nr + 1,
+                &mut ed_model.code_lines,
+                &mut ed_model.grid_node_map,
+            )?;
+
+            insert_new_blank(ed_model, caret_pos, caret_pos.line + 2)?;
+        }
+    }
+
+    ed_model.simple_move_carets_down(2); // one blank line between top level definitions
+
+    Ok(InputOutcome::Accepted)
+}
+
+pub fn insert_new_blank(
+    ed_model: &mut EdModel,
+    caret_pos: &TextPos,
+    insert_on_line_nr: usize,
+) -> EdResult<()> {
+    let new_line_blank = Def2::Blank;
+    let new_line_blank_id = ed_model.module.env.pool.add(new_line_blank);
+
+    let prev_def_mn_id = ed_model
+        .grid_node_map
+        .get_def_mark_node_id_before_line(caret_pos.line + 1, &ed_model.mark_node_pool)?;
+    let prev_def_mn_id_indx = index_of(prev_def_mn_id, &ed_model.markup_ids)?;
+    ed_model
+        .module
+        .ast
+        .def_ids
+        .insert(prev_def_mn_id_indx, new_line_blank_id);
+
+    let blank_mn_id = ed_model.add_mark_node(new_blank_mn_w_nls(
+        ASTNodeId::ADefId(new_line_blank_id),
+        None,
+        2,
+    ));
+
+    ed_model
+        .markup_ids
+        .insert(prev_def_mn_id_indx + 1, blank_mn_id); // + 1 because first markup node is header
+
+    ed_model.insert_all_between_line(
+        insert_on_line_nr, // one blank line between top level definitions
+        0,
+        &[blank_mn_id],
+    )?;
+
+    Ok(())
+}

editor/src/editor/mvc/ed_model.rs

@@ -1,27 +1,22 @@
 use crate::editor::code_lines::CodeLines;
 use crate::editor::grid_node_map::GridNodeMap;
+use crate::editor::markup::nodes::ast_to_mark_nodes;
 use crate::editor::slow_pool::{MarkNodeId, SlowPool};
-use crate::editor::syntax_highlight::HighlightStyle;
 use crate::editor::{
-    ed_error::EdError::ParseError,
-    ed_error::{EdResult, MissingParent, NoNodeAtCaretPosition},
-    markup::attribute::Attributes,
-    markup::nodes::{expr2_to_markup, set_parent_for_all, MarkupNode},
+    ed_error::SrcParseError,
+    ed_error::{EdResult, EmptyCodeString, MissingParent, NoNodeAtCaretPosition},
 };
 use crate::graphics::primitives::rect::Rect;
-use crate::lang::ast::{Expr2, ExprId};
-use crate::lang::expr::{str_to_expr2, Env};
+use crate::lang::expr::Env;
+use crate::lang::parse::{ASTNodeId, AST};
 use crate::lang::pool::PoolStr;
-use crate::lang::scope::Scope;
-use crate::ui::text::caret_w_select::CaretWSelect;
+use crate::ui::text::caret_w_select::{CaretPos, CaretWSelect};
 use crate::ui::text::lines::SelectableLines;
 use crate::ui::text::text_pos::TextPos;
 use crate::ui::ui_error::UIResult;
-use bumpalo::collections::String as BumpString;
 use bumpalo::Bump;
 use nonempty::NonEmpty;
-use roc_module::symbol::Interns;
-use roc_region::all::Region;
+use roc_load::file::LoadedModule;
 use std::path::Path;
 
 #[derive(Debug)]
@@ -31,83 +26,98 @@ pub struct EdModel<'a> {
     pub code_lines: CodeLines,
     // allows us to map window coordinates to MarkNodeId's
     pub grid_node_map: GridNodeMap,
-    pub markup_root_id: MarkNodeId,
-    pub markup_node_pool: SlowPool,
+    pub markup_ids: Vec<MarkNodeId>, // one root node for every expression
+    pub mark_node_pool: SlowPool,
     // contains single char dimensions, used to calculate line height, column width...
     pub glyph_dim_rect_opt: Option<Rect>,
     pub has_focus: bool,
     pub caret_w_select_vec: NonEmpty<(CaretWSelect, Option<MarkNodeId>)>,
-    pub selected_expr_opt: Option<SelectedExpression>,
-    pub interns: &'a Interns, // this should eventually come from LoadedModule, see #1442
+    pub selected_block_opt: Option<SelectedBlock>,
+    pub loaded_module: LoadedModule,
     pub show_debug_view: bool,
     // EdModel is dirty if it has changed since the previous render.
     pub dirty: bool,
 }
 
 #[derive(Debug, Copy, Clone)]
-pub struct SelectedExpression {
-    pub ast_node_id: ExprId,
+pub struct SelectedBlock {
+    pub ast_node_id: ASTNodeId,
     pub mark_node_id: MarkNodeId,
     pub type_str: PoolStr,
 }
 
 pub fn init_model<'a>(
-    code_str: &'a BumpString,
+    code_str: &'a str,
     file_path: &'a Path,
     env: Env<'a>,
-    interns: &'a Interns,
+    loaded_module: LoadedModule,
     code_arena: &'a Bump,
+    caret_pos: CaretPos, // to set caret position
 ) -> EdResult<EdModel<'a>> {
     let mut module = EdModule::new(code_str, env, code_arena)?;
 
-    let ast_root_id = module.ast_root_id;
-    let mut markup_node_pool = SlowPool::new();
+    let mut mark_node_pool = SlowPool::new();
 
-    let markup_root_id = if code_str.is_empty() {
-        let blank_root = MarkupNode::Blank {
-            ast_node_id: ast_root_id,
-            attributes: Attributes::new(),
-            syn_high_style: HighlightStyle::Blank,
-            parent_id_opt: None,
-        };
-
-        markup_node_pool.add(blank_root)
+    let markup_ids = if code_str.is_empty() {
+        EmptyCodeString {}.fail()
     } else {
-        let ast_root = &module.env.pool.get(ast_root_id);
-
-        let temp_markup_root_id = expr2_to_markup(
+        ast_to_mark_nodes(
             code_arena,
             &mut module.env,
-            ast_root,
-            ast_root_id,
-            &mut markup_node_pool,
-        );
-        set_parent_for_all(temp_markup_root_id, &mut markup_node_pool);
+            &module.ast,
+            &mut mark_node_pool,
+            &loaded_module.interns,
+        )
+    }?;
 
-        temp_markup_root_id
+    let mut code_lines = CodeLines::default();
+    let mut grid_node_map = GridNodeMap::default();
+
+    let mut line_nr = 0;
+    let mut col_nr = 0;
+
+    for mark_node_id in &markup_ids {
+        EdModel::insert_mark_node_between_line(
+            &mut line_nr,
+            &mut col_nr,
+            *mark_node_id,
+            &mut grid_node_map,
+            &mut code_lines,
+            &mark_node_pool,
+        )?
+    }
+
+    let caret = match caret_pos {
+        CaretPos::Start => CaretWSelect::default(),
+        CaretPos::Exact(txt_pos) => CaretWSelect::new(txt_pos, None),
+        CaretPos::End => CaretWSelect::new(code_lines.end_txt_pos(), None),
     };
 
-    let code_lines = EdModel::build_code_lines_from_markup(markup_root_id, &markup_node_pool)?;
-    let grid_node_map = EdModel::build_node_map_from_markup(markup_root_id, &markup_node_pool)?;
-
     Ok(EdModel {
         module,
         file_path,
         code_lines,
         grid_node_map,
-        markup_root_id,
-        markup_node_pool,
+        markup_ids,
+        mark_node_pool,
         glyph_dim_rect_opt: None,
         has_focus: true,
-        caret_w_select_vec: NonEmpty::new((CaretWSelect::default(), None)),
-        selected_expr_opt: None,
-        interns,
+        caret_w_select_vec: NonEmpty::new((caret, None)),
+        selected_block_opt: None,
+        loaded_module,
         show_debug_view: false,
         dirty: true,
     })
 }
 
 impl<'a> EdModel<'a> {
+    pub fn get_carets(&self) -> Vec<TextPos> {
+        self.caret_w_select_vec
+            .iter()
+            .map(|tup| tup.0.caret_pos)
+            .collect()
+    }
+
     pub fn get_curr_mark_node_id(&self) -> UIResult<MarkNodeId> {
         let caret_pos = self.get_caret();
         self.grid_node_map.get_id_at_row_col(caret_pos)
@@ -138,11 +148,11 @@ impl<'a> EdModel<'a> {
     pub fn get_curr_child_indices(&self) -> EdResult<(usize, usize)> {
         if self.node_exists_at_caret() {
             let curr_mark_node_id = self.get_curr_mark_node_id()?;
-            let curr_mark_node = self.markup_node_pool.get(curr_mark_node_id);
+            let curr_mark_node = self.mark_node_pool.get(curr_mark_node_id);
 
             if let Some(parent_id) = curr_mark_node.get_parent_id_opt() {
-                let parent = self.markup_node_pool.get(parent_id);
-                parent.get_child_indices(curr_mark_node_id, &self.markup_node_pool)
+                let parent = self.mark_node_pool.get(parent_id);
+                parent.get_child_indices(curr_mark_node_id, &self.mark_node_pool)
             } else {
                 MissingParent {
                     node_id: curr_mark_node_id,
@@ -161,38 +171,26 @@ impl<'a> EdModel<'a> {
 #[derive(Debug)]
 pub struct EdModule<'a> {
     pub env: Env<'a>,
-    pub ast_root_id: ExprId,
+    pub ast: AST,
 }
 
 // for debugging
-// use crate::lang::ast::expr2_to_string;
+//use crate::lang::ast::expr2_to_string;
 
 impl<'a> EdModule<'a> {
     pub fn new(code_str: &'a str, mut env: Env<'a>, ast_arena: &'a Bump) -> EdResult<EdModule<'a>> {
         if !code_str.is_empty() {
-            let mut scope = Scope::new(env.home, env.pool, env.var_store);
+            let parse_res = AST::parse_from_string(code_str, &mut env, ast_arena);
 
-            let region = Region::new(0, 0, 0, 0);
-
-            let expr2_result = str_to_expr2(ast_arena, code_str, &mut env, &mut scope, region);
-
-            match expr2_result {
-                Ok((expr2, _output)) => {
-                    let ast_root_id = env.pool.add(expr2);
-
-                    // for debugging
-                    // dbg!(expr2_to_string(ast_root_id, env.pool));
-
-                    Ok(EdModule { env, ast_root_id })
-                }
-                Err(err) => Err(ParseError {
+            match parse_res {
+                Ok(ast) => Ok(EdModule { env, ast }),
+                Err(err) => SrcParseError {
                     syntax_err: format!("{:?}", err),
-                }),
+                }
+                .fail(),
             }
         } else {
-            let ast_root_id = env.pool.add(Expr2::Blank);
-
-            Ok(EdModule { env, ast_root_id })
+            EmptyCodeString {}.fail()
         }
     }
 }
@@ -200,40 +198,49 @@ impl<'a> EdModule<'a> {
 #[cfg(test)]
 pub mod test_ed_model {
     use crate::editor::ed_error::EdResult;
+    use crate::editor::main::load_module;
     use crate::editor::mvc::ed_model;
+    use crate::editor::resources::strings::HELLO_WORLD;
     use crate::lang::expr::Env;
     use crate::lang::pool::Pool;
     use crate::ui::text::caret_w_select::test_caret_w_select::convert_dsl_to_selection;
     use crate::ui::text::caret_w_select::test_caret_w_select::convert_selection_to_dsl;
+    use crate::ui::text::caret_w_select::CaretPos;
     use crate::ui::text::lines::SelectableLines;
+    use crate::ui::text::text_pos::TextPos;
     use crate::ui::ui_error::UIResult;
-    use bumpalo::collections::String as BumpString;
     use bumpalo::Bump;
     use ed_model::EdModel;
-    use roc_module::symbol::{IdentIds, Interns, ModuleIds};
+    use roc_load::file::LoadedModule;
+    use roc_module::symbol::IdentIds;
+    use roc_module::symbol::ModuleIds;
     use roc_types::subs::VarStore;
+    use std::fs::File;
+    use std::io::Write;
     use std::path::Path;
+    use std::path::PathBuf;
+    use tempfile::tempdir;
+    use uuid::Uuid;
 
     pub fn init_dummy_model<'a>(
-        code_str: &'a BumpString,
+        code_str: &'a str,
+        loaded_module: LoadedModule,
+        module_ids: &'a ModuleIds,
         ed_model_refs: &'a mut EdModelRefs,
+        code_arena: &'a Bump,
     ) -> EdResult<EdModel<'a>> {
         let file_path = Path::new("");
 
         let dep_idents = IdentIds::exposed_builtins(8);
         let exposed_ident_ids = IdentIds::default();
-        let mod_id = ed_model_refs
-            .interns
-            .module_ids
-            .get_or_insert(&"ModId123".into());
 
         let env = Env::new(
-            mod_id,
+            loaded_module.module_id,
             &ed_model_refs.env_arena,
             &mut ed_model_refs.env_pool,
             &mut ed_model_refs.var_store,
             dep_idents,
-            &ed_model_refs.interns.module_ids,
+            module_ids,
             exposed_ident_ids,
         );
 
@@ -241,43 +248,69 @@ pub mod test_ed_model {
             code_str,
             file_path,
             env,
-            &ed_model_refs.interns,
-            &ed_model_refs.code_arena,
+            loaded_module,
+            code_arena,
+            CaretPos::End,
         )
     }
 
     pub struct EdModelRefs {
-        code_arena: Bump,
         env_arena: Bump,
         env_pool: Pool,
         var_store: VarStore,
-        interns: Interns,
     }
 
     pub fn init_model_refs() -> EdModelRefs {
         EdModelRefs {
-            code_arena: Bump::new(),
             env_arena: Bump::new(),
             env_pool: Pool::with_capacity(1024),
             var_store: VarStore::default(),
-            interns: Interns {
-                module_ids: ModuleIds::default(),
-                all_ident_ids: IdentIds::exposed_builtins(8),
-            },
         }
     }
 
     pub fn ed_model_from_dsl<'a>(
-        clean_code_str: &'a BumpString,
-        code_lines: &[&str],
+        clean_code_str: &'a mut String,
+        code_lines: Vec<String>,
         ed_model_refs: &'a mut EdModelRefs,
+        module_ids: &'a ModuleIds,
+        code_arena: &'a Bump,
     ) -> Result<EdModel<'a>, String> {
-        let code_lines_vec: Vec<String> = (*code_lines).iter().map(|s| s.to_string()).collect();
-        let caret_w_select = convert_dsl_to_selection(&code_lines_vec)?;
+        let full_code = vec![HELLO_WORLD, clean_code_str.as_str()];
+        *clean_code_str = full_code.join("\n");
 
-        let mut ed_model = init_dummy_model(clean_code_str, ed_model_refs)?;
+        let temp_dir = tempdir().expect("Failed to create temporary directory for test.");
+        let temp_file_path_buf =
+            PathBuf::from([Uuid::new_v4().to_string(), ".roc".to_string()].join(""));
+        let temp_file_full_path = temp_dir.path().join(temp_file_path_buf);
 
-        ed_model.set_caret(caret_w_select.caret_pos);
+        let mut file = File::create(temp_file_full_path.clone()).expect(&format!(
+            "Failed to create temporary file for path {:?}",
+            temp_file_full_path
+        ));
+        writeln!(file, "{}", clean_code_str).expect(&format!(
+            "Failed to write {:?} to file: {:?}",
+            clean_code_str, file
+        ));
+
+        let loaded_module = load_module(&temp_file_full_path);
+
+        let mut ed_model = init_dummy_model(
+            clean_code_str,
+            loaded_module,
+            module_ids,
+            ed_model_refs,
+            code_arena,
+        )?;
+
+        // adjust for header and main function
+        let nr_hello_world_lines = HELLO_WORLD.matches('\n').count() - 2;
+        let caret_w_select = convert_dsl_to_selection(&code_lines)?;
+        let adjusted_caret_pos = TextPos {
+            line: caret_w_select.caret_pos.line + nr_hello_world_lines,
+            column: caret_w_select.caret_pos.column,
+        };
+
+        ed_model.set_caret(adjusted_caret_pos);
 
         Ok(ed_model)
     }

editor/src/editor/mvc/ed_view.rs

@@ -1,7 +1,7 @@
 use super::ed_model::EdModel;
 use crate::editor::config::Config;
 use crate::editor::ed_error::EdResult;
-use crate::editor::mvc::ed_model::SelectedExpression;
+use crate::editor::mvc::ed_model::SelectedBlock;
 use crate::editor::render_ast::build_code_graphics;
 use crate::editor::render_debug::build_debug_graphics;
 use crate::editor::resources::strings::START_TIP;
@@ -20,33 +20,49 @@ use winit::dpi::PhysicalSize;
 
 #[derive(Debug)]
 pub struct RenderedWgpu {
-    pub text_sections: Vec<glyph_brush::OwnedSection>,
-    pub rects: Vec<Rect>,
+    pub text_sections_behind: Vec<glyph_brush::OwnedSection>, // displayed in front of rect_behind, behind everything else
+    pub text_sections_front: Vec<glyph_brush::OwnedSection>,  // displayed in front of everything
+    pub rects_behind: Vec<Rect>,                              // displayed at lowest depth
+    pub rects_front: Vec<Rect>, // displayed in front of text_sections_behind, behind text_sections_front
 }
 
 impl RenderedWgpu {
     pub fn new() -> Self {
         Self {
-            text_sections: Vec::new(),
-            rects: Vec::new(),
+            text_sections_behind: Vec::new(),
+            text_sections_front: Vec::new(),
+            rects_behind: Vec::new(),
+            rects_front: Vec::new(),
         }
     }
 
-    pub fn add_text(&mut self, new_text_section: glyph_brush::OwnedSection) {
-        self.text_sections.push(new_text_section);
+    pub fn add_text_behind(&mut self, new_text_section: glyph_brush::OwnedSection) {
+        self.text_sections_behind.push(new_text_section);
     }
 
-    pub fn add_rect(&mut self, new_rect: Rect) {
-        self.rects.push(new_rect);
+    pub fn add_text_front(&mut self, new_text_section: glyph_brush::OwnedSection) {
+        self.text_sections_front.push(new_text_section);
     }
 
-    pub fn add_rects(&mut self, new_rects: Vec<Rect>) {
-        self.rects.extend(new_rects);
+    pub fn add_rect_behind(&mut self, new_rect: Rect) {
+        self.rects_behind.push(new_rect);
+    }
+
+    pub fn add_rects_behind(&mut self, new_rects: Vec<Rect>) {
+        self.rects_behind.extend(new_rects);
+    }
+
+    pub fn add_rect_front(&mut self, new_rect: Rect) {
+        self.rects_front.push(new_rect);
     }
 
     pub fn extend(&mut self, rendered_wgpu: RenderedWgpu) {
-        self.text_sections.extend(rendered_wgpu.text_sections);
-        self.rects.extend(rendered_wgpu.rects);
+        self.text_sections_behind
+            .extend(rendered_wgpu.text_sections_behind);
+        self.text_sections_front
+            .extend(rendered_wgpu.text_sections_front);
+        self.rects_behind.extend(rendered_wgpu.rects_behind);
+        self.rects_front.extend(rendered_wgpu.rects_front);
     }
 }
 
@@ -61,7 +77,10 @@ pub fn model_to_wgpu<'a>(
 
     let mut all_rendered = RenderedWgpu::new();
 
-    let tip_txt_coords = (txt_coords.x, txt_coords.y - 4.0 * config.code_font_size);
+    let tip_txt_coords = (
+        txt_coords.x,
+        txt_coords.y - (START_TIP.matches('\n').count() as f32 + 1.0) * config.code_font_size,
+    );
 
     let start_tip_text = owned_section_from_text(&Text {
         position: tip_txt_coords.into(),
@@ -72,15 +91,15 @@ pub fn model_to_wgpu<'a>(
         ..Default::default()
     });
 
-    all_rendered.add_text(start_tip_text);
+    all_rendered.add_text_behind(start_tip_text);
 
     let rendered_code_graphics = build_code_graphics(
-        ed_model.markup_node_pool.get(ed_model.markup_root_id),
+        &ed_model.markup_ids,
         size,
         txt_coords,
         config,
         glyph_dim_rect,
-        &ed_model.markup_node_pool,
+        &ed_model.mark_node_pool,
     )?;
 
     all_rendered.extend(rendered_code_graphics);
@@ -93,7 +112,7 @@ pub fn model_to_wgpu<'a>(
 
     let rendered_selection = build_selection_graphics(
         caret_w_sel_vec,
-        &ed_model.selected_expr_opt,
+        &ed_model.selected_block_opt,
         txt_coords,
         config,
         glyph_dim_rect,
@@ -103,7 +122,7 @@ pub fn model_to_wgpu<'a>(
     all_rendered.extend(rendered_selection);
 
     if ed_model.show_debug_view {
-        all_rendered.add_text(build_debug_graphics(size, txt_coords, config, ed_model)?);
+        all_rendered.add_text_behind(build_debug_graphics(size, txt_coords, config, ed_model)?);
     }
 
     Ok(all_rendered)
@@ -111,7 +130,7 @@ pub fn model_to_wgpu<'a>(
 
 pub fn build_selection_graphics(
     caret_w_select_vec: Vec<CaretWSelect>,
-    selected_expr_opt: &Option<SelectedExpression>,
+    selected_expr_opt: &Option<SelectedBlock>,
     txt_coords: Vector2<f32>,
     config: &Config,
     glyph_dim_rect: Rect,
@@ -139,7 +158,7 @@ pub fn build_selection_graphics(
             let width =
                 ((end_pos.column as f32) * char_width) - ((start_pos.column as f32) * char_width);
 
-            all_rendered.add_rect(make_selection_rect(
+            all_rendered.add_rect_behind(make_selection_rect(
                 sel_rect_x,
                 sel_rect_y,
                 width,
@@ -158,12 +177,12 @@ pub fn build_selection_graphics(
                 let (tip_rect, tip_text) =
                     tooltip.render_tooltip(&glyph_dim_rect, &config.ed_theme.ui_theme);
 
-                all_rendered.add_rect(tip_rect);
-                all_rendered.add_text(tip_text);
+                all_rendered.add_rect_front(tip_rect);
+                all_rendered.add_text_front(tip_text);
             }
         }
 
-        all_rendered.add_rect(make_caret_rect(
+        all_rendered.add_rect_front(make_caret_rect(
             top_left_x,
             top_left_y,
             &glyph_dim_rect,

editor/src/editor/mvc/int_update.rs

@@ -25,6 +25,7 @@ pub fn start_new_int(ed_model: &mut EdModel, digit_char: &char) -> EdResult<Inpu
     } = get_node_context(ed_model)?;
 
     let is_blank_node = curr_mark_node.is_blank();
+    let curr_mark_node_nls = curr_mark_node.get_newlines_at_end();
 
     let int_var = ed_model.module.env.var_store.fresh();
 
@@ -37,7 +38,11 @@ pub fn start_new_int(ed_model: &mut EdModel, digit_char: &char) -> EdResult<Inpu
         text: PoolStr::new(&digit_string, &mut ed_model.module.env.pool),
     };
 
-    ed_model.module.env.pool.set(ast_node_id, expr2_node);
+    ed_model
+        .module
+        .env
+        .pool
+        .set(ast_node_id.to_expr_id()?, expr2_node);
 
     let int_node = MarkupNode::Text {
         content: digit_string,
@@ -45,11 +50,12 @@ pub fn start_new_int(ed_model: &mut EdModel, digit_char: &char) -> EdResult<Inpu
         syn_high_style: HighlightStyle::Number,
         attributes: Attributes::new(),
         parent_id_opt,
+        newlines_at_end: curr_mark_node_nls,
     };
 
     if is_blank_node {
         ed_model
-            .markup_node_pool
+            .mark_node_pool
             .replace_node(curr_mark_node_id, int_node);
 
         // remove data corresponding to Blank node
@@ -59,11 +65,13 @@ pub fn start_new_int(ed_model: &mut EdModel, digit_char: &char) -> EdResult<Inpu
         ed_model.simple_move_carets_right(char_len);
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             &digit_char.to_string(),
             curr_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         Ok(InputOutcome::Accepted)
@@ -85,7 +93,7 @@ pub fn update_int(
             .grid_node_map
             .get_offset_to_node_id(old_caret_pos, int_mark_node_id)?;
 
-        let int_mark_node = ed_model.markup_node_pool.get_mut(int_mark_node_id);
+        let int_mark_node = ed_model.mark_node_pool.get_mut(int_mark_node_id);
         let int_ast_node_id = int_mark_node.get_ast_node_id();
 
         let content_str_mut = int_mark_node.get_content_mut()?;
@@ -98,19 +106,25 @@ pub fn update_int(
         } else {
             content_str_mut.insert(node_caret_offset, *ch);
 
-            let content_str = int_mark_node.get_content()?;
+            let content_str = int_mark_node.get_content();
 
             // update GridNodeMap and CodeLines
-            ed_model.insert_between_line(
+            EdModel::insert_between_line(
                 old_caret_pos.line,
                 old_caret_pos.column,
                 &ch.to_string(),
                 int_mark_node_id,
+                &mut ed_model.grid_node_map,
+                &mut ed_model.code_lines,
             )?;
 
             // update ast
             let new_pool_str = PoolStr::new(&content_str, ed_model.module.env.pool);
-            let int_ast_node = ed_model.module.env.pool.get_mut(int_ast_node_id);
+            let int_ast_node = ed_model
+                .module
+                .env
+                .pool
+                .get_mut(int_ast_node_id.to_expr_id()?);
             match int_ast_node {
                 SmallInt { number, text, .. } => {
                     update_small_int_num(number, &content_str)?;

editor/src/editor/mvc/let_update.rs

@@ -0,0 +1,185 @@
+use roc_module::symbol::Symbol;
+
+use crate::editor::ed_error::EdResult;
+use crate::editor::markup::attribute::Attributes;
+use crate::editor::markup::common_nodes::new_blank_mn_w_nls;
+use crate::editor::markup::common_nodes::new_equals_mn;
+use crate::editor::markup::nodes::MarkupNode;
+use crate::editor::mvc::app_update::InputOutcome;
+use crate::editor::mvc::ed_model::EdModel;
+use crate::editor::mvc::ed_update::get_node_context;
+use crate::editor::mvc::ed_update::NodeContext;
+use crate::editor::syntax_highlight::HighlightStyle;
+use crate::lang::ast::{Expr2, ValueDef};
+use crate::lang::parse::ASTNodeId;
+use crate::lang::pattern::Pattern2;
+
+pub fn start_new_let_value(ed_model: &mut EdModel, new_char: &char) -> EdResult<InputOutcome> {
+    let NodeContext {
+        old_caret_pos,
+        curr_mark_node_id,
+        curr_mark_node,
+        parent_id_opt,
+        ast_node_id,
+    } = get_node_context(ed_model)?;
+
+    let is_blank_node = curr_mark_node.is_blank();
+    let curr_mark_node_nls = curr_mark_node.get_newlines_at_end();
+
+    let val_name_string = new_char.to_string();
+    // safe unwrap because our ArrString has a 30B capacity
+    let val_expr2_node = Expr2::Blank;
+    let val_expr_id = ed_model.module.env.pool.add(val_expr2_node);
+
+    let ident_id = ed_model
+        .module
+        .env
+        .ident_ids
+        .add(val_name_string.clone().into());
+    let var_symbol = Symbol::new(ed_model.module.env.home, ident_id);
+    let body = Expr2::Var(var_symbol);
+    let body_id = ed_model.module.env.pool.add(body);
+
+    let pattern = Pattern2::Identifier(var_symbol);
+    let pattern_id = ed_model.module.env.pool.add(pattern);
+
+    let value_def = ValueDef::NoAnnotation {
+        pattern_id,
+        expr_id: val_expr_id,
+        expr_var: ed_model.module.env.var_store.fresh(),
+    };
+    let def_id = ed_model.module.env.pool.add(value_def);
+
+    let expr2_node = Expr2::LetValue {
+        def_id,
+        body_id,
+        body_var: ed_model.module.env.var_store.fresh(),
+    };
+
+    ed_model
+        .module
+        .env
+        .pool
+        .set(ast_node_id.to_expr_id()?, expr2_node);
+
+    let val_name_mark_node = MarkupNode::Text {
+        content: val_name_string,
+        ast_node_id,
+        syn_high_style: HighlightStyle::Variable,
+        attributes: Attributes::new(),
+        parent_id_opt: Some(curr_mark_node_id),
+        newlines_at_end: curr_mark_node_nls,
+    };
+
+    let val_name_mn_id = ed_model.add_mark_node(val_name_mark_node);
+
+    let equals_mn_id = ed_model.add_mark_node(new_equals_mn(ast_node_id, Some(curr_mark_node_id)));
+
+    let body_mn_id = ed_model.add_mark_node(new_blank_mn_w_nls(
+        ASTNodeId::AExprId(val_expr_id),
+        Some(curr_mark_node_id),
+        1,
+    ));
+
+    let val_mark_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![val_name_mn_id, equals_mn_id, body_mn_id],
+        parent_id_opt,
+        newlines_at_end: 1,
+    };
+
+    if is_blank_node {
+        ed_model
+            .mark_node_pool
+            .replace_node(curr_mark_node_id, val_mark_node);
+
+        // remove data corresponding to Blank node
+        ed_model.del_blank_expr_node(old_caret_pos)?;
+
+        let char_len = 1;
+        ed_model.simple_move_carets_right(char_len);
+
+        // update GridNodeMap and CodeLines
+        ed_model.insert_all_between_line(
+            old_caret_pos.line,
+            old_caret_pos.column,
+            &[val_name_mn_id, equals_mn_id, body_mn_id],
+        )?;
+
+        Ok(InputOutcome::Accepted)
+    } else {
+        Ok(InputOutcome::Ignored)
+    }
+}
+
+// TODO reenable this for updating non-top level value defs
+/*
+pub fn update_let_value(
+    val_name_mn_id: MarkNodeId,
+    def_id: NodeId<ValueDef>,
+    body_id: NodeId<Expr2>,
+    ed_model: &mut EdModel,
+    new_char: &char,
+) -> EdResult<InputOutcome> {
+    if new_char.is_ascii_alphanumeric() {
+        let old_caret_pos = ed_model.get_caret();
+
+        // update markup
+        let val_name_mn_mut = ed_model.mark_node_pool.get_mut(val_name_mn_id);
+        let content_str_mut = val_name_mn_mut.get_content_mut()?;
+
+        let old_val_name = content_str_mut.clone();
+
+        let node_caret_offset = ed_model
+            .grid_node_map
+            .get_offset_to_node_id(old_caret_pos, val_name_mn_id)?;
+
+        if node_caret_offset <= content_str_mut.len() {
+            content_str_mut.insert(node_caret_offset, *new_char);
+
+            // update ast
+            let value_def = ed_model.module.env.pool.get(def_id);
+            let value_ident_pattern_id = value_def.get_pattern_id();
+
+            // TODO no unwrap
+            let ident_id = ed_model
+                .module
+                .env
+                .ident_ids
+                .update_key(&old_val_name, content_str_mut)
+                .unwrap();
+
+            let new_var_symbol = Symbol::new(ed_model.module.env.home, ident_id);
+
+            ed_model
+                .module
+                .env
+                .pool
+                .set(value_ident_pattern_id, Pattern2::Identifier(new_var_symbol));
+
+            ed_model
+                .module
+                .env
+                .pool
+                .set(body_id, Expr2::Var(new_var_symbol));
+
+            // update GridNodeMap and CodeLines
+            ed_model.insert_between_line(
+                old_caret_pos.line,
+                old_caret_pos.column,
+                &new_char.to_string(),
+                val_name_mn_id,
+            )?;
+
+            // update caret
+            ed_model.simple_move_carets_right(1);
+
+            Ok(InputOutcome::Accepted)
+        } else {
+            Ok(InputOutcome::Ignored)
+        }
+    } else {
+        Ok(InputOutcome::Ignored)
+    }
+}
+*/

editor/src/editor/mvc/list_update.rs

@@ -1,6 +1,8 @@
 use crate::editor::ed_error::EdResult;
 use crate::editor::ed_error::{MissingParent, UnexpectedASTNode};
-use crate::editor::markup::attribute::Attributes;
+use crate::editor::markup::common_nodes::{
+    new_blank_mn, new_comma_mn, new_left_square_mn, new_right_square_mn,
+};
 use crate::editor::markup::nodes;
 use crate::editor::markup::nodes::MarkupNode;
 use crate::editor::mvc::app_update::InputOutcome;
@@ -8,9 +10,9 @@ use crate::editor::mvc::ed_model::EdModel;
 use crate::editor::mvc::ed_update::get_node_context;
 use crate::editor::mvc::ed_update::NodeContext;
 use crate::editor::slow_pool::MarkNodeId;
-use crate::editor::syntax_highlight::HighlightStyle;
-use crate::lang::ast::Expr2;
-use crate::lang::ast::{expr2_to_string, ExprId};
+use crate::lang::ast::ExprId;
+use crate::lang::ast::{ast_node_to_string, Expr2};
+use crate::lang::parse::ASTNodeId;
 use crate::lang::pool::PoolVec;
 use crate::ui::text::text_pos::TextPos;
 
@@ -24,63 +26,62 @@ pub fn start_new_list(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
     } = get_node_context(ed_model)?;
 
     let is_blank_node = curr_mark_node.is_blank();
+    let curr_mark_node_nls = curr_mark_node.get_newlines_at_end();
 
     let expr2_node = Expr2::List {
         elem_var: ed_model.module.env.var_store.fresh(),
         elems: PoolVec::empty(ed_model.module.env.pool),
     };
 
-    let mark_node_pool = &mut ed_model.markup_node_pool;
+    ed_model
+        .module
+        .env
+        .pool
+        .set(ast_node_id.to_expr_id()?, expr2_node);
 
-    ed_model.module.env.pool.set(ast_node_id, expr2_node);
+    let left_bracket_node_id = ed_model.add_mark_node(new_left_square_mn(
+        ast_node_id.to_expr_id()?,
+        Some(curr_mark_node_id),
+    ));
 
-    let left_bracket_node = MarkupNode::Text {
-        content: nodes::LEFT_SQUARE_BR.to_owned(),
-        ast_node_id,
-        syn_high_style: HighlightStyle::Bracket,
-        attributes: Attributes::new(),
-        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
-    };
-
-    let left_bracket_node_id = mark_node_pool.add(left_bracket_node);
-
-    let right_bracket_node = MarkupNode::Text {
-        content: nodes::RIGHT_SQUARE_BR.to_owned(),
-        ast_node_id,
-        syn_high_style: HighlightStyle::Bracket,
-        attributes: Attributes::new(),
-        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
-    };
-
-    let right_bracket_node_id = mark_node_pool.add(right_bracket_node);
+    let right_bracket_node_id = ed_model.add_mark_node(new_right_square_mn(
+        ast_node_id.to_expr_id()?,
+        Some(curr_mark_node_id),
+    ));
 
     let nested_node = MarkupNode::Nested {
         ast_node_id,
         children_ids: vec![left_bracket_node_id, right_bracket_node_id],
         parent_id_opt,
+        newlines_at_end: curr_mark_node_nls,
     };
 
     if is_blank_node {
-        mark_node_pool.replace_node(curr_mark_node_id, nested_node);
+        ed_model
+            .mark_node_pool
+            .replace_node(curr_mark_node_id, nested_node);
 
-        // remove data corresponding to Blank node
-        ed_model.del_at_line(old_caret_pos.line, old_caret_pos.column)?;
+        ed_model.del_blank_expr_node(old_caret_pos)?;
 
         ed_model.simple_move_carets_right(nodes::LEFT_SQUARE_BR.len());
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             nodes::LEFT_SQUARE_BR,
             left_bracket_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column + nodes::LEFT_SQUARE_BR.len(),
             nodes::RIGHT_SQUARE_BR,
             right_bracket_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         Ok(InputOutcome::Accepted)
@@ -105,10 +106,10 @@ pub fn add_blank_child(
 
     let trip_result: EdResult<(ExprId, ExprId, MarkNodeId)> = if let Some(parent_id) = parent_id_opt
     {
-        let parent = ed_model.markup_node_pool.get(parent_id);
+        let parent = ed_model.mark_node_pool.get(parent_id);
 
         let list_ast_node_id = parent.get_ast_node_id();
-        let list_ast_node = ed_model.module.env.pool.get(list_ast_node_id);
+        let list_ast_node = ed_model.module.env.pool.get(list_ast_node_id.to_expr_id()?);
 
         match list_ast_node {
             Expr2::List {
@@ -118,11 +119,11 @@ pub fn add_blank_child(
                 let blank_elt = Expr2::Blank;
                 let blank_elt_id = ed_model.module.env.pool.add(blank_elt);
 
-                Ok((blank_elt_id, list_ast_node_id, parent_id))
+                Ok((blank_elt_id, list_ast_node_id.to_expr_id()?, parent_id))
             }
             _ => UnexpectedASTNode {
                 required_node_type: "List".to_string(),
-                encountered_node_type: expr2_to_string(ast_node_id, ed_model.module.env.pool),
+                encountered_node_type: ast_node_to_string(ast_node_id, ed_model.module.env.pool),
             }
             .fail(),
         }
@@ -159,7 +160,7 @@ pub fn add_blank_child(
         }
         _ => UnexpectedASTNode {
             required_node_type: "List".to_string(),
-            encountered_node_type: expr2_to_string(ast_node_id, ed_model.module.env.pool),
+            encountered_node_type: ast_node_to_string(ast_node_id, ed_model.module.env.pool),
         }
         .fail(),
     }?;
@@ -173,7 +174,7 @@ pub fn add_blank_child(
         ed_model,
     )?;
 
-    let parent = ed_model.markup_node_pool.get_mut(parent_id);
+    let parent = ed_model.mark_node_pool.get_mut(parent_id);
 
     for (indx, child) in new_mark_children.iter().enumerate() {
         parent.add_child_at_index(new_child_index + indx, *child)?;
@@ -191,35 +192,26 @@ pub fn update_mark_children(
     parent_id_opt: Option<MarkNodeId>,
     ed_model: &mut EdModel,
 ) -> EdResult<Vec<MarkNodeId>> {
-    let blank_mark_node = MarkupNode::Blank {
-        ast_node_id: blank_elt_id,
-        syn_high_style: HighlightStyle::Blank,
-        attributes: Attributes::new(),
+    let blank_mark_node_id = ed_model.add_mark_node(new_blank_mn(
+        ASTNodeId::AExprId(blank_elt_id),
         parent_id_opt,
-    };
-
-    let blank_mark_node_id = ed_model.markup_node_pool.add(blank_mark_node);
+    ));
 
     let mut children: Vec<MarkNodeId> = vec![];
 
     if new_child_index > 1 {
-        let comma_mark_node = MarkupNode::Text {
-            content: nodes::COMMA.to_owned(),
-            ast_node_id: list_ast_node_id,
-            syn_high_style: HighlightStyle::Blank,
-            attributes: Attributes::new(),
-            parent_id_opt,
-        };
-
-        let comma_mark_node_id = ed_model.markup_node_pool.add(comma_mark_node);
+        let comma_mark_node_id =
+            ed_model.add_mark_node(new_comma_mn(list_ast_node_id, parent_id_opt));
 
         ed_model.simple_move_carets_right(nodes::COMMA.len());
 
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             nodes::COMMA,
             comma_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         children.push(comma_mark_node_id);
@@ -234,11 +226,13 @@ pub fn update_mark_children(
     };
 
     // update GridNodeMap and CodeLines
-    ed_model.insert_between_line(
+    EdModel::insert_between_line(
         old_caret_pos.line,
         old_caret_pos.column + comma_shift,
         nodes::BLANK_PLACEHOLDER,
         blank_mark_node_id,
+        &mut ed_model.grid_node_map,
+        &mut ed_model.code_lines,
     )?;
 
     Ok(children)

editor/src/editor/mvc/lookup_update.rs

@@ -10,7 +10,7 @@ pub fn update_invalid_lookup(
     input_str: &str,
     old_pool_str: &PoolStr,
     curr_mark_node_id: MarkNodeId,
-    ast_node_id: ExprId,
+    expr_id: ExprId,
     ed_model: &mut EdModel,
 ) -> EdResult<InputOutcome> {
     if input_str.chars().all(|ch| ch.is_ascii_alphanumeric()) {
@@ -32,10 +32,10 @@ pub fn update_invalid_lookup(
             .module
             .env
             .pool
-            .set(ast_node_id, Expr2::InvalidLookup(new_pool_str));
+            .set(expr_id, Expr2::InvalidLookup(new_pool_str));
 
         // update MarkupNode
-        let curr_mark_node_mut = ed_model.markup_node_pool.get_mut(curr_mark_node_id);
+        let curr_mark_node_mut = ed_model.mark_node_pool.get_mut(curr_mark_node_id);
         let content_str_mut = curr_mark_node_mut.get_content_mut()?;
         content_str_mut.insert_str(caret_offset, input_str);
 
@@ -43,11 +43,13 @@ pub fn update_invalid_lookup(
         ed_model.simple_move_carets_right(input_str.len());
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             input_str,
             curr_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         Ok(InputOutcome::Accepted)

editor/src/editor/mvc/mod.rs

@@ -1,10 +1,13 @@
 pub mod app_model;
 pub mod app_update;
+mod break_line;
 pub mod ed_model;
 pub mod ed_update;
 pub mod ed_view;
 mod int_update;
+mod let_update;
 mod list_update;
 mod lookup_update;
 mod record_update;
 mod string_update;
+pub mod tld_value_update;

editor/src/editor/mvc/record_update.rs

@@ -2,6 +2,9 @@ use crate::editor::ed_error::EdResult;
 use crate::editor::ed_error::MissingParent;
 use crate::editor::ed_error::RecordWithoutFields;
 use crate::editor::markup::attribute::Attributes;
+use crate::editor::markup::common_nodes::new_blank_mn;
+use crate::editor::markup::common_nodes::new_left_accolade_mn;
+use crate::editor::markup::common_nodes::new_right_accolade_mn;
 use crate::editor::markup::nodes;
 use crate::editor::markup::nodes::MarkupNode;
 use crate::editor::mvc::app_update::InputOutcome;
@@ -12,6 +15,7 @@ use crate::editor::slow_pool::MarkNodeId;
 use crate::editor::syntax_highlight::HighlightStyle;
 use crate::editor::util::index_of;
 use crate::lang::ast::{Expr2, ExprId, RecordField};
+use crate::lang::parse::ASTNodeId;
 use crate::lang::pool::{PoolStr, PoolVec};
 use crate::ui::text::text_pos::TextPos;
 use snafu::OptionExt;
@@ -26,61 +30,44 @@ pub fn start_new_record(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
     } = get_node_context(ed_model)?;
 
     let is_blank_node = curr_mark_node.is_blank();
+    let curr_mark_node_nls = curr_mark_node.get_newlines_at_end();
 
     let ast_pool = &mut ed_model.module.env.pool;
     let expr2_node = Expr2::EmptyRecord;
 
-    let mark_node_pool = &mut ed_model.markup_node_pool;
+    ast_pool.set(ast_node_id.to_expr_id()?, expr2_node);
 
-    ast_pool.set(ast_node_id, expr2_node);
+    let left_bracket_node_id = ed_model.add_mark_node(new_left_accolade_mn(
+        ast_node_id.to_expr_id()?,
+        Some(curr_mark_node_id),
+    ));
 
-    let left_bracket_node = MarkupNode::Text {
-        content: nodes::LEFT_ACCOLADE.to_owned(),
-        ast_node_id,
-        syn_high_style: HighlightStyle::Bracket,
-        attributes: Attributes::new(),
-        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
-    };
-
-    let left_bracket_node_id = mark_node_pool.add(left_bracket_node);
-
-    let right_bracket_node = MarkupNode::Text {
-        content: nodes::RIGHT_ACCOLADE.to_owned(),
-        ast_node_id,
-        syn_high_style: HighlightStyle::Bracket,
-        attributes: Attributes::new(),
-        parent_id_opt: Some(curr_mark_node_id), // current node will be replaced with nested one
-    };
-
-    let right_bracket_node_id = mark_node_pool.add(right_bracket_node);
+    let right_bracket_node_id = ed_model.add_mark_node(new_right_accolade_mn(
+        ast_node_id.to_expr_id()?,
+        Some(curr_mark_node_id),
+    ));
 
     let nested_node = MarkupNode::Nested {
         ast_node_id,
         children_ids: vec![left_bracket_node_id, right_bracket_node_id],
         parent_id_opt,
+        newlines_at_end: curr_mark_node_nls,
     };
 
     if is_blank_node {
-        mark_node_pool.replace_node(curr_mark_node_id, nested_node);
+        ed_model
+            .mark_node_pool
+            .replace_node(curr_mark_node_id, nested_node);
 
-        // remove data corresponding to Blank node
-        ed_model.del_at_line(old_caret_pos.line, old_caret_pos.column)?;
+        ed_model.del_blank_expr_node(old_caret_pos)?;
 
         ed_model.simple_move_carets_right(nodes::LEFT_ACCOLADE.len());
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        ed_model.insert_all_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
-            nodes::LEFT_ACCOLADE,
-            left_bracket_node_id,
-        )?;
-
-        ed_model.insert_between_line(
-            old_caret_pos.line,
-            old_caret_pos.column + nodes::LEFT_ACCOLADE.len(),
-            nodes::RIGHT_ACCOLADE,
-            right_bracket_node_id,
+            &[left_bracket_node_id, right_bracket_node_id],
         )?;
 
         Ok(InputOutcome::Accepted)
@@ -100,7 +87,7 @@ pub fn update_empty_record(
     if input_chars.all(|ch| ch.is_ascii_alphabetic())
         && input_chars.all(|ch| ch.is_ascii_lowercase())
     {
-        let prev_mark_node = ed_model.markup_node_pool.get(prev_mark_node_id);
+        let prev_mark_node = ed_model.mark_node_pool.get(prev_mark_node_id);
 
         let NodeContext {
             old_caret_pos,
@@ -110,8 +97,8 @@ pub fn update_empty_record(
             ast_node_id,
         } = get_node_context(ed_model)?;
 
-        if prev_mark_node.get_content()? == nodes::LEFT_ACCOLADE
-            && curr_mark_node.get_content()? == nodes::RIGHT_ACCOLADE
+        if prev_mark_node.get_content() == nodes::LEFT_ACCOLADE
+            && curr_mark_node.get_content() == nodes::RIGHT_ACCOLADE
         {
             // update AST
             let record_var = ed_model.module.env.var_store.fresh();
@@ -124,7 +111,11 @@ pub fn update_empty_record(
 
             let new_ast_node = Expr2::Record { record_var, fields };
 
-            ed_model.module.env.pool.set(ast_node_id, new_ast_node);
+            ed_model
+                .module
+                .env
+                .pool
+                .set(ast_node_id.to_expr_id()?, new_ast_node);
 
             // update Markup
 
@@ -134,12 +125,13 @@ pub fn update_empty_record(
                 syn_high_style: HighlightStyle::RecordField,
                 attributes: Attributes::new(),
                 parent_id_opt,
+                newlines_at_end: 0,
             };
 
-            let record_field_node_id = ed_model.markup_node_pool.add(record_field_node);
+            let record_field_node_id = ed_model.add_mark_node(record_field_node);
 
             if let Some(parent_id) = parent_id_opt {
-                let parent = ed_model.markup_node_pool.get_mut(parent_id);
+                let parent = ed_model.mark_node_pool.get_mut(parent_id);
 
                 let new_child_index = index_of(curr_mark_node_id, &sibling_ids)?;
 
@@ -155,11 +147,13 @@ pub fn update_empty_record(
             ed_model.simple_move_carets_right(1);
 
             // update GridNodeMap and CodeLines
-            ed_model.insert_between_line(
+            EdModel::insert_between_line(
                 old_caret_pos.line,
                 old_caret_pos.column,
                 new_input,
                 record_field_node_id,
+                &mut ed_model.grid_node_map,
+                &mut ed_model.code_lines,
             )?;
 
             Ok(InputOutcome::Accepted)
@@ -183,23 +177,22 @@ pub fn update_record_colon(
         ast_node_id,
     } = get_node_context(ed_model)?;
     if let Some(parent_id) = parent_id_opt {
-        let curr_ast_node = ed_model.module.env.pool.get(ast_node_id);
+        let curr_ast_node = ed_model.module.env.pool.get(ast_node_id.to_expr_id()?);
 
         let prev_mark_node_id_opt = ed_model.get_prev_mark_node_id()?;
         if let Some(prev_mark_node_id) = prev_mark_node_id_opt {
-            let prev_mark_node = ed_model.markup_node_pool.get(prev_mark_node_id);
+            let prev_mark_node = ed_model.mark_node_pool.get(prev_mark_node_id);
 
-            let prev_ast_node = ed_model
-                .module
-                .env
-                .pool
-                .get(prev_mark_node.get_ast_node_id());
+            match prev_mark_node.get_ast_node_id() {
+                ASTNodeId::ADefId(_) => Ok(InputOutcome::Ignored),
+                ASTNodeId::AExprId(prev_expr_id) => {
+                    let prev_expr = ed_model.module.env.pool.get(prev_expr_id);
 
                     // current and prev node should always point to record when in valid position to add ':'
-            if matches!(prev_ast_node, Expr2::Record { .. })
+                    if matches!(prev_expr, Expr2::Record { .. })
                         && matches!(curr_ast_node, Expr2::Record { .. })
                     {
-                let sibling_ids = curr_mark_node.get_sibling_ids(&ed_model.markup_node_pool);
+                        let sibling_ids = curr_mark_node.get_sibling_ids(&ed_model.mark_node_pool);
 
                         let new_child_index = index_of(curr_mark_node_id, &sibling_ids)?;
 
@@ -213,11 +206,13 @@ pub fn update_record_colon(
                                 if ed_model.node_exists_at_caret() {
                                     let next_mark_node_id =
                                         ed_model.grid_node_map.get_id_at_row_col(old_caret_pos)?;
-                            let next_mark_node = ed_model.markup_node_pool.get(next_mark_node_id);
-                            if next_mark_node.get_content()? == nodes::RIGHT_ACCOLADE {
+                                    let next_mark_node =
+                                        ed_model.mark_node_pool.get(next_mark_node_id);
+                                    if next_mark_node.get_content() == nodes::RIGHT_ACCOLADE {
                                         // update AST node
                                         let new_field_val = Expr2::Blank;
-                                let new_field_val_id = ed_model.module.env.pool.add(new_field_val);
+                                        let new_field_val_id =
+                                            ed_model.module.env.pool.add(new_field_val);
 
                                         let first_field_mut = fields
                                             .iter_mut(ed_model.module.env.pool)
@@ -235,30 +230,31 @@ pub fn update_record_colon(
 
                                         let record_colon_node = MarkupNode::Text {
                                             content: record_colon.to_owned(),
-                                    ast_node_id: record_ast_node_id,
+                                            ast_node_id: ASTNodeId::AExprId(record_ast_node_id),
                                             syn_high_style: HighlightStyle::Operator,
                                             attributes: Attributes::new(),
                                             parent_id_opt: Some(parent_id),
+                                            newlines_at_end: 0,
                                         };
 
                                         let record_colon_node_id =
-                                    ed_model.markup_node_pool.add(record_colon_node);
+                                            ed_model.add_mark_node(record_colon_node);
                                         ed_model
-                                    .markup_node_pool
+                                            .mark_node_pool
                                             .get_mut(parent_id)
-                                    .add_child_at_index(new_child_index, record_colon_node_id)?;
-
-                                let record_blank_node = MarkupNode::Blank {
-                                    ast_node_id: new_field_val_id,
-                                    syn_high_style: HighlightStyle::Blank,
-                                    attributes: Attributes::new(),
-                                    parent_id_opt: Some(parent_id),
-                                };
+                                            .add_child_at_index(
+                                                new_child_index,
+                                                record_colon_node_id,
+                                            )?;
 
                                         let record_blank_node_id =
-                                    ed_model.markup_node_pool.add(record_blank_node);
+                                            ed_model.add_mark_node(new_blank_mn(
+                                                ASTNodeId::AExprId(new_field_val_id),
+                                                Some(parent_id),
+                                            ));
+
                                         ed_model
-                                    .markup_node_pool
+                                            .mark_node_pool
                                             .get_mut(parent_id)
                                             .add_child_at_index(
                                                 new_child_index + 1,
@@ -269,18 +265,10 @@ pub fn update_record_colon(
                                         ed_model.simple_move_carets_right(record_colon.len());
 
                                         // update GridNodeMap and CodeLines
-                                ed_model.insert_between_line(
+                                        ed_model.insert_all_between_line(
                                             old_caret_pos.line,
                                             old_caret_pos.column,
-                                    nodes::COLON,
-                                    record_colon_node_id,
-                                )?;
-
-                                ed_model.insert_between_line(
-                                    old_caret_pos.line,
-                                    old_caret_pos.column + nodes::COLON.len(),
-                                    nodes::BLANK_PLACEHOLDER,
-                                    record_blank_node_id,
+                                            &[record_colon_node_id, record_blank_node_id],
                                         )?;
 
                                         Ok(InputOutcome::Accepted)
@@ -296,6 +284,8 @@ pub fn update_record_colon(
                     } else {
                         Ok(InputOutcome::Ignored)
                     }
+                }
+            }
         } else {
             Ok(InputOutcome::Ignored)
         }
@@ -315,7 +305,7 @@ pub fn update_record_field(
     ed_model: &mut EdModel,
 ) -> EdResult<InputOutcome> {
     // update MarkupNode
-    let curr_mark_node_mut = ed_model.markup_node_pool.get_mut(curr_mark_node_id);
+    let curr_mark_node_mut = ed_model.mark_node_pool.get_mut(curr_mark_node_id);
     let content_str_mut = curr_mark_node_mut.get_content_mut()?;
     let node_caret_offset = ed_model
         .grid_node_map
@@ -337,11 +327,13 @@ pub fn update_record_field(
     ed_model.simple_move_carets_right(new_input.len());
 
     // update GridNodeMap and CodeLines
-    ed_model.insert_between_line(
+    EdModel::insert_between_line(
         old_caret_pos.line,
         old_caret_pos.column,
         new_input,
         curr_mark_node_id,
+        &mut ed_model.grid_node_map,
+        &mut ed_model.code_lines,
     )?;
 
     // update AST Node

editor/src/editor/mvc/string_update.rs

@@ -7,6 +7,7 @@ use crate::editor::mvc::ed_model::EdModel;
 use crate::editor::mvc::ed_update::get_node_context;
 use crate::editor::mvc::ed_update::NodeContext;
 use crate::editor::syntax_highlight::HighlightStyle;
+use crate::lang::ast::update_str_expr;
 use crate::lang::ast::ArrString;
 use crate::lang::ast::Expr2;
 use crate::lang::pool::PoolStr;
@@ -27,7 +28,7 @@ pub fn update_small_string(
     let new_input = &new_char.to_string();
 
     // update markup
-    let curr_mark_node_mut = ed_model.markup_node_pool.get_mut(curr_mark_node_id);
+    let curr_mark_node_mut = ed_model.mark_node_pool.get_mut(curr_mark_node_id);
     let content_str_mut = curr_mark_node_mut.get_content_mut()?;
     let node_caret_offset = ed_model
         .grid_node_map
@@ -36,7 +37,7 @@ pub fn update_small_string(
     if node_caret_offset != 0 && node_caret_offset < content_str_mut.len() {
         if old_array_str.len() < ArrString::capacity() {
             if let Expr2::SmallStr(ref mut mut_array_str) =
-                ed_model.module.env.pool.get_mut(ast_node_id)
+                ed_model.module.env.pool.get_mut(ast_node_id.to_expr_id()?)
             {
                 // safe because we checked the length
                 unsafe {
@@ -51,17 +52,23 @@ pub fn update_small_string(
 
             let new_ast_node = Expr2::Str(PoolStr::new(&new_str, ed_model.module.env.pool));
 
-            ed_model.module.env.pool.set(ast_node_id, new_ast_node);
+            ed_model
+                .module
+                .env
+                .pool
+                .set(ast_node_id.to_expr_id()?, new_ast_node);
         }
 
         content_str_mut.insert_str(node_caret_offset, new_input);
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             new_input,
             curr_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         // update caret
@@ -73,11 +80,7 @@ pub fn update_small_string(
     }
 }
 
-pub fn update_string(
-    new_input: &str,
-    old_pool_str: &PoolStr,
-    ed_model: &mut EdModel,
-) -> EdResult<InputOutcome> {
+pub fn update_string(new_char: char, ed_model: &mut EdModel) -> EdResult<InputOutcome> {
     let NodeContext {
         old_caret_pos,
         curr_mark_node_id,
@@ -87,31 +90,32 @@ pub fn update_string(
     } = get_node_context(ed_model)?;
 
     // update markup
-    let curr_mark_node_mut = ed_model.markup_node_pool.get_mut(curr_mark_node_id);
+    let curr_mark_node_mut = ed_model.mark_node_pool.get_mut(curr_mark_node_id);
     let content_str_mut = curr_mark_node_mut.get_content_mut()?;
     let node_caret_offset = ed_model
         .grid_node_map
         .get_offset_to_node_id(old_caret_pos, curr_mark_node_id)?;
 
     if node_caret_offset != 0 && node_caret_offset < content_str_mut.len() {
-        content_str_mut.insert_str(node_caret_offset, new_input);
+        content_str_mut.insert(node_caret_offset, new_char);
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
-            new_input,
+            &new_char.to_string(),
             curr_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         // update ast
-        let mut new_string = old_pool_str.as_str(ed_model.module.env.pool).to_owned();
-        new_string.push_str(new_input);
-
-        let new_pool_str = PoolStr::new(&new_string, &mut ed_model.module.env.pool);
-        let new_ast_node = Expr2::Str(new_pool_str);
-
-        ed_model.module.env.pool.set(ast_node_id, new_ast_node);
+        update_str_expr(
+            ast_node_id.to_expr_id()?,
+            new_char,
+            node_caret_offset - 1, // -1 because offset was calculated with quotes
+            &mut ed_model.module.env.pool,
+        )?;
 
         // update caret
         ed_model.simple_move_carets_right(1);
@@ -133,8 +137,13 @@ pub fn start_new_string(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
 
     if curr_mark_node.is_blank() {
         let new_expr2_node = Expr2::SmallStr(arraystring::ArrayString::new());
+        let curr_mark_node_nls = curr_mark_node.get_newlines_at_end();
 
-        ed_model.module.env.pool.set(ast_node_id, new_expr2_node);
+        ed_model
+            .module
+            .env
+            .pool
+            .set(ast_node_id.to_expr_id()?, new_expr2_node);
 
         let new_string_node = MarkupNode::Text {
             content: nodes::STRING_QUOTES.to_owned(),
@@ -142,21 +151,24 @@ pub fn start_new_string(ed_model: &mut EdModel) -> EdResult<InputOutcome> {
             syn_high_style: HighlightStyle::String,
             attributes: Attributes::new(),
             parent_id_opt,
+            newlines_at_end: curr_mark_node_nls,
         };
 
         ed_model
-            .markup_node_pool
+            .mark_node_pool
             .replace_node(curr_mark_node_id, new_string_node);
 
         // remove data corresponding to Blank node
-        ed_model.del_at_line(old_caret_pos.line, old_caret_pos.column)?;
+        ed_model.del_blank_expr_node(old_caret_pos)?;
 
         // update GridNodeMap and CodeLines
-        ed_model.insert_between_line(
+        EdModel::insert_between_line(
             old_caret_pos.line,
             old_caret_pos.column,
             nodes::STRING_QUOTES,
             curr_mark_node_id,
+            &mut ed_model.grid_node_map,
+            &mut ed_model.code_lines,
         )?;
 
         ed_model.simple_move_carets_right(1);

editor/src/editor/mvc/tld_value_update.rs

@@ -0,0 +1,206 @@
+use roc_module::symbol::{Interns, Symbol};
+
+use crate::{
+    editor::{
+        ed_error::{EdResult, FailedToUpdateIdentIdName, KeyNotFound},
+        markup::{
+            attribute::Attributes,
+            common_nodes::{new_blank_mn_w_nls, new_equals_mn},
+            nodes::{set_parent_for_all, MarkupNode},
+        },
+        slow_pool::{MarkNodeId, SlowPool},
+        syntax_highlight::HighlightStyle,
+    },
+    lang::{
+        ast::{Def2, Expr2},
+        expr::Env,
+        parse::ASTNodeId,
+        pattern::{get_identifier_string, Pattern2},
+        pool::NodeId,
+    },
+    ui::text::text_pos::TextPos,
+};
+
+use super::{
+    app_update::InputOutcome,
+    ed_model::EdModel,
+    ed_update::{get_node_context, NodeContext},
+};
+
+// Top Level Defined Value. example: `main = "Hello, World!"`
+
+pub fn tld_mark_node<'a>(
+    identifier_id: NodeId<Pattern2>,
+    expr_mark_node_id: MarkNodeId,
+    ast_node_id: ASTNodeId,
+    mark_node_pool: &mut SlowPool,
+    env: &Env<'a>,
+    interns: &Interns,
+) -> EdResult<MarkupNode> {
+    let pattern2 = env.pool.get(identifier_id);
+    let val_name = get_identifier_string(pattern2, interns)?;
+
+    let val_name_mn = MarkupNode::Text {
+        content: val_name,
+        ast_node_id,
+        syn_high_style: HighlightStyle::Variable,
+        attributes: Attributes::new(),
+        parent_id_opt: None,
+        newlines_at_end: 0,
+    };
+
+    let val_name_mn_id = mark_node_pool.add(val_name_mn);
+
+    let equals_mn_id = mark_node_pool.add(new_equals_mn(ast_node_id, None));
+
+    let full_let_node = MarkupNode::Nested {
+        ast_node_id,
+        children_ids: vec![val_name_mn_id, equals_mn_id, expr_mark_node_id],
+        parent_id_opt: None,
+        newlines_at_end: 2,
+    };
+
+    Ok(full_let_node)
+}
+
+pub fn start_new_tld_value(ed_model: &mut EdModel, new_char: &char) -> EdResult<InputOutcome> {
+    let NodeContext {
+        old_caret_pos,
+        curr_mark_node_id,
+        curr_mark_node: _,
+        parent_id_opt: _,
+        ast_node_id,
+    } = get_node_context(ed_model)?;
+
+    let val_expr_node = Expr2::Blank;
+    let val_expr_id = ed_model.module.env.pool.add(val_expr_node);
+
+    let val_expr_mn = new_blank_mn_w_nls(ASTNodeId::AExprId(val_expr_id), None, 0);
+    let val_expr_mn_id = ed_model.mark_node_pool.add(val_expr_mn);
+
+    let val_name_string = new_char.to_string();
+
+    let ident_id = ed_model
+        .module
+        .env
+        .ident_ids
+        .add(val_name_string.clone().into());
+
+    let module_ident_ids_opt = ed_model
+        .loaded_module
+        .interns
+        .all_ident_ids
+        .get_mut(&ed_model.module.env.home);
+
+    if let Some(module_ident_ids_ref) = module_ident_ids_opt {
+        // this might create different IdentId for interns and env.ident_ids which may be a problem
+        module_ident_ids_ref.add(val_name_string.into());
+    } else {
+        KeyNotFound {
+            key_str: format!("{:?}", ed_model.module.env.home),
+        }
+        .fail()?
+    }
+
+    let val_symbol = Symbol::new(ed_model.module.env.home, ident_id);
+
+    let patt2 = Pattern2::Identifier(val_symbol);
+    let patt2_id = ed_model.module.env.pool.add(patt2);
+
+    let tld_mark_node = tld_mark_node(
+        patt2_id,
+        val_expr_mn_id,
+        ast_node_id,
+        &mut ed_model.mark_node_pool,
+        &ed_model.module.env,
+        &ed_model.loaded_module.interns,
+    )?;
+
+    let new_ast_node = Def2::ValueDef {
+        identifier_id: patt2_id,
+        expr_id: val_expr_id,
+    };
+
+    ed_model
+        .module
+        .env
+        .pool
+        .set(ast_node_id.to_def_id()?, new_ast_node);
+
+    ed_model
+        .mark_node_pool
+        .replace_node(curr_mark_node_id, tld_mark_node);
+
+    set_parent_for_all(curr_mark_node_id, &mut ed_model.mark_node_pool);
+
+    // remove data corresponding to old Blank node
+    ed_model.del_line(old_caret_pos.line + 1)?;
+    ed_model.del_line(old_caret_pos.line)?;
+
+    let char_len = 1;
+    ed_model.simple_move_carets_right(char_len);
+
+    let mut curr_line = old_caret_pos.line;
+    let mut curr_column = old_caret_pos.column;
+
+    EdModel::insert_mark_node_between_line(
+        &mut curr_line,
+        &mut curr_column,
+        curr_mark_node_id,
+        &mut ed_model.grid_node_map,
+        &mut ed_model.code_lines,
+        &ed_model.mark_node_pool,
+    )?;
+
+    Ok(InputOutcome::Accepted)
+}
+
+pub fn update_tld_val_name(
+    val_name_mn_id: MarkNodeId,
+    old_caret_pos: TextPos,
+    ed_model: &mut EdModel,
+    new_char: &char,
+) -> EdResult<InputOutcome> {
+    if new_char.is_ascii_alphanumeric() {
+        // update markup
+        let val_name_mn_mut = ed_model.mark_node_pool.get_mut(val_name_mn_id);
+        let content_str_mut = val_name_mn_mut.get_content_mut()?;
+
+        let old_val_name = content_str_mut.clone();
+
+        let node_caret_offset = ed_model
+            .grid_node_map
+            .get_offset_to_node_id(old_caret_pos, val_name_mn_id)?;
+
+        if node_caret_offset <= content_str_mut.len() {
+            content_str_mut.insert(node_caret_offset, *new_char);
+
+            let update_val_name_res = ed_model
+                .module
+                .env
+                .ident_ids
+                .update_key(&old_val_name, content_str_mut);
+
+            if let Err(err_str) = update_val_name_res {
+                FailedToUpdateIdentIdName { err_str }.fail()?;
+            }
+
+            EdModel::insert_between_line(
+                old_caret_pos.line,
+                old_caret_pos.column,
+                &new_char.to_string(),
+                val_name_mn_id,
+                &mut ed_model.grid_node_map,
+                &mut ed_model.code_lines,
+            )?;
+
+            ed_model.simple_move_caret_right(old_caret_pos, 1);
+
+            Ok(InputOutcome::Accepted)
+        } else {
+            Ok(InputOutcome::Ignored)
+        }
+    } else {
+        Ok(InputOutcome::Ignored)
+    }
+}

editor/src/editor/render_ast.rs

@@ -1,4 +1,5 @@
 use super::markup::nodes::{MarkupNode, BLANK_PLACEHOLDER};
+use super::slow_pool::MarkNodeId;
 use crate::editor::mvc::ed_view::RenderedWgpu;
 use crate::editor::slow_pool::SlowPool;
 use crate::editor::{ed_error::EdResult, theme::EdTheme, util::map_get};
@@ -10,37 +11,49 @@ use winit::dpi::PhysicalSize;
 use crate::{editor::config::Config, graphics::colors};
 
 pub fn build_code_graphics<'a>(
-    markup_node: &'a MarkupNode,
+    markup_ids: &[MarkNodeId],
     size: &PhysicalSize<u32>,
     txt_coords: Vector2<f32>,
     config: &Config,
     glyph_dim_rect: Rect,
-    markup_node_pool: &'a SlowPool,
+    mark_node_pool: &'a SlowPool,
 ) -> EdResult<RenderedWgpu> {
     let area_bounds = (size.width as f32, size.height as f32);
     let layout = wgpu_glyph::Layout::default().h_align(wgpu_glyph::HorizontalAlign::Left);
     let mut rendered_wgpu = RenderedWgpu::new();
 
-    let (glyph_text_vec, rects) = markup_to_wgpu(
-        markup_node,
+    let mut all_glyph_text_vec = vec![];
+    let mut all_rects = vec![];
+    let mut txt_row_col = (0, 0);
+
+    for markup_id in markup_ids.iter() {
+        let mark_node = mark_node_pool.get(*markup_id);
+
+        let (mut glyph_text_vec, mut rects) = markup_to_wgpu(
+            mark_node,
             &CodeStyle {
                 ed_theme: &config.ed_theme,
                 font_size: config.code_font_size,
                 txt_coords,
                 glyph_dim_rect,
             },
-        markup_node_pool,
+            &mut txt_row_col,
+            mark_node_pool,
         )?;
 
+        all_glyph_text_vec.append(&mut glyph_text_vec);
+        all_rects.append(&mut rects)
+    }
+
     let section = gr_text::owned_section_from_glyph_texts(
-        glyph_text_vec,
+        all_glyph_text_vec,
         txt_coords.into(),
         area_bounds,
         layout,
     );
 
-    rendered_wgpu.add_rects(rects);
-    rendered_wgpu.add_text(section);
+    rendered_wgpu.add_rects_behind(all_rects); // currently only rects for Blank
+    rendered_wgpu.add_text_behind(section);
 
     Ok(rendered_wgpu)
 }
@@ -55,51 +68,57 @@ struct CodeStyle<'a> {
 fn markup_to_wgpu<'a>(
     markup_node: &'a MarkupNode,
     code_style: &CodeStyle,
-    markup_node_pool: &'a SlowPool,
+    txt_row_col: &mut (usize, usize),
+    mark_node_pool: &'a SlowPool,
 ) -> EdResult<(Vec<glyph_brush::OwnedText>, Vec<Rect>)> {
     let mut wgpu_texts: Vec<glyph_brush::OwnedText> = Vec::new();
     let mut rects: Vec<Rect> = Vec::new();
 
-    let mut txt_row_col = (0, 0);
-
     markup_to_wgpu_helper(
         markup_node,
         &mut wgpu_texts,
         &mut rects,
         code_style,
-        &mut txt_row_col,
-        markup_node_pool,
+        txt_row_col,
+        mark_node_pool,
     )?;
 
     Ok((wgpu_texts, rects))
 }
 
-// TODO use text_row
 fn markup_to_wgpu_helper<'a>(
     markup_node: &'a MarkupNode,
     wgpu_texts: &mut Vec<glyph_brush::OwnedText>,
     rects: &mut Vec<Rect>,
     code_style: &CodeStyle,
     txt_row_col: &mut (usize, usize),
-    markup_node_pool: &'a SlowPool,
+    mark_node_pool: &'a SlowPool,
 ) -> EdResult<()> {
     match markup_node {
         MarkupNode::Nested {
             ast_node_id: _,
             children_ids,
             parent_id_opt: _,
+            newlines_at_end,
         } => {
             for child_id in children_ids.iter() {
-                let child = markup_node_pool.get(*child_id);
+                let child = mark_node_pool.get(*child_id);
                 markup_to_wgpu_helper(
                     child,
                     wgpu_texts,
                     rects,
                     code_style,
                     txt_row_col,
-                    markup_node_pool,
+                    mark_node_pool,
                 )?;
             }
+
+            for _ in 0..*newlines_at_end {
+                wgpu_texts.push(newline(code_style.font_size));
+
+                txt_row_col.0 += 1;
+                txt_row_col.1 = 0;
+            }
         }
         MarkupNode::Text {
             content,
@@ -107,14 +126,23 @@ fn markup_to_wgpu_helper<'a>(
             syn_high_style,
             attributes: _,
             parent_id_opt: _,
+            newlines_at_end,
         } => {
             let highlight_color = map_get(&code_style.ed_theme.syntax_high_map, syn_high_style)?;
 
-            let glyph_text = glyph_brush::OwnedText::new(content)
+            let full_content = markup_node.get_full_content();
+
+            let glyph_text = glyph_brush::OwnedText::new(full_content)
                 .with_color(colors::to_slice(*highlight_color))
                 .with_scale(code_style.font_size);
 
             txt_row_col.1 += content.len();
+
+            for _ in 0..*newlines_at_end {
+                txt_row_col.0 += 1;
+                txt_row_col.1 = 0;
+            }
+
             wgpu_texts.push(glyph_text);
         }
         MarkupNode::Blank {
@@ -122,8 +150,11 @@ fn markup_to_wgpu_helper<'a>(
             attributes: _,
             syn_high_style,
             parent_id_opt: _,
+            newlines_at_end,
         } => {
-            let glyph_text = glyph_brush::OwnedText::new(BLANK_PLACEHOLDER)
+            let full_content = markup_node.get_full_content();
+
+            let glyph_text = glyph_brush::OwnedText::new(full_content)
                 .with_color(colors::to_slice(colors::WHITE))
                 .with_scale(code_style.font_size);
 
@@ -132,7 +163,7 @@ fn markup_to_wgpu_helper<'a>(
             let char_width = code_style.glyph_dim_rect.width;
             let char_height = code_style.glyph_dim_rect.height;
 
-            let hole_rect = Rect {
+            let blank_rect = Rect {
                 top_left_coords: (
                     code_style.txt_coords.x + (txt_row_col.1 as f32) * char_width,
                     code_style.txt_coords.y
@@ -144,12 +175,21 @@ fn markup_to_wgpu_helper<'a>(
                 height: char_height,
                 color: *highlight_color,
             };
-            rects.push(hole_rect);
+            rects.push(blank_rect);
 
             txt_row_col.1 += BLANK_PLACEHOLDER.len();
             wgpu_texts.push(glyph_text);
+
+            for _ in 0..*newlines_at_end {
+                txt_row_col.0 += 1;
+                txt_row_col.1 = 0;
+            }
         }
     };
 
     Ok(())
 }
+
+fn newline(font_size: f32) -> glyph_brush::OwnedText {
+    glyph_brush::OwnedText::new("\n").with_scale(font_size)
+}

editor/src/editor/render_debug.rs

@@ -4,7 +4,7 @@ use crate::editor::mvc::ed_model::EdModel;
 use crate::graphics::colors;
 use crate::graphics::colors::from_hsb;
 use crate::graphics::primitives::text as gr_text;
-use crate::lang::ast::expr2_to_string;
+use crate::lang::ast::def2_to_string;
 use cgmath::Vector2;
 use winit::dpi::PhysicalSize;
 
@@ -19,36 +19,49 @@ pub fn build_debug_graphics(
     let area_bounds = (size.width as f32, size.height as f32);
     let layout = wgpu_glyph::Layout::default().h_align(wgpu_glyph::HorizontalAlign::Left);
 
-    let debug_txt_coords: Vector2<f32> = (txt_coords.x, txt_coords.y * 3.0).into();
+    let debug_txt_coords: Vector2<f32> = (txt_coords.x * 20.0, txt_coords.y).into();
+
+    let carets_text =
+        glyph_brush::OwnedText::new(format!("carets: {:?}\n\n", ed_model.get_carets()))
+            .with_color(colors::to_slice(from_hsb(0, 0, 100)))
+            .with_scale(config.debug_font_size);
 
     let grid_node_map_text = glyph_brush::OwnedText::new(format!("{}", ed_model.grid_node_map))
         .with_color(colors::to_slice(from_hsb(20, 41, 100)))
-        .with_scale(config.code_font_size);
+        .with_scale(config.debug_font_size);
 
     let code_lines_text = glyph_brush::OwnedText::new(format!("{}", ed_model.code_lines))
         .with_color(colors::to_slice(from_hsb(0, 49, 96)))
-        .with_scale(config.code_font_size);
+        .with_scale(config.debug_font_size);
 
-    let mark_node_tree_text = glyph_brush::OwnedText::new(tree_as_string(
-        ed_model.markup_root_id,
-        &ed_model.markup_node_pool,
-    ))
+    let mut mark_node_trees_string = "\nmark node trees:".to_owned();
+
+    for mark_id in ed_model.markup_ids[1..].iter() {
+        // 1.. -> skip header
+        mark_node_trees_string.push_str(&tree_as_string(*mark_id, &ed_model.mark_node_pool));
+    }
+
+    let mark_node_tree_text = glyph_brush::OwnedText::new(mark_node_trees_string)
         .with_color(colors::to_slice(from_hsb(266, 31, 96)))
-    .with_scale(config.code_font_size);
+        .with_scale(config.debug_font_size);
 
-    let mark_node_pool_text = glyph_brush::OwnedText::new(format!("{}", ed_model.markup_node_pool))
+    let mark_node_pool_text = glyph_brush::OwnedText::new(format!("{}", ed_model.mark_node_pool))
         .with_color(colors::to_slice(from_hsb(110, 45, 82)))
-        .with_scale(config.code_font_size);
+        .with_scale(config.debug_font_size);
 
-    let ast_node_text = glyph_brush::OwnedText::new(format!(
-        "\n\n(ast_root)\n{}",
-        expr2_to_string(ed_model.module.ast_root_id, ed_model.module.env.pool)
-    ))
+    let mut ast_node_text_str = "AST:\n".to_owned();
+
+    for def_id in ed_model.module.ast.def_ids.iter() {
+        ast_node_text_str.push_str(&def2_to_string(*def_id, ed_model.module.env.pool))
+    }
+
+    let ast_node_text = glyph_brush::OwnedText::new(ast_node_text_str)
         .with_color(colors::to_slice(from_hsb(211, 80, 100)))
-    .with_scale(config.code_font_size);
+        .with_scale(config.debug_font_size);
 
     let section = gr_text::owned_section_from_glyph_texts(
         vec![
+            carets_text,
             grid_node_map_text,
             code_lines_text,
             mark_node_tree_text,

editor/src/editor/resources/strings.rs

@@ -1,3 +1,27 @@
-pub const NOTHING_OPENED: &str = "Opening files is not yet supported, execute `cargo run edit` from the root folder of the repo to try the editor.";
-pub const START_TIP: &str =
-    "Start by typing '[', '{', '\"' or a number.\nInput chars that would create parse errors will be ignored.";
+#![allow(dead_code)]
+
+pub const NOTHING_OPENED: &str =
+    "Execute `cargo run edit` from the root folder of the repo to try the editor.";
+
+pub const START_TIP: &str = r#"Currently supported: lists, records, string, numbers and value definitions.
+
+Use `Ctrl+Shift+Up` or `Cmd+Shift+Up` to select surrounding expression.
+Use backspace after `Ctrl+Shift+Up` to delete the selected expression.
+
+`Ctrl+S` or `Cmd+S` to save.
+`Ctrl+R` to run.
+
+Input chars that would create parse errors or change formatting will be ignored.
+For convenience and consistency, there is only one way to format roc.
+"#;
+
+pub const HELLO_WORLD: &str = r#"
+app "test-app"
+    packages { base: "platform" }
+    imports []
+    provides [ main ] to base
+
+main = "Hello, world!"
+
+
+"#;

editor/src/editor/slow_pool.rs

@@ -63,7 +63,7 @@ impl fmt::Display for SlowPool {
                 "{}: {} ({}) ast_id {:?} {}",
                 index,
                 node.node_type_as_string(),
-                node.get_content().unwrap_or_else(|_| "".to_string()),
+                node.get_content(),
                 ast_node_id.parse::<usize>().unwrap(),
                 child_str
             )?;

editor/src/editor/style.rs

@@ -1 +1 @@
-pub const CODE_TXT_XY: (f32, f32) = (40.0, 130.0);
+pub const CODE_TXT_XY: (f32, f32) = (40.0, 350.0);

editor/src/editor/syntax_highlight.rs

@@ -14,6 +14,8 @@ pub enum HighlightStyle {
     PackageRelated, // app, packages, imports, exposes, provides...
     Variable,
     RecordField,
+    Import,
+    Provides,
     Blank,
 }
 
@@ -31,6 +33,8 @@ pub fn default_highlight_map() -> HashMap<HighlightStyle, RgbaTup> {
         (PackageRelated, gr_colors::WHITE),
         (Variable, gr_colors::WHITE),
         (RecordField, from_hsb(258, 50, 90)),
+        (Import, from_hsb(185, 50, 75)),
+        (Provides, from_hsb(185, 50, 75)),
         (Blank, from_hsb(258, 50, 90)),
         // comment from_hsb(285, 6, 47) or 186, 35, 40
     ]

editor/src/lang/ast.rs

@@ -3,6 +3,7 @@
 use std::collections::{HashMap, HashSet};
 use std::hash::BuildHasherDefault;
 
+use crate::editor::ed_error::{EdResult, UnexpectedASTNode};
 use crate::lang::pattern::{Pattern2, PatternId};
 use crate::lang::pool::Pool;
 use crate::lang::pool::{NodeId, PoolStr, PoolVec, ShallowClone};
@@ -222,6 +223,17 @@ pub enum Expr2 {
     RuntimeError(/* TODO make a version of RuntimeError that fits in 15B */),
 }
 
+// A top level definition, not inside a function. For example: `main = "Hello, world!"`
+#[derive(Debug)]
+pub enum Def2 {
+    // ValueDef example: `main = "Hello, world!"`. identifier -> `main`, expr -> "Hello, world!"
+    ValueDef {
+        identifier_id: NodeId<Pattern2>,
+        expr_id: NodeId<Expr2>,
+    },
+    Blank,
+}
+
 #[derive(Debug)]
 pub enum ValueDef {
     WithAnnotation {
@@ -267,6 +279,48 @@ impl ShallowClone for ValueDef {
     }
 }
 
+impl ValueDef {
+    pub fn get_expr_id(&self) -> ExprId {
+        match self {
+            ValueDef::WithAnnotation { expr_id, .. } => *expr_id,
+            ValueDef::NoAnnotation { expr_id, .. } => *expr_id,
+        }
+    }
+
+    pub fn get_pattern_id(&self) -> NodeId<Pattern2> {
+        match self {
+            ValueDef::WithAnnotation { pattern_id, .. } => *pattern_id,
+            ValueDef::NoAnnotation { pattern_id, .. } => *pattern_id,
+        }
+    }
+}
+
+pub fn value_def_to_string(val_def: &ValueDef, pool: &Pool) -> String {
+    match val_def {
+        ValueDef::WithAnnotation {
+            pattern_id,
+            expr_id,
+            type_id,
+            rigids,
+            expr_var,
+        } => {
+            format!("WithAnnotation {{ pattern_id: {:?}, expr_id: {:?}, type_id: {:?}, rigids: {:?}, expr_var: {:?}}}", pool.get(*pattern_id), expr2_to_string(*expr_id, pool), pool.get(*type_id), rigids, expr_var)
+        }
+        ValueDef::NoAnnotation {
+            pattern_id,
+            expr_id,
+            expr_var,
+        } => {
+            format!(
+                "NoAnnotation {{ pattern_id: {:?}, expr_id: {:?}, expr_var: {:?}}}",
+                pool.get(*pattern_id),
+                expr2_to_string(*expr_id, pool),
+                expr_var
+            )
+        }
+    }
+}
+
 #[derive(Debug)]
 pub enum FunctionDef {
     WithAnnotation {
@@ -402,7 +456,11 @@ pub struct WhenBranch {
 // TODO make the inner types private?
 pub type ExprId = NodeId<Expr2>;
 
+pub type DefId = NodeId<Def2>;
+
 use RecordField::*;
+
+use super::parse::ASTNodeId;
 impl RecordField {
     pub fn get_record_field_var(&self) -> &Variable {
         match self {
@@ -437,6 +495,13 @@ impl RecordField {
     }
 }
 
+pub fn ast_node_to_string(node_id: ASTNodeId, pool: &Pool) -> String {
+    match node_id {
+        ASTNodeId::ADefId(def_id) => def2_to_string(def_id, pool),
+        ASTNodeId::AExprId(expr_id) => expr2_to_string(expr_id, pool),
+    }
+}
+
 pub fn expr2_to_string(node_id: ExprId, pool: &Pool) -> String {
     let mut full_string = String::new();
     let expr2 = pool.get(node_id);
@@ -550,16 +615,118 @@ fn expr2_to_string_helper(
         Expr2::SmallInt { text, .. } => {
             out_string.push_str(&format!("SmallInt({})", text.as_str(pool)));
         }
+        Expr2::LetValue {
+            def_id, body_id, ..
+        } => {
+            out_string.push_str(&format!(
+                "LetValue(def_id: >>{:?}), body_id: >>{:?})",
+                value_def_to_string(pool.get(*def_id), pool),
+                pool.get(*body_id)
+            ));
+        }
         other => todo!("Implement for {:?}", other),
     }
 
     out_string.push('\n');
 }
 
+pub fn def2_to_string(node_id: DefId, pool: &Pool) -> String {
+    let mut full_string = String::new();
+    let def2 = pool.get(node_id);
+
+    match def2 {
+        Def2::ValueDef {
+            identifier_id,
+            expr_id,
+        } => {
+            full_string.push_str(&format!(
+                "Def2::ValueDef(identifier_id: >>{:?}), expr_id: >>{:?})",
+                pool.get(*identifier_id),
+                expr2_to_string(*expr_id, pool)
+            ));
+        }
+        Def2::Blank => {
+            full_string.push_str("Def2::Blank");
+        }
+    }
+
+    full_string
+}
+
 fn var_to_string(some_var: &Variable, indent_level: usize) -> String {
     format!("{}Var({:?})\n", get_spacing(indent_level + 1), some_var)
 }
 
+// get string from SmallStr or Str
+pub fn get_string_from_expr2(node_id: ExprId, pool: &Pool) -> EdResult<String> {
+    match pool.get(node_id) {
+        Expr2::SmallStr(arr_string) => Ok(arr_string.as_str().to_string()),
+        Expr2::Str(pool_str) => Ok(pool_str.as_str(pool).to_owned()),
+        other => UnexpectedASTNode {
+            required_node_type: "SmallStr or Str",
+            encountered_node_type: format!("{:?}", other),
+        }
+        .fail()?,
+    }
+}
+
+pub fn update_str_expr(
+    node_id: ExprId,
+    new_char: char,
+    insert_index: usize,
+    pool: &mut Pool,
+) -> EdResult<()> {
+    let str_expr = pool.get_mut(node_id);
+
+    enum Either {
+        MyString(String),
+        MyPoolStr(PoolStr),
+        Done,
+    }
+
+    let insert_either = match str_expr {
+        Expr2::SmallStr(arr_string) => {
+            let insert_res = arr_string.try_insert(insert_index as u8, new_char);
+
+            match insert_res {
+                Ok(_) => Either::Done,
+                _ => {
+                    let mut new_string = arr_string.as_str().to_string();
+                    new_string.insert(insert_index, new_char);
+
+                    Either::MyString(new_string)
+                }
+            }
+        }
+        Expr2::Str(old_pool_str) => Either::MyPoolStr(*old_pool_str),
+        other => UnexpectedASTNode {
+            required_node_type: "SmallStr or Str",
+            encountered_node_type: format!("{:?}", other),
+        }
+        .fail()?,
+    };
+
+    match insert_either {
+        Either::MyString(new_string) => {
+            let new_pool_str = PoolStr::new(&new_string, pool);
+
+            pool.set(node_id, Expr2::Str(new_pool_str))
+        }
+        Either::MyPoolStr(old_pool_str) => {
+            let mut new_string = old_pool_str.as_str(pool).to_owned();
+
+            new_string.insert(insert_index, new_char);
+
+            let new_pool_str = PoolStr::new(&new_string, pool);
+
+            pool.set(node_id, Expr2::Str(new_pool_str))
+        }
+        Either::Done => (),
+    }
+
+    Ok(())
+}
+
 #[test]
 fn size_of_expr() {
     assert_eq!(std::mem::size_of::<Expr2>(), crate::lang::pool::NODE_BYTES);

editor/src/lang/expr.rs

@@ -3,10 +3,11 @@
 #![allow(unused_imports)]
 use bumpalo::{collections::Vec as BumpVec, Bump};
 use std::collections::HashMap;
+use std::iter::FromIterator;
 
 use crate::lang::ast::{
-    expr2_to_string, ClosureExtra, Expr2, ExprId, FloatVal, IntStyle, IntVal, RecordField,
-    WhenBranch,
+    expr2_to_string, value_def_to_string, ClosureExtra, Def2, Expr2, ExprId, FloatVal, IntStyle,
+    IntVal, RecordField, ValueDef, WhenBranch,
 };
 use crate::lang::def::{
     canonicalize_defs, sort_can_defs, CanDefs, Declaration, Def, PendingDef, References,
@@ -288,6 +289,25 @@ pub fn to_expr_id<'a>(
     (env.add(expr, region), output)
 }
 
+pub fn str_to_def2<'a>(
+    arena: &'a Bump,
+    input: &'a str,
+    env: &mut Env<'a>,
+    scope: &mut Scope,
+    region: Region,
+) -> Result<Vec<Def2>, SyntaxError<'a>> {
+    match roc_parse::test_helpers::parse_defs_with(arena, input.trim()) {
+        Ok(vec_loc_def) => Ok(defs_to_defs2(
+            arena,
+            env,
+            scope,
+            arena.alloc(vec_loc_def),
+            region,
+        )),
+        Err(fail) => Err(fail),
+    }
+}
+
 pub fn str_to_expr2<'a>(
     arena: &'a Bump,
     input: &'a str,
@@ -296,20 +316,23 @@ pub fn str_to_expr2<'a>(
     region: Region,
 ) -> Result<(Expr2, self::Output), SyntaxError<'a>> {
     match roc_parse::test_helpers::parse_loc_with(arena, input.trim()) {
-        Ok(loc_expr) => {
-            let desugared_loc_expr = desugar_expr(arena, arena.alloc(loc_expr));
-
-            Ok(to_expr2(
-                env,
-                scope,
-                arena.alloc(desugared_loc_expr.value),
-                region,
-            ))
-        }
+        Ok(loc_expr) => Ok(loc_expr_to_expr2(arena, loc_expr, env, scope, region)),
         Err(fail) => Err(fail),
     }
 }
 
+fn loc_expr_to_expr2<'a>(
+    arena: &'a Bump,
+    loc_expr: Located<Expr<'a>>,
+    env: &mut Env<'a>,
+    scope: &mut Scope,
+    region: Region,
+) -> (Expr2, self::Output) {
+    let desugared_loc_expr = desugar_expr(arena, arena.alloc(loc_expr));
+
+    to_expr2(env, scope, arena.alloc(desugared_loc_expr.value), region)
+}
+
 pub fn to_expr2<'a>(
     env: &mut Env<'a>,
     scope: &mut Scope,
@@ -967,6 +990,74 @@ pub fn to_expr2<'a>(
     }
 }
 
+pub fn defs_to_defs2<'a>(
+    arena: &'a Bump,
+    env: &mut Env<'a>,
+    scope: &mut Scope,
+    parsed_defs: &'a BumpVec<roc_region::all::Loc<roc_parse::ast::Def<'a>>>,
+    region: Region,
+) -> Vec<Def2> {
+    use roc_parse::ast::Expr::*;
+
+    parsed_defs
+        .iter()
+        .map(|loc| to_def2_from_def(arena, env, scope, &loc.value, region))
+        .collect()
+}
+
+pub fn to_def2_from_def<'a>(
+    arena: &'a Bump,
+    env: &mut Env<'a>,
+    scope: &mut Scope,
+    parsed_def: &'a roc_parse::ast::Def<'a>,
+    region: Region,
+) -> Def2 {
+    use roc_parse::ast::Def::*;
+
+    match parsed_def {
+        SpaceBefore(inner_def, _) => to_def2_from_def(arena, env, scope, inner_def, region),
+        SpaceAfter(inner_def, _) => to_def2_from_def(arena, env, scope, inner_def, region),
+        Body(&loc_pattern, &loc_expr) => {
+            // TODO loc_pattern use identifier
+            let expr2 = loc_expr_to_expr2(arena, loc_expr, env, scope, region).0;
+            let expr_id = env.pool.add(expr2);
+
+            use roc_parse::ast::Pattern::*;
+
+            match loc_pattern.value {
+                Identifier(_) => {
+                    let (_, pattern2) = to_pattern2(
+                        env,
+                        scope,
+                        PatternType::TopLevelDef,
+                        &loc_pattern.value,
+                        region,
+                    );
+                    let pattern_id = env.pool.add(pattern2);
+
+                    // TODO support with annotation
+                    Def2::ValueDef {
+                        identifier_id: pattern_id,
+                        expr_id,
+                    }
+                }
+                other => {
+                    unimplemented!(
+                        "I don't yet know how to convert the pattern {:?} into an expr2",
+                        other
+                    )
+                }
+            }
+        }
+        other => {
+            unimplemented!(
+                "I don't know how to make an expr2 from this def yet: {:?}",
+                other
+            )
+        }
+    }
+}
+
 fn flatten_str_literal<'a>(
     env: &mut Env<'a>,
     scope: &mut Scope,
@@ -1412,6 +1503,7 @@ fn decl_to_let(pool: &mut Pool, var_store: &mut VarStore, decl: Declaration, ret
             Def::AnnotationOnly { .. } => todo!(),
             Def::Value(value_def) => {
                 let def_id = pool.add(value_def);
+
                 let body_id = pool.add(ret);
 
                 Expr2::LetValue {

editor/src/lang/mod.rs

@@ -3,7 +3,8 @@ pub mod constrain;
 mod def;
 pub mod expr;
 mod module;
-mod pattern;
+pub mod parse;
+pub mod pattern;
 pub mod pool;
 pub mod roc_file;
 pub mod scope;

editor/src/lang/parse.rs

@@ -0,0 +1,98 @@
+use std::fmt::Debug;
+
+use crate::{
+    editor::ed_error::ASTNodeIdWithoutExprId, editor::ed_error::EdResult, lang::scope::Scope,
+};
+use bumpalo::Bump;
+use roc_parse::parser::SyntaxError;
+use roc_region::all::Region;
+
+use super::{
+    ast::{DefId, Expr2, ExprId},
+    expr::{str_to_def2, Env},
+};
+
+#[derive(Debug)]
+pub struct AST {
+    pub header: AppHeader,
+    pub def_ids: Vec<DefId>,
+}
+
+#[derive(Debug, PartialEq, Copy, Clone)]
+pub enum ASTNodeId {
+    ADefId(DefId),
+    AExprId(ExprId),
+}
+
+impl ASTNodeId {
+    pub fn to_expr_id(&self) -> EdResult<ExprId> {
+        match self {
+            ASTNodeId::AExprId(expr_id) => Ok(*expr_id),
+            _ => ASTNodeIdWithoutExprId { ast_node_id: *self }.fail()?,
+        }
+    }
+
+    pub fn to_def_id(&self) -> EdResult<DefId> {
+        match self {
+            ASTNodeId::ADefId(def_id) => Ok(*def_id),
+            _ => ASTNodeIdWithoutExprId { ast_node_id: *self }.fail()?,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct AppHeader {
+    pub app_name: String,
+    pub packages_base: String,
+    pub imports: Vec<String>,
+    pub provides: Vec<String>,
+    pub ast_node_id: ExprId, // TODO probably want to use HeaderId
+}
+
+impl AST {
+    pub fn parse_from_string<'a>(
+        code_str: &'a str,
+        env: &mut Env<'a>,
+        ast_arena: &'a Bump,
+    ) -> Result<AST, SyntaxError<'a>> {
+        let blank_line_indx = code_str
+            .find("\n\n")
+            .expect("I was expecting a double newline to split header and rest of code.");
+
+        let header_str = &code_str[0..blank_line_indx];
+        let tail_str = &code_str[blank_line_indx..];
+
+        let mut scope = Scope::new(env.home, env.pool, env.var_store);
+        let region = Region::new(0, 0, 0, 0);
+
+        let mut def_ids = Vec::<DefId>::new();
+
+        let def2_vec = str_to_def2(ast_arena, tail_str, env, &mut scope, region)?;
+
+        for def2 in def2_vec {
+            let def_id = env.pool.add(def2);
+
+            def_ids.push(def_id);
+        }
+
+        let ast_node_id = env.pool.add(Expr2::Blank);
+
+        Ok(AST {
+            header: AppHeader::parse_from_string(header_str, ast_node_id),
+            def_ids,
+        })
+    }
+}
+
+impl AppHeader {
+    // TODO don't use mock struct and actually parse string
+    pub fn parse_from_string(_header_str: &str, ast_node_id: ExprId) -> Self {
+        AppHeader {
+            app_name: "\"untitled-app\"".to_owned(),
+            packages_base: "\"platform\"".to_owned(),
+            imports: vec![],
+            provides: vec!["main".to_owned()],
+            ast_node_id,
+        }
+    }
+}

editor/src/lang/pattern.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::all)]
 #![allow(dead_code)]
 #![allow(unused_imports)]
+use crate::editor::ed_error::{EdResult, UnexpectedPattern2Variant};
 use crate::lang::ast::{ExprId, FloatVal, IntVal};
 use crate::lang::expr::{to_expr_id, Env, Output};
 use crate::lang::pool::{NodeId, Pool, PoolStr, PoolVec, ShallowClone};
@@ -9,7 +10,7 @@ use bumpalo::collections::Vec as BumpVec;
 use roc_can::expr::unescape_char;
 use roc_can::num::{finish_parsing_base, finish_parsing_float, finish_parsing_int};
 use roc_collections::all::BumpMap;
-use roc_module::symbol::Symbol;
+use roc_module::symbol::{Interns, Symbol};
 use roc_parse::ast::{StrLiteral, StrSegment};
 use roc_parse::pattern::PatternType;
 use roc_problem::can::{MalformedPatternProblem, Problem, RuntimeError};
@@ -482,6 +483,17 @@ pub fn symbols_from_pattern(pool: &Pool, initial: &Pattern2) -> Vec<Symbol> {
     symbols
 }
 
+pub fn get_identifier_string(pattern: &Pattern2, interns: &Interns) -> EdResult<String> {
+    match pattern {
+        Pattern2::Identifier(symbol) => Ok(symbol.ident_str(interns).to_string()),
+        other => UnexpectedPattern2Variant {
+            required_pattern2: "Identifier".to_string(),
+            encountered_pattern2: format!("{:?}", other),
+        }
+        .fail()?,
+    }
+}
+
 pub fn symbols_and_variables_from_pattern(
     pool: &Pool,
     initial: &Pattern2,

editor/src/lib.rs

@@ -17,6 +17,6 @@ mod window;
 use std::io;
 use std::path::Path;
 
-pub fn launch(filepaths: &[&Path]) -> io::Result<()> {
-    editor::main::launch(filepaths)
+pub fn launch(project_dir_path_opt: Option<&Path>) -> io::Result<()> {
+    editor::main::launch(project_dir_path_opt)
 }

editor/src/ui/text/caret_w_select.rs

@@ -12,6 +12,12 @@ pub struct CaretWSelect {
     pub selection_opt: Option<Selection>,
 }
 
+pub enum CaretPos {
+    Start,
+    Exact(TextPos),
+    End,
+}
+
 fn mk_some_sel(start_pos: TextPos, end_pos: TextPos) -> UIResult<Option<Selection>> {
     if start_pos == end_pos {
         Ok(None)
@@ -146,7 +152,7 @@ pub mod test_caret_w_select {
 
     // Retrieve selection and position from formatted string
     pub fn convert_dsl_to_selection(lines: &[String]) -> Result<CaretWSelect, String> {
-        let lines_str: String = lines.join("");
+        let lines_str: String = lines.join("\n");
 
         let parsed = LineParser::parse(Rule::linesWithSelect, &lines_str)
             .expect("Selection test DSL parsing failed");

editor/src/ui/text/lines.rs

@@ -13,14 +13,12 @@ use crate::ui::text::{
 use crate::ui::ui_error::UIResult;
 use crate::ui::util::is_newline;
 use crate::window::keyboard_input::Modifiers;
-use bumpalo::collections::String as BumpString;
-use bumpalo::Bump;
 use std::cmp::max;
 use std::cmp::min;
 use winit::event::VirtualKeyCode;
 
 pub trait Lines {
-    fn get_line(&self, line_nr: usize) -> UIResult<&str>;
+    fn get_line_ref(&self, line_nr: usize) -> UIResult<&str>;
 
     fn line_len(&self, line_nr: usize) -> UIResult<usize>;
 
@@ -28,7 +26,7 @@ pub trait Lines {
 
     fn nr_of_chars(&self) -> usize;
 
-    fn all_lines<'a>(&self, arena: &'a Bump) -> BumpString<'a>;
+    fn all_lines_as_string(&self) -> String;
 
     fn is_last_line(&self, line_nr: usize) -> bool;
 
@@ -83,7 +81,7 @@ pub trait MutSelectableLines {
 
     fn insert_str(&mut self, new_str: &str) -> UIResult<()>;
 
-    fn pop_char(&mut self) -> UIResult<()>;
+    fn backspace(&mut self) -> UIResult<()>;
 
     fn del_selection(&mut self) -> UIResult<()>;
 }
@@ -114,7 +112,7 @@ pub fn move_caret_left<T: Lines>(
         } else {
             let curr_line_len = lines.line_len(old_line_nr - 1)?;
 
-            (old_line_nr - 1, curr_line_len - 1)
+            (old_line_nr - 1, curr_line_len)
         }
     } else {
         (old_line_nr, old_col_nr - 1)
@@ -185,7 +183,7 @@ pub fn move_caret_right<T: Lines>(
         let is_last_line = lines.is_last_line(old_line_nr);
 
         if !is_last_line {
-            if old_col_nr + 1 > curr_line_len - 1 {
+            if old_col_nr + 1 > curr_line_len {
                 (old_line_nr + 1, 0)
             } else {
                 (old_line_nr, old_col_nr + 1)
@@ -263,7 +261,9 @@ pub fn move_caret_up<T: Lines>(
         let prev_line_len = lines.line_len(old_line_nr - 1)?;
 
         if prev_line_len <= old_col_nr {
-            (old_line_nr - 1, prev_line_len - 1)
+            let new_column = if prev_line_len > 0 { prev_line_len } else { 0 };
+
+            (old_line_nr - 1, new_column)
         } else {
             (old_line_nr - 1, old_col_nr)
         }
@@ -331,7 +331,9 @@ pub fn move_caret_down<T: Lines>(
 
         if next_line_len <= old_col_nr {
             if !is_last_line {
-                (old_line_nr + 1, next_line_len - 1)
+                let new_column = if next_line_len > 0 { next_line_len } else { 0 };
+
+                (old_line_nr + 1, new_column)
             } else {
                 (old_line_nr + 1, next_line_len)
             }
@@ -382,7 +384,7 @@ pub fn move_caret_home<T: Lines>(
     let curr_line_nr = caret_w_select.caret_pos.line;
     let old_col_nr = caret_w_select.caret_pos.column;
 
-    let curr_line_str = lines.get_line(curr_line_nr)?;
+    let curr_line_str = lines.get_line_ref(curr_line_nr)?;
     let line_char_iter = curr_line_str.chars();
 
     let mut first_no_space_char_col = 0;

editor/src/ui/text/mod.rs

@@ -2,4 +2,5 @@ pub mod big_text_area;
 pub mod caret_w_select;
 pub mod lines;
 pub mod selection;
+mod text_buffer;
 pub mod text_pos;

editor/src/ui/text/text_buffer.rs

@@ -0,0 +1,193 @@
+use std::path::Path;
+
+use crate::ui::{
+    ui_error::{OutOfBounds, TextBufReadFailed, UIResult},
+    util::{path_to_string, reader_from_path},
+};
+use snafu::ensure;
+
+use super::{selection::Selection, text_pos::TextPos};
+use std::io::BufRead;
+
+// Do not use for large amounts of text.
+// This should become a trait in the future and be implemented by a SmallTextBuffer and Rope(for large amounts of text)
+#[derive(Debug)]
+pub struct TextBuffer {
+    pub lines: Vec<String>,
+}
+
+impl TextBuffer {
+    pub fn from_path(path: &Path) -> UIResult<Self> {
+        let buf_reader = reader_from_path(path)?;
+        let mut lines: Vec<String> = Vec::new();
+
+        for line in buf_reader.lines() {
+            match line {
+                Ok(line_str) => lines.push(line_str),
+                Err(e) => {
+                    TextBufReadFailed {
+                        path_str: path_to_string(path),
+                        err_msg: e.to_string(),
+                    }
+                    .fail()?;
+                }
+            }
+        }
+
+        Ok(TextBuffer { lines })
+    }
+
+    pub fn nr_of_chars(&self) -> usize {
+        let mut nr_of_chars = 0;
+
+        for line in self.lines.iter() {
+            nr_of_chars += line.len();
+        }
+
+        nr_of_chars
+    }
+
+    pub fn nr_of_lines(&self) -> usize {
+        self.lines.len()
+    }
+
+    pub fn get_line_ref(&self, line_nr: usize) -> UIResult<&str> {
+        self.ensure_bounds(line_nr)?;
+        // safe unwrap because we checked the length
+        Ok(self.lines.get(line_nr).unwrap())
+    }
+
+    pub fn line_len(&self, line_nr: usize) -> UIResult<usize> {
+        Ok(self.get_line_ref(line_nr)?.len())
+    }
+
+    fn ensure_bounds(&self, line_nr: usize) -> UIResult<()> {
+        ensure!(
+            line_nr < self.nr_of_lines(),
+            OutOfBounds {
+                index: line_nr,
+                collection_name: "TextBuffer",
+                len: self.nr_of_lines(),
+            }
+        );
+
+        Ok(())
+    }
+
+    fn ensure_bounds_txt_pos(&self, txt_pos: TextPos) -> UIResult<()> {
+        ensure!(
+            txt_pos.line < self.nr_of_lines(),
+            OutOfBounds {
+                index: txt_pos.line,
+                collection_name: "TextBuffer",
+                len: self.nr_of_lines(),
+            }
+        );
+
+        let line_ref = self.get_line_ref(txt_pos.line)?;
+        let line_len = line_ref.len();
+
+        ensure!(
+            txt_pos.column <= line_len,
+            OutOfBounds {
+                index: txt_pos.column,
+                collection_name: format!("Line in TextBuffer: {}", line_ref),
+                len: line_len,
+            }
+        );
+
+        Ok(())
+    }
+
+    pub fn all_lines_ref(&self) -> &[String] {
+        &self.lines
+    }
+
+    pub fn get_selected_str(&self, selection: Selection) -> UIResult<String> {
+        let start_line_nr = selection.start_pos.line;
+        let start_col_nr = selection.start_pos.column;
+
+        let end_line_nr = selection.end_pos.line;
+        let end_col_nr = selection.end_pos.column;
+
+        let mut selected_str = String::new();
+
+        if end_line_nr > start_line_nr {
+            selected_str.push_str(&self.get_line_ref(start_line_nr)?[start_col_nr..]);
+
+            for line_nr in start_line_nr + 1..end_line_nr - 1 {
+                selected_str.push_str(self.get_line_ref(line_nr)?);
+            }
+
+            selected_str.push_str(&self.get_line_ref(end_line_nr)?[..end_col_nr]);
+        } else {
+            // start_line_nr == end_line_nr
+            selected_str.push_str(&self.get_line_ref(start_line_nr)?[start_col_nr..end_col_nr]);
+        }
+
+        Ok(selected_str)
+    }
+
+    pub fn insert_str(&mut self, txt_pos: TextPos, new_str: &str) -> UIResult<()> {
+        self.ensure_bounds_txt_pos(txt_pos)?;
+
+        // safe unwrap because we checked the length
+        self.lines
+            .get_mut(txt_pos.line)
+            .unwrap()
+            .insert_str(txt_pos.column, new_str);
+
+        Ok(())
+    }
+
+    pub fn backspace_char(&mut self, txt_pos: TextPos) -> UIResult<()> {
+        if txt_pos.column > 0 {
+            let prev_col_pos = TextPos {
+                line: txt_pos.line,
+                column: txt_pos.column - 1,
+            };
+
+            self.ensure_bounds_txt_pos(prev_col_pos)?;
+
+            let line_ref = self.lines.get_mut(prev_col_pos.line).unwrap(); // safe because of earlier bounds check
+
+            line_ref.remove(prev_col_pos.column);
+        } else if txt_pos.line > 0 {
+            self.lines.remove(txt_pos.line);
+        }
+
+        Ok(())
+    }
+
+    pub fn del_selection(&mut self, selection: Selection) -> UIResult<()> {
+        self.ensure_bounds_txt_pos(selection.start_pos)?;
+        self.ensure_bounds_txt_pos(selection.end_pos)?;
+
+        let start_line_nr = selection.start_pos.line;
+        let start_col_nr = selection.start_pos.column;
+        let end_line_nr = selection.end_pos.line;
+        let end_col_nr = selection.end_pos.column;
+
+        if end_line_nr > start_line_nr {
+            // remove in reverse to prevent shifting indices
+            if end_col_nr == self.line_len(end_line_nr)? {
+                self.lines.remove(end_line_nr);
+            } else {
+                let line_ref = self.lines.get_mut(end_line_nr).unwrap(); // safe because of earlier bounds check
+                line_ref.replace_range(..end_col_nr, "");
+            }
+
+            self.lines.drain(start_line_nr + 1..end_line_nr);
+
+            let line_ref = self.lines.get_mut(start_line_nr).unwrap(); // safe because of earlier bounds check
+            line_ref.replace_range(start_col_nr.., "")
+        } else {
+            // selection.end_pos.line == selection.start_pos.line
+            let line_ref = self.lines.get_mut(selection.start_pos.line).unwrap(); // safe because of earlier bounds check
+
+            line_ref.replace_range(selection.start_pos.column..selection.end_pos.column, "")
+        }
+
+        Ok(())
+    }
+}

editor/src/ui/ui_error.rs

@@ -1,3 +1,5 @@
+use std::io;
+
 use snafu::{Backtrace, Snafu};
 
 //import errors as follows:
@@ -8,6 +10,16 @@ use snafu::{Backtrace, Snafu};
 #[derive(Debug, Snafu)]
 #[snafu(visibility(pub))]
 pub enum UIError {
+    #[snafu(display(
+        "LineInsertionFailed: line_nr ({}) needs to be <= nr_of_lines ({}).",
+        line_nr,
+        nr_of_lines
+    ))]
+    LineInsertionFailed {
+        line_nr: usize,
+        nr_of_lines: usize,
+        backtrace: Backtrace,
+    },
     #[snafu(display(
         "OutOfBounds: index {} was out of bounds for {} with length {}.",
         index,
@@ -34,6 +46,13 @@ pub enum UIError {
     ))]
     FileOpenFailed { path_str: String, err_msg: String },
 
+    #[snafu(display(
+        "FileWriteFailed: failed to write to file with path {}, I got this IO error: {}.",
+        path_str,
+        source
+    ))]
+    FileWriteFailed { source: io::Error, path_str: String },
+
     #[snafu(display("TextBufReadFailed: the file {} could be opened but we encountered the following error while trying to read it: {}.", path_str, err_msg))]
     TextBufReadFailed { path_str: String, err_msg: String },
 

editor/src/ui/util.rs

@@ -1,6 +1,6 @@
-use super::ui_error::{OutOfBounds, UIResult};
-use snafu::OptionExt;
-use std::slice::SliceIndex;
+use super::ui_error::{FileOpenFailed, FileWriteFailed, OutOfBounds, UIResult};
+use snafu::{OptionExt, ResultExt};
+use std::{fs::File, io::BufReader, path::Path, slice::SliceIndex};
 
 pub fn is_newline(char_ref: &char) -> bool {
     let newline_codes = vec!['\u{d}', '\n'];
@@ -33,3 +33,27 @@ pub fn slice_get_mut<T>(
 
     Ok(elt_ref)
 }
+
+pub fn reader_from_path(path: &Path) -> UIResult<BufReader<File>> {
+    match File::open(path) {
+        Ok(file) => Ok(BufReader::new(file)),
+        Err(e) => FileOpenFailed {
+            path_str: path_to_string(path),
+            err_msg: e.to_string(),
+        }
+        .fail()?,
+    }
+}
+
+pub fn path_to_string(path: &Path) -> String {
+    let mut path_str = String::new();
+    path_str.push_str(&path.to_string_lossy());
+
+    path_str
+}
+
+pub fn write_to_file(path: &Path, content: &str) -> UIResult<()> {
+    std::fs::write(path, content).with_context(|| FileWriteFailed {
+        path_str: path_to_string(path),
+    })
+}

editor/src/window/keyboard_input.rs

@@ -27,6 +27,17 @@ impl Modifiers {
 
         active
     }
+
+    // returns true if modifiers are active that can be active when the user wants to insert a new char; e.g.: shift+a to make A
+    pub fn new_char_modifiers(&self) -> bool {
+        self.no_modifiers()
+        || (self.shift && !self.ctrl && !self.alt && !self.logo) // e.g.: shift+a to make A 
+        || (self.cmd_or_ctrl() && self.alt) // e.g.: ctrl+alt+2 to make @ on azerty keyboard
+    }
+
+    fn no_modifiers(&self) -> bool {
+        !self.shift && !self.ctrl && !self.alt && !self.logo
+    }
 }
 
 pub fn no_mods() -> Modifiers {

editor/tests/README.md

@@ -0,0 +1,5 @@
+
+# Where are the tests?
+
+We have a lot of tests at the end of source files, this allows us to test functions that are not exposed by the editor itself.
+`editor/mvc/ed_update.rs` and `editor/ui/text/big_text_area.rs` have many important tests.

editor/tests/selection.pest

@@ -1,4 +1,4 @@
-text = { (ASCII_ALPHANUMERIC | " " | "\t" | "\n" | "{" | "}" | "," | "." | "[" | "]" | ":" | "<" | ">" | "-" | "\"" )* }
+text = { (ASCII_ALPHANUMERIC | " " | "\t" | "\n" | "{" | "}" | "," | "." | "[" | "]" | ":" | "<" | ">" | "-" | "\"" | "=" )* }
 
 caret = {"┃"}