if guard empty condition message

compiler/parse/src/expr.rs

@@ -2,8 +2,8 @@ use crate::ast::{
     AssignedField, Attempting, CommentOrNewline, Def, Expr, Pattern, Spaceable, TypeAnnotation,
 };
 use crate::blankspace::{
-    line_comment, space0, space0_after, space0_around, space0_before, space0_before_e, space1,
-    space1_around, space1_before, spaces_exactly,
+    line_comment, space0, space0_after, space0_around, space0_around_e, space0_before,
+    space0_before_e, space1, space1_around, space1_before, spaces_exactly,
 };
 use crate::ident::{global_tag_or_ident, ident, lowercase_ident, Ident};
 use crate::keyword;
@@ -11,10 +11,10 @@ use crate::number_literal::number_literal;
 use crate::parser::{
     self, allocated, and_then_with_indent_level, ascii_char, ascii_string, attempt, backtrackable,
     fail, map, newline_char, not, not_followed_by, optional, sep_by1, specialize, specialize_ref,
-    then, unexpected, unexpected_eof, word2, EExpr, Either, ParseResult, Parser, State,
+    then, unexpected, unexpected_eof, word1, word2, EExpr, Either, ParseResult, Parser, State,
     SyntaxError, When,
 };
-use crate::pattern::{loc_closure_param, loc_pattern};
+use crate::pattern::loc_closure_param;
 use crate::type_annotation;
 use bumpalo::collections::string::String;
 use bumpalo::collections::Vec;
@@ -1150,19 +1150,43 @@ mod when {
         min_indent: u16,
     ) -> impl Parser<'a, (Vec<'a, Located<Pattern<'a>>>, Option<Located<Expr<'a>>>), SyntaxError<'a>>
     {
+        specialize(
+            |e, r, c| SyntaxError::Expr(EExpr::When(e, r, c)),
+            branch_alternatives_help(min_indent),
+        )
+    }
+
+    fn branch_alternatives_help<'a>(
+        min_indent: u16,
+    ) -> impl Parser<'a, (Vec<'a, Located<Pattern<'a>>>, Option<Located<Expr<'a>>>), When<'a>> {
         and!(
             sep_by1(
-                ascii_char(b'|'),
-                space0_around(loc_pattern(min_indent), min_indent),
+                word1(b'|', When::Bar),
+                space0_around_e(
+                    specialize(When::Pattern, crate::pattern::loc_pattern_help(min_indent)),
+                    min_indent,
+                    When::Space,
+                    When::IndentPattern
                 ),
-            optional(skip_first!(
-                parser::keyword(keyword::IF, min_indent),
+            ),
+            one_of![
+                map!(
+                    skip_first!(
+                        parser::keyword_e(keyword::IF, When::IfToken),
                         // TODO we should require space before the expression but not after
-                space1_around(
-                    loc!(move |arena, state| parse_expr(min_indent, arena, state)),
-                    min_indent
+                        space0_around_e(
+                            loc!(specialize_ref(When::IfGuard, move |arena, state| {
+                                parse_expr(min_indent, arena, state)
+                            })),
+                            min_indent,
+                            When::Space,
+                            When::IndentIfGuard,
                         )
-            ))
+                    ),
+                    Some
+                ),
+                |_, s| Ok((NoProgress, None, s))
+            ]
         )
     }
 

compiler/parse/src/parser.rs

@@ -390,6 +390,10 @@ pub enum When<'a> {
     Is(Row, Col),
     Pattern(EPattern<'a>, Row, Col),
     Arrow(Row, Col),
+    Bar(Row, Col),
+    IfToken(Row, Col),
+    // TODO make EExpr
+    IfGuard(&'a SyntaxError<'a>, Row, Col),
     Condition(&'a EExpr<'a>, Row, Col),
     Branch(&'a EExpr<'a>, Row, Col),
     Syntax(&'a SyntaxError<'a>, Row, Col),
@@ -399,6 +403,7 @@ pub enum When<'a> {
     IndentPattern(Row, Col),
     IndentArrow(Row, Col),
     IndentBranch(Row, Col),
+    IndentIfGuard(Row, Col),
     PatternAlignment(u16, Row, Col),
 }
 
@@ -967,16 +972,17 @@ pub fn keyword<'a>(
     }
 }
 
-pub fn keyword_e<'a, E>(keyword: &'static str, if_error: E) -> impl Parser<'a, (), E>
+pub fn keyword_e<'a, ToError, E>(keyword: &'static str, if_error: ToError) -> impl Parser<'a, (), E>
 where
-    E: 'a + Clone,
+    ToError: Fn(Row, Col) -> E,
+    E: 'a,
 {
     move |arena, state: State<'a>| {
         let initial_state = state.clone();
         // first parse the keyword characters
         let (_, _, after_keyword_state) = ascii_string(keyword)
             .parse(arena, state)
-            .map_err(|(_, _, state)| (NoProgress, if_error.clone(), state))?;
+            .map_err(|(_, _, state)| (NoProgress, if_error(state.line, state.column), state))?;
 
         // then we must have at least one space character
         // TODO this is potentially wasteful if there are a lot of spaces
@@ -990,7 +996,11 @@ where
                 // this is not a keyword, maybe it's `whence` or `iffy`
                 // anyway, make no progress and return the initial state
                 // so we can try something else
-                Err((NoProgress, if_error.clone(), initial_state))
+                Err((
+                    NoProgress,
+                    if_error(initial_state.line, initial_state.column),
+                    initial_state,
+                ))
             }
         }
     }

compiler/parse/src/pattern.rs

@@ -62,7 +62,9 @@ pub fn loc_pattern<'a>(min_indent: u16) -> impl Parser<'a, Located<Pattern<'a>>,
     )
 }
 
-fn loc_pattern_help<'a>(min_indent: u16) -> impl Parser<'a, Located<Pattern<'a>>, EPattern<'a>> {
+pub fn loc_pattern_help<'a>(
+    min_indent: u16,
+) -> impl Parser<'a, Located<Pattern<'a>>, EPattern<'a>> {
     one_of!(
         specialize(EPattern::PInParens, loc_pattern_in_parens_help(min_indent)),
         loc!(underscore_pattern_help()),

compiler/parse/src/type_annotation.rs

@@ -66,7 +66,7 @@ fn term<'a>(min_indent: u16) -> impl Parser<'a, Located<TypeAnnotation<'a>>, Typ
                     and!(
                         skip_second!(
                             backtrackable(space0_e(min_indent, Type::TSpace, Type::TIndentEnd)),
-                            crate::parser::keyword_e(keyword::AS, Type::TEnd(0, 0))
+                            crate::parser::keyword_e(keyword::AS, Type::TEnd)
                         ),
                         space0_before_e(
                             term(min_indent),

compiler/reporting/src/error/parse.rs

@@ -145,6 +145,59 @@ fn to_syntax_report<'a>(
         }
         Type(typ) => to_type_report(alloc, filename, &typ, 0, 0),
         Pattern(pat) => to_pattern_report(alloc, filename, &pat, 0, 0),
+        Expr(expr) => to_expr_report(alloc, filename, &expr, 0, 0),
+        _ => todo!("unhandled parse error: {:?}", parse_problem),
+    }
+}
+
+fn to_expr_report<'a>(
+    alloc: &'a RocDocAllocator<'a>,
+    filename: PathBuf,
+    parse_problem: &roc_parse::parser::EExpr<'a>,
+    _start_row: Row,
+    _start_col: Col,
+) -> Report<'a> {
+    use roc_parse::parser::EExpr;
+
+    match parse_problem {
+        EExpr::When(when, row, col) => to_when_report(alloc, filename, &when, *row, *col),
+        _ => todo!("unhandled parse error: {:?}", parse_problem),
+    }
+}
+
+fn to_when_report<'a>(
+    alloc: &'a RocDocAllocator<'a>,
+    filename: PathBuf,
+    parse_problem: &roc_parse::parser::When<'a>,
+    start_row: Row,
+    start_col: Col,
+) -> Report<'a> {
+    use roc_parse::parser::When;
+
+    match *parse_problem {
+        When::IfGuard(nested, row, col) => match what_is_next(alloc.src_lines, row, col) {
+            Next::Token("->") => {
+                let surroundings = Region::from_rows_cols(start_row, start_col, row, col);
+                let region = Region::from_row_col(row, col);
+
+                let doc = alloc.stack(vec![
+                    alloc.reflow(
+                        r"I just started parsing an if guard, but there is no guard condition:",
+                    ),
+                    alloc.region_with_subregion(surroundings, region),
+                    alloc.concat(vec![
+                        alloc.reflow("Try adding an expression before the arrow!")
+                    ]),
+                ]);
+
+                Report {
+                    filename,
+                    doc,
+                    title: "IF GUARD NO CONDITION".to_string(),
+                }
+            }
+            _ => to_syntax_report(alloc, filename, nested, row, col),
+        },
         _ => todo!("unhandled parse error: {:?}", parse_problem),
     }
 }
@@ -1289,6 +1342,7 @@ enum Next<'a> {
     Keyword(&'a str),
     // Operator(&'a str),
     Close(&'a str, char),
+    Token(&'a str),
     Other(Option<char>),
 }
 
@@ -1299,18 +1353,20 @@ fn what_is_next<'a>(source_lines: &'a [&'a str], row: Row, col: Col) -> Next<'a>
         None => Next::Other(None),
         Some(line) => {
             let chars = &line[col_index..];
+            let mut it = chars.chars();
 
             match roc_parse::keyword::KEYWORDS
                 .iter()
                 .find(|keyword| starts_with_keyword(chars, keyword))
             {
                 Some(keyword) => Next::Keyword(keyword),
-                None => match chars.chars().next() {
+                None => match it.next() {
                     None => Next::Other(None),
                     Some(c) => match c {
                         ')' => Next::Close("parenthesis", ')'),
                         ']' => Next::Close("square bracket", ']'),
                         '}' => Next::Close("curly brace", '}'),
+                        '-' if it.next() == Some('>') => Next::Token("->"),
                         // _ if is_symbol(c) => todo!("it's an operator"),
                         _ => Next::Other(Some(c)),
                     },
@@ -1331,6 +1387,10 @@ fn starts_with_keyword(rest_of_line: &str, keyword: &str) -> bool {
     }
 }
 
+fn next_is_arrow(rest_of_line: &str) -> bool {
+    rest_of_line.starts_with("->")
+}
+
 fn next_line_starts_with_close_curly(source_lines: &[&str], row: Row) -> Option<(Row, Col)> {
     match source_lines.get(row as usize + 1) {
         None => None,

compiler/reporting/tests/test_reporting.rs

@@ -4790,12 +4790,15 @@ mod test_reporting {
             ),
             indoc!(
                 r#"
-                ── PARSE PROBLEM ───────────────────────────────────────────────────────────────
+                ── IF GUARD NO CONDITION ───────────────────────────────────────────────────────
                 
-                Unexpected token :
+                I just started parsing an if guard, but there is no guard condition:
                 
+                1│  when Just 4 is
                 2│      Just if ->
                                 ^
+                
+                Try adding an expression before the arrow!
             "#
             ),
         )