Implement 'roc format <dir_or_files>' command

cli/src/format.rs

@@ -0,0 +1,39 @@
+use std::path::PathBuf;
+
+use bumpalo::collections::String;
+use bumpalo::Bump;
+use roc_fmt::def::fmt_def;
+use roc_fmt::module::fmt_module;
+use roc_parse::{
+    module::{self, module_defs},
+    parser::{Parser, State},
+};
+use roc_reporting::user_error;
+
+pub fn format(files: Vec<PathBuf>) {
+    for file in files {
+        let arena = Bump::new();
+
+        let src = std::fs::read_to_string(&file).unwrap();
+
+        match module::parse_header(&arena, State::new(src.as_bytes())) {
+            Ok((result, state)) => {
+                let mut buf = String::new_in(&arena);
+
+                fmt_module(&mut buf, &result);
+
+                match module_defs().parse(&arena, state) {
+                    Ok((_, loc_defs, _)) => {
+                        for loc_def in loc_defs {
+                            fmt_def(&mut buf, arena.alloc(loc_def.value), 0);
+                        }
+                    }
+                    Err(error) => user_error!("Unexpected parse failure when parsing this for defs formatting:\n\n{:?}\n\nParse error was:\n\n{:?}\n\n", src, error)
+                }
+
+                std::fs::write(&file, buf).unwrap();
+            }
+            Err(error) => user_error!("Unexpected parse failure when parsing this for module header formatting:\n\n{:?}\n\nParse error was:\n\n{:?}\n\n", src, error)
+        };
+    }
+}