Better syntax errors for [| and |>

CHANGELOG.md

@@ -28,6 +28,8 @@
 
 #### :nail_care: Polish
 
+- Dedicated error messages for old Reason array literal syntax (`[|` and `|]`), and for the old pipe (`|>`). Primarly intended to help LLMs that might try to use old code patterns. https://github.com/rescript-lang/rescript/pull/8010
+
 #### :house: Internal
 
 - Rename Core to Stdlib in tests/tests. https://github.com/rescript-lang/rescript/pull/8005

compiler/syntax/src/res_diagnostics.ml

@@ -137,12 +137,43 @@ let print_report ?(custom_intro = None) ?(formatter = Format.err_formatter)
  match diagnostics with
  | [] -> ()
  | d :: rest ->
+ (* A few specializations for best-effort error messages for old syntax etc. *)
+ let msg =
+ match d.category with
+ | Unexpected {token = Token.Bar; _} ->
+ let idx_prev = d.start_pos.pos_cnum - 1 in
+ let idx_next = d.end_pos.pos_cnum in
+ if
+ idx_prev >= 0
+ && idx_prev < String.length src
+ && String.get src idx_prev = '['
+ then
+ let base = explain d in
+ base
+ ^ "\n\n\
+ \ Did you mean to write an array literal? Arrays are written \
+ with `[ ... ]` (not `[| ... |]`)."
+ ^ "\n Quick fix: replace `[|` with `[` and `|]` with `]`."
+ ^ "\n Example: `[|1, 2, 3|]` -> `[1, 2, 3]`"
+ else if
+ idx_next >= 0
+ && idx_next < String.length src
+ && String.get src idx_next = '>'
+ then
+ let base = explain d in
+ base
+ ^ "\n\n\
+ \ The old data-last pipe `|>` has been removed from the language.\n\
+ \ Use a data first `->` pipe instead."
+ else explain d
+ | _ -> explain d
+ in
  Location.report_error ~custom_intro ~src:(Some src) formatter
  Location.
  {
  loc =
  {loc_start = d.start_pos; loc_end = d.end_pos; loc_ghost = false};
- msg = explain d;
+ msg;
  sub = [];
  if_highlight = "";
  };

tests/syntax_tests/data/parsing/errors/expressions/expected/oldArraySyntax.res.txt

@@ -0,0 +1,16 @@
+
+ Syntax error!
+ syntax_tests/data/parsing/errors/expressions/oldArraySyntax.res:2:10
+
+ 1 │ // Old Reason array literal
+ 2 │ let s = [|1, 2, 3|]
+ 3 │ 
+ 4 │ 
+
+ I'm not sure what to parse here when looking at "|".
+
+ Did you mean to write an array literal? Arrays are written with `[ ... ]` (not `[| ... |]`).
+ Quick fix: replace `[|` with `[` and `|]` with `]`.
+ Example: `[|1, 2, 3|]` -> `[1, 2, 3]`
+
+let s = [|1;2;3|]

tests/syntax_tests/data/parsing/errors/expressions/expected/oldDataLastPipe.res.txt

@@ -0,0 +1,16 @@
+
+ Syntax error!
+ syntax_tests/data/parsing/errors/expressions/oldDataLastPipe.res:2:16
+
+ 1 │ // Old data-last pipe
+ 2 │ let x = f => f |> String.length
+ 3 │ 
+ 4 │ 
+
+ I'm not sure what to parse here when looking at "|".
+
+ The old data-last pipe `|>` has been removed from the language.
+ Use a data first `->` pipe instead.
+
+let x [arity:1]f = f
+;;String.length

tests/syntax_tests/data/parsing/errors/expressions/oldArraySyntax.res

@@ -0,0 +1,3 @@
+// Old Reason array literal
+let s = [|1, 2, 3|]
+

tests/syntax_tests/data/parsing/errors/expressions/oldDataLastPipe.res

@@ -0,0 +1,3 @@
+// Old data-last pipe
+let x = f => f |> String.length
+