todo-better-errors.md

Error messages that could be better

Expected to see type constructor

Problem: The parser sees (..., ...) as the beginning of a type, and thinks that there is a missing type constructor. It's likely however that the programmer forgot to use product syntax (with *) for tuple types.

Ideal Solution: Highlight the (..., ...) with an appropriate error message. Something like: "Invalid or incomplete type". Explain the * syntax in below the source reference.

Unexpected token.

foo.sml
  | 
1 | val x: (int, int) = (42, 15210)
  |                   ^

Expected to see a type constructor.

Equal-sign looks like identifier

Problem: The parser tries to use '=' as the name of a variable, but gets mad because '=' is infixed but does not appear in infix position. Perhaps another reasonable error would be to say that '=' cannot be rebound.

Ideal Solution: Say something like "missing variable or pattern binder" and point to the whitespace, if possible.

Infix identifier not prefaced by 'op'

input.sml
  |
1 | val = 5
  |     ^

Double comma/semicolon

Problem: A common mistake when writing lists or tuples or sequences of expressions is having two commas/semicolons in a row. The parser currently doesn't do anything to detect this.

Ideal Solution: Explicitly say "two X in a row" and point to both.

Unexpected token.

test.sml
  |
3 |   , foo
  |   ^

Expected beginning of expression.

Extra comma/semicolon at end

Problem: Similar to the double comma/semicolon problem, it's common to accidentally put a superfluous comma or semicolon at the end of a tuple/list/sequence.

Ideal Solution: Explicitly point to the extra comma/semicolon.

Unexpected token.

test.sml
  |
1 | val x = (a, b, c, )
  |                   ^

Expected beginning of expression.