Show helpful error message when calling a function in a constant value

[nino]

This should resolve #2983.

Attempting to call a function in a constant definition will now show a more helpful error message:

error: Syntax error
  ┌─ /src/parse/error.gleam:3:18
  │
3 │ const wib: Int = wibble(1, "wobble")
  │                  ^^^^^^^ Functions can only be called within other functions

error: Syntax error
  ┌─ /src/one/two.gleam:1:15
  │
1 │ const first = list.at([1], 0)
  │               ^^^^^^^^ Functions can only be called within other functions

(the second test is at compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__errors__functions_called_outside_module.snap, which already existed)

[lpil]

Thank you. Could you add tests for calling functions from other modules please? For example list.map(items, to_string).

[nino]

@lpil Turns out, there already was such a test case. I've updated the code to make the error message look the same whether or not the function is from another module. (i.e., the error highlights the whole function call from beginning of name to end of arguments)

[lpil]

I'm a bit unsure about using the expression function args parsing method here as it emits errors relating to expressions, not constants. This will be inaccurate at times.

Previously we stopped when we saw a ( as this is enough to know that it's a function call. How come you decided to move away from that here?

[nino]

@lpil

as it emits errors relating to expressions, not constants

That's a good point. I'd decided to parse the function params because that would make the error message nicer, by highlighting the whole function call, rather than just the name and opening paren. (Also hadn't realized that the parser already checks for ( after imported identifiers (const a = b.c() already showed a reasonable error).)

I guess, to get the error message to highlight the whole function call, we'd have to wrap parse_fn_args in some custom error handling logic or something. Which is probably overkill.

I've now changed the code for const a = b() to match the existing code for const a = b.c(). Now the error message only highlights up to the opening paren, but it won't show wrong error messages inside the parens.

[lpil]

Ah! OK cool that sounds great.

Thank you once again! This should help people a lot 💜