Improve use error messages with the expanded code

[sobolevn]

use is a rather new concept in programming languages. It is rather hard to understand.

When getting an error, it is not really clear how to solve it.

Example of an invalid code:

fn my_function(callback) {
  callback()  # <- should be `callback(1, 2)`
}

fn next(obj) {
  echo obj
}

pub fn main() {
  use a, b <- my_function
  next(a)
  next(b)
}

Produces:

error: Incorrect arity
   ┌─ /src/main.gleam:11:15
   │
11 │   use a, b <- my_function
   │       ----    ^^^^^^^^^^^
   │       │        
   │       Expected no arguments, got 2

This function takes a callback that expects no arguments. But 2 were
provided on the left hand side of `<-`.

See: https://tour.gleam.run/advanced-features/use/

It would be awesome to see this:

error: Incorrect arity
   ┌─ /src/main.gleam:11:15
   │
11 │   use a, b <- my_function
   │       ----    ^^^^^^^^^^^
   │       │        
   │       Expected no arguments, got 2

This function takes a callback that expects no arguments. But 2 were
provided on the left hand side of `<-`.

Current expanded version:

my_function(fn (a, b) {
  next(a)
  next(b)
})

See: https://tour.gleam.run/advanced-features/use/

How hard would it be? What do others think - is it useful?

[lpil]

It doesn't look so bad with this trivial example (if a bit verbose), but in real code it could produce hundreds of lines of code, resulting in an impossible to read error message. How might we deal with that?

[ankddev]

Maybe we can do collapse some lines like in inexhaustive pattern error:

error: Inexhaustive patterns
   ┌─ C:\Users\user\projects\test_gleam\src\test_gleam.gleam:6:3
   │
 6 │ ╭   case str {
 7 │ │     "hello" -> todo
 8 │ │     "hello, " <> name -> todo
 9 │ │     "buy" -> todo
   · │
16 │ │     "yesterday" -> todo
17 │ │   }
   │ ╰───^

This case expression does not have a pattern for all possible values. If it
is run on one of the values without a pattern then it will crash.

The missing patterns are:

    _

Or, how Rust sows error message for both mutable and immutable borrow:

error[E0502]: cannot borrow `string` as immutable because it is also borrowed as mutable
  --> src\main.rs:6:20
   |
4  |     let mut second_string = &mut string;
   |                             ----------- mutable borrow occurs here
5  |
6  |     println!("{}", string);
   |                    ^^^^^^ immutable borrow occurs here
...
23 |     println!("{}", second_string);
   |                    ------------- mutable borrow later used here
   |
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

So, error message for your sample code may be something like this:

error: Incorrect arity
   ┌─ /src/main.gleam:11:15
   │
11 │   use a, b <- my_function
   │       ----    ^^^^^^^^^^^
   │       │        
   │       Expected no arguments, got 2

This function takes a callback that expects no arguments. But 2 were
provided on the left hand side of `<-`.

Current expanded version:

		my_function(fn (a, b) {
		  next(a)
		  next(b)
		  next(c)
		  next(d)
		  next(e)
		  ...
		  next(f)
		})

See: https://tour.gleam.run/advanced-features/use/

The main concern here is how we should display skipped lines? Maybe we also want to show line numbers here?