"Extract Variable" doesn't properly work with use and case

[GearsDatapacks]

The "extract variable" code action is offered in a couple of places where it shouldn't be. Inside use callbacks:

import gleam/result

pub fn main() {
  use x <- result.try(todo)
  Ok(x + 1)
}

Which transforms into:

import gleam/result

pub fn main() {
  let value = Ok(x + 1)
  use x <- result.try(todo)
  value
}

And also inside a case body:

pub fn main(result: Result(Int, String)) {
  case result {
    Ok(value) -> value + 1
    Error(_) -> panic
  }
}

Which becomes:

pub fn main(result: Result(Int, String)) {
  let int = value + 1
  case result {
    Ok(value) -> int
    Error(_) -> panic
  }
}

In both of these cases, a variable is referenced outside of the scope which it is created in, producing invalid Gleam code.

[lpil]

Oh dear! Thank you

[matiascr]

I would like to take a look at this. I've also noticed that it's reproducible in blocks.

[lpil]

Thank you

[matiascr]

I'm going to assume that (if the code action is allowed in this position) this selection

pub fn main(result: Result(Int, String)) {
  case result {
    Ok(value) -> value + 1
                 ^^^^^^^^^
    Error(_) -> panic
  }
}

Should extract variable in the following way

pub fn main(result: Result(Int, String)) {
  case result {
    Ok(value) -> {
      let int = value + 1
      int
    }
    Error(_) -> panic
  }
}

Currently, "extract variable" is not allowed top level statements. Perhaps the first statemet in a block (or the right hand side of the case clause) should also be considered a top level statement, and thus, not get extracted.

Does it still makes sense to offer the code action that creates the block (i. e. to start a refactor)? If we allow that, maybe creating an "add braces to arm expression" like the one in Rust is also a possibility.

[lpil]

That would be a good code action to have, please open an issue for it

[matiascr]

Then, should we still offer the code action for these example?

pub fn main(result) {
  case result {
    Ok(value) -> value + 1
                 ^^^^^^^^^
    Error(_) -> panic
  }
}

I ask, because the code action is not offered (on purpose) when selecting the whole right-hand side of the assignment in such cases for assignments, like in

pub fn main() {
  let v = value + 1
          ^^^^^^^^^
  let w = 1
          ^
  v + w
}

I would suggest that this selection should not be allowed inside case clauses either when selecting the whole expression

case result {
  Ok(value) -> value + 1
               ^^^^^^^^^
  Error(_) -> panic
}

But the following should (and wrap it in the block and all that)

case result {
  Ok(value) -> 2 * value + 1
                   ^^^^^^^^^
  Error(_) -> panic
}

[lpil]

I think the most useful thing would be to always offer it, and to wrap the expression in a block if-need-be.