allow more whitespace inconsistencies in case statements

[avh4]

Elm itself requires the following of case branches:

• all branches have the same indentation
• the indentation of branches is greater than the indentation of case

Ideally, elm-format should relax both of those requirements as much as possible (while maintaining good parsing performance for already-formatted code, and having decent parser performance for lenient code), allowing:

f x =
  case x of
      _ -> ()
          _ -> {- different indentation -} ()

f x =
  case x of
 _ -> {- indentation less than `case` -} ()

This one might be harder to do safely (performantly), as "fresh lines" (newline with no trailing whitespace) are handled specially when parsing whitespace:

f x =
  case x of
_ -> {- branch starts after a fresh line -} ()

Ambiguity

When case statements are nested, relaxing these rules can result in ambiguity.

f x =
    case x of
        Just y ->
            case y of
                _ -> ()
           _ -> {- which case is this attached to? -} ()

f x =
    case x of
        Just y ->
            case y of
                _ -> ()
            _ -> {- which case is this attached to? -} ()

f x =
    case x of
        Just y -> case y of
                _ -> ()
            _ -> {- which case is this attached to? -} ()

f x =
    case x of
        Just y -> case y of
                _ -> ()
        _ -> {- which case is this attached to? -} ()

... other examples?

Open questions

• [ ] are there other possible parser ambiguities that could result from relaxing these rules?
  • other syntax that uses ->
    • [ ] lambda functions
• [ ] how does this interact with other whitespace-sensitive syntax?
  • [ ] let expressions
  • things that response to "fresh lines"
    • [ ] top-level definitions

[avh4]

Here's an example of an ambiguity that would be introduced by relaxing the whitespace requirements:

case x of
    Nothing -> f
  Just
    y -> ()

does this mean Nothing -> f Just; y -> (), or does it mean Nothing -> f; Just y -> () ?

[avh4]

use cases via lydell:

• Pasting (parts of) a case expression into another, and ending up with messed up code
• Adding a new branch and accidentally adding one space too much or too little