Missing LambdaCase [GHC-51179]

Language extension: LambdaCase

The Lambda Case extension enables a syntactic sugar for a lambda function that pattern matches its argument immediately. Using LambdaCase,

\case {"1" -> 1; "2" -> 2}

is equivalent to

\x -> case x of {"1" -> 1; "2" -> 2}

When the extension is missing, the expression is interpreted as a regular lambda expression that is incorrectly formatted.

Examples

Missing LambdaCase language extension

Cannot use lambda case expressions - LambdaCase language extension is missing.

error: [GHC-51179]
    Illegal \case
    Suggested fix: Perhaps you intended to use LambdaCase
  |
3 | f = \case 
  |      ^^^^

ErrLambdaCase.hs

Before

module ErrLambdaCase where

f = \case 
      "1" -> 1
      "2" -> 2
      _   -> 3

After

{-# LANGUAGE LambdaCase #-}
module ErrLambdaCase where

f = \case 
      "1" -> 1
      "2" -> 2
      _   -> 3