Missing space after tilde `~` [GHC-27207]

The ~ character can be used for multiple purposes:

• For type equality in a function signature: a ~ b
• As a binary operator: (~) :: a -> b -> c
• For defining non-strict fields in the presence of the StrictData extension: data Foo = MkFoo Int ~Char
• For making irrefutable pattern matches lazy again in the presence of the Strict extension: f ~x = 5
• As a lazy pattern to turn refutable patterns into irrefutable patterns: case undefined of ~(a,b) -> length [a,b]

If no space is placed between ~ and the expression that follows it, this is interpreted as the last bullet; in expression contexts, the lazy pattern is not allowed. The most likely case is that you wish to use ~ as a binary operator, in which case a space is needed following it.

Examples

MissingSpaceLazyPattern.hs:11:9: error:
    Lazy pattern in expression context: ~1
    Did you mean to add a space after the '~'?
   |
11 | f x = x ~1
   |         ^^

MissingSpaceLazyPattern.hs

module MissingSpaceLazyPattern where

(~) :: Int -> Int -> Bool
(~) a b = a == b

infixl 5 ~

f :: Int -> Bool
f x = x ~1

module MissingSpaceLazyPattern where

(~) :: Int -> Int -> Bool
(~) a b = a == b

infixl 5 ~

f :: Int -> Bool
f x = x ~ 1