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Consider the following module, which declares a record with two fields (X), an instance of this record (x), and then (at the top level) pattern matches the fields of x to create two new functions.

{-# LANGUAGE RankNTypes #-}
module Foo where

data X
  = X { a :: forall a . a -> a , b :: Int -> Int }

x :: X
x = X { a = id, b = succ }

succ' :: Int -> Int

X { a = id', b = succ' } = x

If we ask ghci what the type of id' is, we get told:

:t id'
id' :: a -> a

However adding this type explicitly, gives the following:

Foo.hs:15:9:
    Couldn't match expected type `forall a. a -> a'
                with actual type `a -> a'
    In the pattern: X {a = id', b = succ'}
    In a pattern binding: X {a = id', b = succ'} = x
Failed, modules loaded: none.

This can happen in practice when using (e.g.) Parsec's TokenParser.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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