#7268: Explicit type signatures for top level record pattern matches
polymorphism
fail
---------------------------------------+------------------------------------
Reporter: TristanAllwood | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type
checker)
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------------+------------------------------------
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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7268>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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