#7264: Adding GHC's inferred type signatures to a working program can make it
fail
with Rank2Types
------------------------------+---------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
{{{
{-# LANGUAGE Rank2Types #-}
module Test where
data Foo = Foo { unFoo :: forall r . (RealFrac r) => r -> String }
mkFoo1 :: (forall r. RealFrac r => r -> String) -> Maybe Foo
mkFoo1 val = Just $ Foo val
--mkFoo2 :: (forall r. RealFrac r => r -> String) -> Maybe Foo
mkFoo2 val = Foo `fmap` Just val
}}}
Without the commented-out type signature, the program typechecks without
complaint. With it, the program fails to compile with the error:
{{{
Test.hs:10:30:
Couldn't match expected type `forall r. RealFrac r => r -> String'
with actual type `r0 -> String'
In the first argument of `Just', namely `val'
In the second argument of `fmap', namely `Just val'
In the expression: Foo `fmap` Just val
}}}
Note that the commented-out type is exactly what's inferred by GHC.
Also note that (with ImpredicativeTypes also enabled), this is fine (but
mkFoo2 still fails if its type signature is included):
{{{
mkFoo3 :: (forall r. RealFrac r => r -> String) -> Maybe Foo
mkFoo3 val = Foo `fmap` (Just :: (forall s. RealFrac s => s -> String) ->
(Maybe (forall s. RealFrac s => s -> String))) val
}}}
It's confusing that adding a valid, GHC-inferred type signature to a
program can break it, and the error message leaves something to be
desired.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7264>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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