#1549: ghc derives different type signature for equal functions with overlapping
instances
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Reporter: int-e | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.7
Severity: normal | Keywords:
Difficulty: Unknown | Os: Linux
Testcase: | Architecture: x86
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The code below derives two different type signatures for the same function
in two different modules.
One of these modules defines a new instance for the `Monad` class. What's
interesting is that this causes the type checker to derive a more general
type than before. I think that's a bug.
Another interesting point is that the code below works without -fallow-
overlapping-instances,
but if the instances from `A.hs` and from `B.hs` are combined in a single
file, the compiler complains very loudly. The derived types are unaffected
by `-fallow-overlapping-instances`.
{{{
==> A.hs <==
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
module A (MaybeT (..), module Control.Monad) where
import Control.Monad
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
instance Monad m => Monad (MaybeT m)
==> B.hs <==
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
module B (qqq) where
import A
data Foo a = Foo
instance Monad (MaybeT Foo) where
qqq _ = runMaybeT (runMaybeT (return 1) >> return 2)
==> C.hs <==
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
module C (rrr) where
import A
rrr _ = runMaybeT (runMaybeT (return 1) >> return 2)
==> D.hs <==
module D (qqq, rrr) where
import B
import C
{-
# ghci -fallow-overlapping-instances D.hs
GHCi, version 6.7.20070716: http://www.haskell.org/ghc/ :? for help
(...)
Ok, modules loaded: C, D, A, B.
*D> :t qqq
qqq :: (Monad (A.MaybeT m), Num t1) => t -> m (Maybe t1)
*D> :t rrr
rrr :: (Monad m, Num t1) => t -> m (Maybe t1)
*D> :i MaybeT
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
-- Defined at A.hs:6:8-13
instance [overlap ok] (Monad m) => Monad (MaybeT m)
-- Defined at A.hs:8:0-35
-}
}}}
I think that the type derived for `qqq` is correct in the presence of
overlapping instances.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1549>
GHC <http://www.haskell.org/ghc/>
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