#4952: Typechecking regression
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Reporter: igloo | Owner:
Type: bug | Status: new
Priority: highest | Milestone: 7.0.2
Component: Compiler (Type checker) | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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7.0.1 accepts this module (cut down from hashed-storage-0.5.4):
{{{
{-# LANGUAGE UndecidableInstances,
MultiParamTypeClasses,
KindSignatures,
FlexibleInstances,
FunctionalDependencies #-}
module Storage.Hashed.Monad () where
class Monad m => TreeRO m where
withDirectory :: (MonadError e m) => Int -> m a -> m a
expandTo :: (MonadError e m) => Int -> m Int
instance (Monad m, MonadError e m) => TreeRO (M m) where
expandTo = undefined
withDirectory dir _ = do
_ <- expandTo dir
undefined
data M (m :: * -> *) a
instance Monad m => Monad (M m) where
(>>=) = undefined
return = undefined
instance MonadError e m => MonadError e (M m)
class Monad m => MonadError e m | m -> e
}}}
but 7.0 branch says:
{{{
[1 of 1] Compiling Storage.Hashed.Monad ( Storage/Hashed/Monad.hs,
interpreted )
Storage/Hashed/Monad.hs:17:12:
Could not deduce (MonadError e1 m) arising from a use of `expandTo'
from the context (Monad m, MonadError e m)
bound by the instance declaration
at Storage/Hashed/Monad.hs:14:10-50
or from (MonadError e1 (M m))
bound by the type signature for
withDirectory :: MonadError e1 (M m) => Int -> M m a -> M
m a
at Storage/Hashed/Monad.hs:(16,5)-(18,15)
Possible fix:
add (MonadError e1 m) to the context of
the type signature for
withDirectory :: MonadError e1 (M m) => Int -> M m a -> M m a
or the instance declaration
In a stmt of a 'do' expression: _ <- expandTo dir
In the expression:
do { _ <- expandTo dir;
undefined }
In an equation for `withDirectory':
withDirectory dir _
= do { _ <- expandTo dir;
undefined }
Failed, modules loaded: none.
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4952>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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