#4259: Relax restrictions on type family instance overlap
----------------------------------------+-----------------------------------
Reporter: lilac | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler (Type checker) | Version: 6.12.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: | Testcase:
Blockedby: | Blocking:
Related: |
----------------------------------------+-----------------------------------
Comment(by duairc):
I'd just like to point out that the following works (using functional
dependencies):
{{{
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
class (Monad (Inner m), Monad m) => MonadLayer m where
type Inner m :: * -> *
lift :: Inner m a -> m a
class (Monad b, Monad m) => MonadBase b m | m -> b where
liftBase :: b a -> m a
instance MonadBase IO IO where
liftBase = id
instance (MonadLayer m, MonadBase b (Inner m)) => MonadBase b m where
liftBase = lift . liftBase
}}}
While the following (using type families) does not:
{{{
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
class (Monad (Inner m), Monad m) => MonadLayer m where
type Inner m :: * -> *
lift :: Inner m a -> m a
class (Monad (Base m), Monad m) => MonadBase m where
type Base m :: * -> *
liftBase :: Base m a -> m a
instance MonadBase IO where
type Base IO = IO
liftBase = id
instance (MonadLayer m, MonadBase (Inner m)) => MonadBase m where
type Base m = Base (Inner m)
liftBase = lift . liftBase
}}}
The example using functional dependencies will work even if the instances
are defined in different modules.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4259#comment:19>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
