#1808: type families: impossible happened, at runtime in GHCi
-----------------------+----------------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8
Severity: normal | Keywords:
Difficulty: Unknown | Os: Linux
Testcase: | Architecture: x86
-----------------------+----------------------------------------------------
in the following module:
{{{
{-# LANGUAGE TypeFamilies #-}
module Bug where
import Prelude hiding (foldr, null)
import qualified Prelude (foldr, null)
import Data.Monoid
type family Elem a :: *
class Foldable t where
fold :: (Monoid a, a ~ (Elem t)) => t -> Elem t
fold = foldMap id
foldMap :: Monoid m => (Elem t -> m) -> t -> m
foldMap f = foldr (mappend . f) mempty
foldr :: (Elem t -> b -> b) -> b -> t -> b
null :: t -> Bool
-- Instances
type instance Elem [a] = a
instance Foldable [a] where
null = Prelude.null
foldr = Prelude.foldr
}}}
Expected behavior, using FunDeps:
{{{
$ ghci Control
GHCi, version 6.8.0.20071028: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Control ( Control.hs, interpreted )
Ok, modules loaded: Control.
*Control> null []
True
}}}
behavior when code converted to type families:
{{{
$ ghci Bug
GHCi, version 6.8.0.20071028: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Ok, modules loaded: Bug.
*Bug> null []
ghc-6.8.0.20071028: panic! (the 'impossible' happened)
(GHC version 6.8.0.20071028 for i386-unknown-linux):
nameModule $dMonoid{v arx}
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
Commenting out the default instance for "fold" stops the error from
occurring.
The same code, using FunDeps:
{{{
{-# LANGUAGE MultiParamTypeClasses,
FunctionalDependencies,
FlexibleContexts,
FlexibleInstances #-}
module Control where
import Prelude hiding (foldr, null)
import qualified Prelude (foldr, null)
import Data.Monoid
class Foldable t a | t -> a where
fold :: (Monoid a) => t -> a
fold = foldMap id
foldMap :: Monoid m => (a -> m) -> t -> m
foldMap f = foldr (mappend . f) mempty
foldr :: (a -> b -> b) -> b -> t -> b
null :: t -> Bool
-- Instances
instance Foldable [a] a where
null = Prelude.null
foldr = Prelude.foldr
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1808>
GHC <http://www.haskell.org/ghc/>
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