type families: impossible happened, at runtime in GHCi

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

Trac metadata

Trac field	Value
Version	6.8
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	aslatter@gmail.com
Operating system
Architecture