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{-# LANGUAGE GADTs, KindSignatures #-}
-- See Trac #1323; crashed GHC 6.6
module Main where
import Data.Kind (Type)
data Zero = Zero
deriving (Show)
-- Change this newtype to a data and all's well
-- and it's not like the class restriction actually restricts
-- the type rather than the constructor
newtype Succ p = Succ p deriving (Show)
class TyNum a where
instance TyNum Zero where
instance (TyNum p) => TyNum (Succ p) where
data List :: Type -> Type -> Type where
Nil :: List a Zero
Cons :: (TyNum p) => a -> List a p -> List a (Succ p)
instance (Show a) => Show (List a l) where
show Nil = "Nil"
show (Cons a t) = "Cons " ++ (show a) ++ " (" ++ (show t) ++ ")"
zipL :: List a l -> List b l -> List (a, b) l
zipL Nil Nil = Nil -- this line is fine
zipL (Cons l ls) (Cons r rs) = Cons (l,r) (zipL ls rs) -- this line blows up
l1 = Cons 5 (Cons 3 Nil)
l2 = Cons True (Cons False Nil)
main = print $ zipL l1 l2
{-
$ ghc --make Test.hs
[1 of 1] Compiling Main ( Test.hs, Test.o )
ghc-6.6: panic! (the 'impossible' happened)
(GHC version 6.6 for x86_64-unknown-linux):
Pattern match failure in do expression at simplCore/Simplify.lhs:1540:8-21
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
-}
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