#7354: Panic with recursion-schemes package and unit
------------------------+---------------------------------------------------
Reporter: amplitwist | Owner:
Type: bug | Status: new
Priority: normal | Component: GHCi
Version: 7.6.1 | Keywords:
Os: Linux | Architecture: x86_64 (amd64)
Failure: GHCi crash | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by amplitwist):
Here's another test case.
{{{
{-# LANGUAGE CPP, TypeFamilies, Rank2Types, FlexibleContexts,
FlexibleInstances, GADTs, StandaloneDeriving, UndecidableInstances #-}
module Main where
type family Base t :: * -> *
data family Prim t :: * -> *
class Functor (Base t) => Unfoldable t where
embed :: Base t t -> t
ana
:: (a -> Base t a) -- ^ a (Base t)-coalgebra
-> a -- ^ seed
-> t -- ^ resulting fixed point
ana g = a where a = embed . fmap a . g
data instance Prim [a] b = Cons a b | Nil deriving (Eq,Ord,Show,Read)
coalg 0 = Nil
coalg n = Cons n (n-1)
alg Nil = 1
alg (Cons a b) = a * b
instance Functor (Prim [a]) where
fmap f (Cons a b) = Cons a (f b)
fmap _ Nil = Nil
}}}
Load into GHCI, and evaluate:
{{{
*Main> ana alg
ghc: panic! (the 'impossible' happened)
(GHC version 7.6.1 for x86_64-unknown-linux):
nameModule
<<details unavailable>>
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7354#comment:2>
GHC <http://www.haskell.org/ghc/>
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