#4272: Typechecker loop with type families
-------------------------------+--------------------------------------------
Reporter: NickSmallbone | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.13 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: Compile-time crash
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The following code (which I think is ill-typed) causes GHC to hang, both
on 6.12.1 and 6.13.20100616:
{{{
{-# LANGUAGE TypeFamilies, ScopedTypeVariables, FlexibleContexts #-}
module Crash where
class Family f where
terms :: f a -> a
class Family (TermFamily a) => TermLike a where
type TermFamily a :: * -> *
laws :: forall a b. TermLike a => TermFamily a a -> b
laws t = prune t (terms (undefined :: TermFamily a a))
prune :: TermLike a => TermFamily a a -> TermFamily a a -> b
prune = undefined
}}}
The compiler still hangs if I remove the typeclass constraints (everything
to the left of a =>, I mean), but everything else seems to be necessary
for the compiler to loop. Using -dshow-passes shows that it's the
typechecker that loops.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4272>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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