#4179: Infinite loop with type function inference
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.3
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Kevin Quick ([email protected]) writes:
I started with the following:
{{{
{-# LANGUAGE TypeFamilies #-}
class DoC a where
type A2 a
op :: a -> A2 a
data Con x = InCon (x (Con x))
type FCon x = x (Con x)
foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f
(fmap (foldDoC f) t)
doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) doCon (InCon x) = op x
fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) fCon = foldDoC
op
}}}
I then changed the rank of op, but forgot to update the foldDoC
accordingly---see below. Attempting to compile this causes GHC to run
forever using 100% cpu. The corrected definition of foldDoC works fine.
Should the GHC (6.12.1) behavior in the face of my foolishness be reported
as a bug or is this a legitimate infinite recursion of type deduction?
{{{
{-# LANGUAGE TypeFamilies #-}
class DoC a where
type A2 a
type A3 a
op :: a -> A2 a -> A3 a
data Con x = InCon (x (Con x))
type FCon x = x (Con x)
-- should have been changed to this, which works
-- foldDoC :: Functor f => (f a -> a) -> A2 (FCon f) -> Con f -> a
-- foldDoC f i (InCon t) = f (fmap (foldDoC f i) t)
-- this original version causes GHC to hang foldDoC :: Functor f => (f a
-> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t)
doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) doCon
(InCon x) = op x
-- note that if this is commented out then there's no hang: presumably
because GHC doesn't have to perform type deduction for foldDoC.
fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x)
fCon = foldDoC op
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4179>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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