id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
4179	Infinite loop with type function inference	simonpj		"Kevin Quick (quick@sparq.org) 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
}}}
"	bug	closed	normal	7.0.1	Compiler (Type checker)	6.12.3	fixed			Unknown/Multiple	Unknown/Multiple	None/Unknown		indexed-types/should_fail/T4179