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
--
-KQ
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