higher-kind deriving ... or not
C T McBride
Tue, 26 Feb 2002 19:23:02 +0000 (GMT)
I'm rather fond of fixpoint constructions like this one:
> newtype Copy a = Copy a deriving Show
> data Wonky f
> = Wonky
> | Manky (f (Wonky f))
> deriving Show
(Clearly this is an ill-motivated example, but the real example which
caused this problem is available on request...)
The trouble is that when I ask either hugs -98 or ghci -fglasgow-exts
show (Wonky :: Wonky Copy)
the poor compiler's brain explodes.
I take it this is a known problem with instance inference, deriving, etc.
Of course, it's easy to write my own show for Wonky Copy, but that's
a tad annoying.
I tried to guess the type of the show instance derived for
Wonky. Being a naive sort of chap, I thought it might be
show :: (forall a. Show a => Show (f a)) => Wonky f -> String
but that's a syntax error.
A little more tinkering, and it looks like it might be
show :: Show (f (Wonky f)) => Wonky f -> String
Is this really the type of show? If so, no wonder there's a problem.
I don't want to start an argument about how to solve this problem. I
do want to suggest that, for the time being, it would be better to
reject `deriving Show' for type constructors like Wonky (ie those with
higher-kind parameters) than to generate instances which break the
Or am I just being a spoilsport?