On Wed, Aug 01, 2007 at 07:57:48AM +0200, Wolfgang Lux wrote:

Jim Apple wrote:

...

data Rec f = In (f (Rec f)) type P f a = f (Rec f, a)

mapP :: Functor f => (a -> b) -> P f a -> P f b mapP g = fmap (\(x,a) -> (x, g a))

instance Functor f => Functor (P f) where fmap = mapP

Why did Gofer have this power while Haskell does not?

Haskell does have the same power as Gofer, it simply does not allow you to define instances for type synonyms (just in order to prevent overlapping instances, I guess). If you use a newtype instead of a type synonym everything works fine:

Quite probably they never bothered to test it. stefan@stefans:/usr/local/src/gofer$ tail -15 standard.prelude openfile f = primFopen f (error ("can't open file "++f)) id -- End of Gofer standard prelude: -------------------------------------------- class Functor f where fmap :: (a -> b) -> f a -> f b data Rec f = In (f (Rec f)) type P f a = f (Rec f, a) mapP :: Functor f => (a -> b) -> P f a -> P f b mapP g = fmap (\(x,a) -> (x, g a)) instance Functor f => Functor (P f) where fmap = mapP stefan@stefans:/usr/local/src/gofer$ src/gofer Gofer Version 2.30b Copyright (c) Mark P Jones 1991-1995 Reading script file "standard.prelude": ERROR "standard.prelude" (line 874): Not enough arguments for type synonym "P" FATAL ERROR: Unable to load prelude stefan@stefans:/usr/local/src/gofer$ Stefan