The problem is that the representation probably does not reduce to a normal
form.
Say, for the case
type instance F (Nest a) x = Either() (a,F a x)
fnn :: F (Nest a) (Nest a)
fnn = Left ()
it compiles ok.
But why can't the representation be infinite, like any other infinite data
type?
Cheers,
hugo
On Mon, Apr 7, 2008 at 10:30 PM, Hugo Pacheco
Hi guys, I have been experimenting some weird stuff (risky, yes I know) but the behaviour was certainly not the one I expected:
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
module Nest where
data Nest a = Nil | Cons a (Nest (a,a))
type family F a x :: * type instance F (Nest a) x = Either () (a,F (Nest (a,a)) x)
fnn :: F (Nest Int) (Nest Int) fnn = Left ()
The following module fails to compile (or better, compilation never ends).
Maybe there is something very bad going on due to the undecidable-instances extension?
Any clue? hugo