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 <hpacheco@gmail.com> wrote:

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