RE: [Haskell-cafe] Re: Type Families: infinite compile process?

You might find it helpful to read our new paper: Type checking with open type functions http://research.microsoft.com/%7Esimonpj/papers/assoc-types/index.htm From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Hugo Pacheco Sent: 07 April 2008 22:47 To: Haskell Cafe Subject: [Haskell-cafe] Re: Type Families: infinite compile process? 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 mailto: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