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

Simon Peyton-Jones simonpj at microsoft.com
Tue Apr 8 09:39:06 EDT 2008

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 at haskell.org [mailto:haskell-cafe-bounces at 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?

On Mon, Apr 7, 2008 at 10:30 PM, Hugo Pacheco <hpacheco at gmail.com<mailto:hpacheco at 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?

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