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

Hugo Pacheco hpacheco at gmail.com
Mon Apr 7 17:30:33 EDT 2008

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

Any clue?
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