On 3/27/06, lee marks <[EMAIL PROTECTED]> wrote:
> So this is legal:
>
> type Fix s a = s a (Fix s a)
>
> fold :: Functor (s a) => (s a b -> b) -> Fix s a -> b
> fold f = f . fmap (fold f)
>
> but this is not:
>
> fold f = f . fmap (fold f)
data Fix s a = Fix {runFix :: s a (Fix s a)}
fold :: Functor (s a) => (s a b -> b) -> Fix s a -> b
fold f g = f . fmap (fold f) . runFix
Yes, we can build infinite types :-) My favorite so far:
data Self a = Self {runSelf :: Self a -> a}
fix f = (\(Self g) -> f (g (Self g))) (Self (\(Self g) -> f (g (Self g))))
That one seems to give GHCi something to think about! Adding a type
annotation doesn't help.
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