Re: [Haskell-cafe] Re: Equirecursive types?

On 3/27/06, lee marks 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.