On Tue, Jan 5, 2010 at 7:49 AM, Steffen Schuldenzucker <[email protected]> wrote: > Hi Paul, > > Paul Brauner wrote: >> Hi, >> >> I'm trying to get a deep feeling of Functors (and then pointed Functors, >> Applicative Functors, etc.). To this end, I try to find lawless >> instances of Functor that satisfy one law but not the other. >> >> I've found one instance that satisfies fmap (f.g) = fmap f . fmap g >> but not fmap id = id: >> [...] >> But I can't come up with an example that satifies law 1 and not law 2. >> I'm beginning to think this isn't possible but I didn't read anything >> saying so, neither do I manage to prove it. >> >> I'm sure someone knows :) > > data Foo a = Foo a > > instance Functor Foo where > fmap f (Foo x) = Foo . f . f $ x
And what is the type of f here? _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
