On Thu, Jun 10, 2010 at 2:10 PM, Maciej Piechotka
data Named a = Named String a
instance Functor Named where f `fmap` (Named s v) = Named s (f v)
instance Applicative Named where pure x = Named "" x (Named s f) <*> (Named t v) = Named (s ++ "(" ++ t ++ ")") (f v)
This is not technically a legal applicative instance, because it is not associative. This can be seen when you try to clean up the usage as we have been discussing: g <.> f = liftA2 (.) g f f = Named "f" (+1) g = Named "g" (*2) h = Named "h" (^3) ghci> f <*> (g <*> (h <*> namedPure 42)) f(g(h(42))) ghci> (f <.> g <.> h) <*> namedPure 42 f(g)(h)(42) The Applicative laws are supposed to guarantee that this refactor is legal. Of course, the latter answer is nonsense. Luke