On Thu, Jun 10, 2010 at 2:10 PM, Maciej Piechotka <uzytkown...@gmail.com> wrote:
> 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
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