On Fri, Oct 01, 2010 at 01:47:09AM +0200, edgar klerks wrote:

Hi All,

I was wondering, why there isn't a composition operator for applicative functors. Of course it is rather trivial to implement, but it is a useful feature:

I think you've answered your own question: it's rather trivial to implement. If we added every single useful function to the standard libraries, we'd be up to our necks.

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-} module ApplicativeComposition where import Control.Applicative

class (Applicative f) => ApplicativeComposition f where (<.>) :: f (b -> c) -> f (a -> b) -> f (a -> c)

instance (Applicative f) => ApplicativeComposition f where (<.>) f g = pure (.) <*> f <*> g

Can this be added to later versions of haskell?

You can always make a formal proposal [1], although judging by past discussions of similar sorts of things I doubt it would be accepted, for the reasons I wrote above. Also, why bother making a new ApplicativeComposition type class? Since you can simply implement (<.>) in terms of existing Applicative methods the new type class doesn't really add anything. -Brent Footnotes: [1] http://www.haskell.org/haskellwiki/Library_submissions