Re: [Haskell-beginners] Applicative Composition

There was another small reason, I did ask it, because the applicative interface isn't as rich as the monad interface. I have read, that is because of historic reasons. They weren't in there at the beginning. But I really like to use applicative functors, if possible and this one seemed to be obvious, so I was a bit puzzeled, why it wasn't there. Are there any plans to enrich the interface for applicative functors? On Fri, Oct 1, 2010 at 11:03 AM, edgar klerks wrote:

Hi Brent and Antione,

On Fri, Oct 1, 2010 at 3:41 AM, Brent Yorgey wrote:

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

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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.

Ah yes I understand that. '

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{-# 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.

Nopes I will refrain from that, I had to search first, before ask. My apologies for that, it was a bit late, when I posted it.

And another thing is, that there are different implementations for such a operator. Better let the user create it themself.

Thnx for the reply.

Greets,

Edgar

On Fri, Oct 1, 2010 at 7:15 AM, Antoine Latter wrote:

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Forwarding to list - it looks like I forgot to reply-all.

---------- Forwarded message ---------- From: Antoine Latter Date: Thu, Sep 30, 2010 at 7:17 PM Subject: Re: [Haskell-beginners] Applicative Composition To: edgar klerks

On Thu, Sep 30, 2010 at 6:47 PM, edgar klerks wrote:

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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:

{-# 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?

Here's the last time this topic came up on the lists: http://www.haskell.org/pipermail/libraries/2010-August/013992.html

Here's the corresponding library proposal ticket: http://hackage.haskell.org/trac/ghc/ticket/4189

There were a few folks against it for a few different reasons.

Antoine _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners