Re: [Haskell] Composing applicative functors?

I just added a Cofunctor instance for type composition, but there are two competing choices: -- | Often useful for \"acceptors\" (consumers, sinks) of values. class Cofunctor acc where cofmap :: (a -> b) -> (acc b -> acc a) -- | Composition of type constructors: unary & unary. Called "g . f" -- in [1], section 5. newtype (g :. http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.: f) a = T_T { runT_T :: g (f a) } instance (Functor http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Functo... g, Cofunctor f) => Cofunctor (g :. http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.: f) where cofmap h (T_T gf) = T_T (fmap http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:fmap (cofmap h) gf) -- Or this alternative. Having both yields "Duplicate instance -- declarations". How to decide between these instances? -- instance (Cofunctor g, Functor f) => Cofunctor (g :.: f) where -- cofmap h (T_T gf) = T_T (cofmap (fmap h) gf) Any comments/suggestions about how or whether to make the choice between these two instances? - Conal On 3/12/07, Conal Elliott wrote:

I'm still interested in comments composition operators at http://haskell.org/haskellwiki/TypeComposition . Now I see that my ":.::" is what [1] calls "StaticArrow".

Mainly, I'd like to hear thoughts on how to make an arrow out of a functor & arrow, with the functor on the inside. My attempt is called " ::.:" on the wiki page.

newtype ((~>) ::.: f) a b = TT_T { runTT_T :: f a ~> f b }

As mentioned in the comments, my Arrow definition isn't really satisfactory. Is there an arrow that corresponds to this sort of composition?

Thanks, - Conal

On 3/8/07, Conal Elliott wrote:

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I made a first pass at a module of type compositions with some instances at http://haskell.org/haskellwiki/TypeComposition and would love to get comments & suggestions.

Is there an existing process for library proposals & discussions?

Cheers, - Conal

On 2/21/07, Ross Paterson < ross@soi.city.ac.uk> wrote:

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[redirecting to libraries] On Wed, Feb 21, 2007 at 04:23:50PM -0800, Conal Elliott wrote:

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Is the composition instance of Control.Applicative defined somewhere? I see it in the McBride & Paterson paper [1], but not in the library docs [2]. I'd rather pull it in from a library than (re)define it myself.

[1] http://www.soi.city.ac.uk/~ross/papers/Applicative.pdfhttp://www.soi.city.ac.uk/%7Eross/papers/Applicative.pdf [2] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicati...

It's not defined in the libraries. The type (and the identity functor) should probably go in the module that defines Functor (currently Control.Monad) with a Functor instance, and an Applicative instance in Control.Applicative.

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