Re: Functor => Pointed => Applicative => Monad

On Mon, Nov 29, 2010 at 7:46 PM, David Menendez wrote:

On Mon, Nov 29, 2010 at 11:26 AM, Isaac Dupree wrote:

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On 11/29/10 03:39, John Smith wrote:

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Is there any intention to reorganise the standard class hierarchy, arranging them logically instead of in order of invention? I plagiarised the following example from http://stackoverflow.com/questions/1634911/can-liftm-differ-from-lifta and Trac:

class Functor f where map :: (a -> b) -> f a -> f b

class Functor f => Pointed f where pure :: a -> f a

Is it useful to have Pointed non-Functors?

Is Pointed useful at all? The last time this discussion came up, I asked for algorithms which were generic over pointed functors (in the same way that traverse is generic over applicative functors) and no one could think of any.

Without <*> or >>=, all we can say about pure is that it's a natural transformation.

It would be useful for the Failure typeclass[1]. As-is, we made Monad the superclass just for ease of use, but there could definitely be some non-Monads for which a Failure instance makes sense. I think the one that occurred to me most recently is forms in Yesod, which have an Applicative but not Monad instance. Michael [1] http://hackage.haskell.org/packages/archive/failure/0.1.0.1/doc/html/Control...