Re: FunctorM

At 00:55 04/06/04 -0700, Ashley Yakeley wrote:

In article <20040528001157.GA1924@momenergy.repetae.net>, John Meacham wrote:

...

class FunctorM f where fmapM :: Monad m => (a -> m b) -> f a -> m (f b)

I have an equivalent (I think) class in HBase in its big library of Functor classes:

class (Functor f) => ExtractableFunctor f where { fextract :: forall g a. (FunctorApplyReturn g) => f (g a) -> g (f a); };

for :: (ExtractableFunctor f,FunctorApplyReturn m) => (a -> m b) -> (f a -> m (f b)); for foo fa = fextract (fmap foo fa);

The important thing here is that a FunctorApplyReturn is more general than Monad, it has only return and "fapply :: f (a -> b) -> f a -> f b".

I think the monadic 'zero' may be useful in applications of FunctorM, in that it provides a way to cleanly handle errors encountered during the "traversal". #g --

Given this, it's also possible to write this function (by using a state monad):

ftolist :: (ExtractableFunctor f) => f a -> [a]

The main Functor classes in HBase are arranged like this:

Functor <= FunctorApply <= FunctorApplyReturn <= Monad

Doubtless if I knew a little more category theory, I'd have better names for them. It would be nice if the standard libraries adopted a superclass arrangement like this...

-- Ashley Yakeley, Seattle WA

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