Collecting values from Functors?

[Tomasz Zielonka]

On Thu, Jun 05, 2003 at 09:08:03AM +1200, Tom Pledger wrote:
>  | I am sorry, I misunderstood the problem.
> 
> You're too modest.  :-)
>
> There *is* a solution in that direction.

Yes, I knew I could use a State monad or a Writer monad, but I thought
that it would be an overkill. Fold is more appropriate here.

> Here's my version of fmapM, which was inspired by something in Tim
> Sheard's paper "Generic Unification via Two-Level Types and
> Parameterized Modules".
> 
>     import Control.Monad.State
> 
>     -- ------------------------------------------------------------
>     -- Functors through which monads may be lifted
> 
>     class Functor f => FunctorSeq f where
>         fseq :: Monad m => f (m a) -> m (f a)
> 
>     instance FunctorSeq [] where
>         fseq = sequence
> 
>     instance FunctorSeq Maybe where
>         fseq Nothing   = return Nothing
>         fseq (Just mx) = do x <- mx; return (Just x)
> 
>     fmapM :: (Monad m, FunctorSeq f) => (a -> m b) -> f a -> m (f b)
>     fmapM f xs = fseq (fmap f xs)
> 
>     fseq2list :: (FunctorSeq f) => f a -> [a]
>     fseq2list fa
>         = reverse (execState (fmapM (\a -> modify (a:)) fa) [])

I like this solution. The fseq function seems to be more general.

> Regards,
> Tom

Regards,
Tom
:)

-- 
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