Collecting values from Functors?

[Tomasz Zielonka]

On Wed, Jun 04, 2003 at 08:38:29PM +0200, Tomasz Zielonka wrote:
> On Wed, Jun 04, 2003 at 11:15:30AM -0700, Hal Daume III wrote:
> > > I'm trying to figure if there's any way I can use (say) monads to collect 
> > > values from a Functor.
> > > 
> > > For example, suppose I have a tree of some values that supports fmap, is 
> > > there any way I can use the fmap function to collect a list of all the node 
> > > values?
> > 
> > No, you need a fold to do that.
> 
> Or a variant of Functor constructor class that I have proposed some time
> ago on comp.lang.functional:
> 
> class FunctorM t where
>     fmapM :: Monad m => (a -> m b) -> (t a -> m (t b))
>     fmapM_ :: Monad m => (a -> m b) -> (t a -> m ())
>     fmapM_ f t = fmapM f t >> return ()
> 
> instance FunctorM [] where
>     fmapM = mapM
>     fmapM_ = mapM_

I am sorry, I misunderstood the problem.

Best regards,
Tom

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