Collecting values from Functors?

Graham Klyne
Fri, 06 Jun 2003 21:54:57 +0100

At 20:52 06/06/03 +0200, Ralf Hinze wrote:
> > 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 ()
>The `fmapM' function is also known as a monadic map. It can be
>defined in a generic way for every Haskell data type. It's in
>the library of Generic Haskell (called mapMl):

So much to learn!  I must try and read it all.  One day.

A brief chase of that link didn't show up anything that was obviously about 
monadic maps... do you have any more specific links?

>As an aside, gmap and friends won't fit the bill, as they work
>on types rather than functors.

As it happens, I think what I described was based on the type rather than 
the Functor (the message subject may have been misleading), because I 
didn't actually use the existing fmap definition, but defined a new 
function with certain similarities.


Graham Klyne
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