[Haskell-beginners] Re: maybe use for functors or arrows?

[Heinrich Apfelmus]

Michael Mossey wrote:
> Is there a way to write the function
> 
> process :: [(Location,Item)] -> [(Location,ValuableItem)]
> 
> 
> given a function indicating which Item's to keep
> 
> transform :: Item -> Maybe ValuableItem
> 
> using functors and arrows? The value for location stays with any item 
> that is kept.
> 
> What I have is
> 
> process inp = catMaybes (map g inp)
>   where g (l,i) = case transform i of
>     Nothing -> Nothing
>     Just v  -> Just (l,v)
> 
> This looks like an arrow situation to me because you want to make a 
> function that acts on the second value in a tuple, and a little bit like 
> a Maybe functor.

    import Control.Arrow ((***))

    process =
       catMaybes . map (uncurry (liftM2 (,)) . (return *** transform))

Whether this is more readable is another question.


Regards,
Heinrich Apfelmus

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