On Sat, 8 Jan 2011, Conor McBride wrote:
Of course, there is an alternative generalisation.
[] and Maybe are both Foldable, hence so is their composition.
There's got to be a thing of type
collapse :: (Foldable f, Alternative a) => f x -> a x
which would do the job.
Nice! It would be
collapse = Data.Foldable.foldr (\a b -> pure a <|> b) empty
and with transformers:Data.Functor.Compose (or the one from TypeCompose)
we get
catMaybes = collapse . Compose
Of course, anything which is both foldable and alternative
certainly has a function with the type of join.
join = collapse . Compose
:-)
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