[Haskell-cafe] Generalizing catMaybes

Conor McBride conor at strictlypositive.org
Sat Jan 8 18:05:48 CET 2011

On 8 Jan 2011, at 15:27, Henning Thielemann wrote:

> On Sat, 8 Jan 2011, Conor McBride wrote:
>> On 8 Jan 2011, at 11:14, Henning Thielemann wrote:
>>> For me, the solutions of Dave Menendez make most sense: Generalize  
>>> Maybe to Foldable and List to MonadPlus.
>> What has it to do with monads? There's no bind in sight.
> I see a '>>=' in front of each of his expressions.

That'll teach me to wake up first. Sorry.

If you have some m (f x), and you make an (m x) from each
inner x, then you do need something joiny.

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.

Of course, anything which is both foldable and alternative
certainly has a function with the type of join.



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