[Haskell-cafe] Generalizing catMaybes

[Brandon S Allbery KF8NH]

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On 1/7/11 21:56 , Tony Morris wrote:
>  I am wondering if it possible to generalise catMaybes:
> 
> (Something f, SomethingElse t) => t (f a) -> t a
> 
> I have being doing some gymnastics with Traversable and Foldable and a
> couple of other things from category-extras to no avail. Perhaps
> someone else's brain is molded into an appropriate shape to reveal an
> answer!

Looks to me like you want something like:

> mtraverse :: (Traversable t, Monoid m) => t m -> m
> mtraverse xs = traverse mappend (mempty:xs)

or possibly the same kind of thing using MonadPlus instead of Monoid.

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- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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