Gregory Crosswhite <gcr...@phys.washington.edu> wrote:
> Or even better,
>
> filter isJust
To make it worse again the original function can be generalized in a few
ways. Here is a generalization from the inner Maybe type:
import Data.Foldable as F
catFoldables :: Foldable t => [t a] -> [a]
catFoldables = concatMap F.toList
Here is a generalization from the outer list type:
joinMaybes :: (Alternative m, Monad m) => m (Maybe a) -> m a
joinMaybes = (>>= maybe empty pure)
And finally the generalization from everything:
import Data.Foldable as F
joinFoldables :: (Alternative m, Foldable t, Monad m) => m (t a) -> m a
joinFoldables = (>>= F.foldr (\x _ -> pure x) empty)
The final function looks a bit scary, but is actually surprisingly easy
to understand, once you realize that 'foldr' is just a generalization of
the 'maybe' function. The structure of Maybe is a list structure with
at most one element after all.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
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