Additional pipes prelude functions

[Andrew Martin]

Would you accept catMaybes and mapMaybe functions that are specialized to work on pipes? They would have the following type signatures:

catMaybes :: Pipe (Maybe a) a m r
mapMaybe :: (a -> b) -> Pipe (Maybe a) b m r

Also worth considering would be lefts and rights, which would be:

lefts :: Pipe (Either a b) a m r
rights :: Pipe (Either a b) b m r

[michaelt]

Note that catMaybes and rights are both specializations of Pipes.Prelude.concat Maybe the documentation for concat should be elaborated more to bring this out? At the moment it just says

  -- | Flatten all 'Foldable' elements flowing downstream

Pipes.Prelude doesn't really use examples, but something like this might make the meaning of 'flatten' clearer

>>> runEffect $ each [[1], [], [2]] >-> P.concat >-> P.print
1
2
>>> runEffect $ each [Just 1, Nothing, Just 2] >-> P.concat >-> P.print
1
2
>>> runEffect $ each [Right 1, Left "message", Right 2] >-> P.concat >-> P.print
1
2

It's a little annoying, by the way, that the generalization of Prelude.concat to Foldable made the opposite decision

 concat :: Foldable t => t [a] -> [a]

rather than

 concat :: Foldable t =>  [t a] -> [a]

[Gabriella Gonzalez]

In addition to what @michaelt said there is Pipes.Prelude.mapFoldable, too

[Mitchell Dalvi Rosen]

How about intersperse?

intersperse :: Monad m => a -> Producer' a m r -> Producer' a m r

[Gabriella Gonzalez]

@mitchellwrosen You can implement intersperse in terms of other utilities:

interperse x p = for p (\y -> do yield x; yield y) >-> Pipes.Prelude.drop 1

[David Feuer]

I didn't see this before, but I just added mapMaybe and wither. @michaelt makes a good point about flattening, but I think it's worth adding a now-common idiom.