2008/8/24 Sean Leather
Does (|||) seem useful to others? Is it already available in some other form (or in a library) of which I'm not aware? If yes and no are the answers, then I wonder if it's useful enough for Data.List (modulo any expected renaming).
There's a generalized version of (|||) called "interleave" defined in
the paper "Backtracking, Interleaving, and Terminating Monad
Transformers"
http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf
The logict package has an implementation
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logict
In short, logict provides a class MonadLogic whose members support
interleave, among other functions:
class (MonadPlus m) => MonadLogic m where
msplit :: m a -> m (Maybe (a, m a))
...
interleave :: m a -> m a -> m a
interleave a b = msplit a >>= maybe b (\(x,a') -> return x
`mplus` interleave b a')
instance MonadLogic [] where
msplit [] = [Nothing]
msplit (x:xs) = [Just (x,xs)]
The main disadvantage interleave and (|||) have, compared to mplus and
(++), is that they aren't associative.
--
Dave Menendez