[Haskell-cafe] MonadPlus and Alternative folds

[David Feuer]

It kind of seems like there should be fold-like things to match some and
many.

foldrAlt :: Alternative f => (a -> b -> b) -> b -> f a -> f b
foldrAlt c n m = foldr1Alt c n m <|> pure n

foldr1Alt :: Alternative f => (a -> b -> b) -> b -> f a -> f b
foldr1Alt c n m = c <$> m <*> foldrAlt c n m

foldlMP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b
foldlMP' f b m = foldl1MP' f b m <|> pure b

foldl1MP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b
foldl1MP' f b m = m >>= \r -> let !b' = f b r in foldlMP' f b' m

--These look a bit iffy
foldlAlt :: Alternative f => (b -> a -> b) -> b -> f a -> f b
foldlAlt f b m = ($ b) <$> foldrAlt (\x r acc -> r (f acc x)) id m

foldl1Alt :: Alternative f => (b -> a -> b) -> b -> f a -> f b
foldl1Alt f b m = ($ b) <$> foldr1Alt (\x r acc -> r (f acc x)) id m

Do these or similar exist somewhere? If not, should they exist somewhere?
I'm particularly interested in whether either of these will be fast, or
will admit a fast implementation, for attoparsec parsers.

Thanks,

David Feuer
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