filterM incompatible with haskell

[danr]

The filterM in this library has this type

filterM :: (Alternative m, Monad m) => (a -> Boolean, m a) -> m a

whereas filterM in Haskell is (reframed slighly):

filterM :: (Monad m, Traversable f) => (a -> m Boolean, f a) -> f a

The important part is that the predicate returns inside the monad. This allows for example succinct implementation of powerset:

powerset :: [a] -> [[a]]
powerset = filterM (const [False, True])

as well as enabling logging/state/etc inside the predicate:

countingFilterM :: (a -> Bool) -> [a] -> State Int [a]
countingFilterM p = filterM (\ x -> modify succ >> return (p x))

Please consider exporting filterM to have the same functionality as in Haskell's Control.Monad.

[davidchambers]

Thanks very much for raising this issue, @danr. :)

Until recently Fantasy Land did not define a Filterable type class, so it was necessary to cobble together a filter function from the type classes available at the time. In #2 we introduced two filter functions:

filter :: (Applicative f, Foldable f, Monoid (f a)) => (a -> Boolean, f a) -> f a
filterM :: (Monad m, Monoid (m a)) => (a -> Boolean, m a) -> m a

The goal was to support as many filterable types as possible by providing two different derivations. The functions were necessarily given different names, with the M suffix hinting at the Monad m constraint. Although the M suffix may have been added for the same reason as in Haskell, Sanctuary's filterM was never intended to mirror its Haskell namesake.

In #37 the type of filterM was changed to permit Z.filterM(odd, Just(4)):

-filterM :: (Monad m, Monoid (m a)) => (a -> Boolean, m a) -> m a
+filterM :: (Alternative m, Monad m) => (a -> Boolean, m a) -> m a

Recently, in #80, the two filter functions were replaced with a single unified function:

filter :: Filterable f => (a -> Boolean, f a) -> f a

Now that the name is available we could provide the Sanctuary equivalent of Haskell's filterM. I find your powerset example extremely compelling! If you're interested in working on a pull request for this addition, please say so. :)