Proposal: add two more Applicative and Monad adapters

[David Feuer]

Also worth considering:

manyM :: (Alternative f, Monad f) => f a -> f [a]
manyM v = many_v
    where
        many_v = do
          ma <- optional v
          case ma of
            Nothing -> pure []
            Just a -> liftA2 (:) a many_v

someM :: (Alternative f, Monad f) => f a -> f [a]
someM v = liftA2 (:) v (manyM v)

Unlike the default definitions, these bound backtracking when (<|>)
represents that.

On Sat, Aug 11, 2018, 1:19 PM David Feuer <david.feuer at gmail.com> wrote:

> We currently offer liftA, liftM, liftM2, and ap to implement Functor and
> Applicative methods in terms of Applicative and Monad ones. But there are a
> couple other functions in that general vein that are missing. I propose
> that we should add at least replaceA, and perhaps also beforeM.
>
> -- (<$) = replaceA
> -- This may be better than the default if there is
> -- an optimized definition of *> (which may be
> -- based on an optimized >>).
> replaceA :: Applicative f => a -> f x -> f a
> replaceA a fa = fa *> pure a
>
> -- (<*) = beforeM
> -- This may be better than the default if there is
> -- an optimized definition of <$, or if <$ is defined as
> -- replaceA and *> is optimized.
> beforeM :: Monad f => f a -> f x -> f a
> beforeM fa fx = fa >>= \a -> a <$ fx
>
> Why a <$ fx and not fx >> pure a? Because <$ could be implemented
> specially, and is unlikely to be implemented by hand using <* if there
> isn't a custom <*.
>
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