Re: Add foldMapM to Data.Foldable

Actually, the most "natural" Applicative version is probably this: newtype Ap f a = Ap {getAp :: f a} instance (Applicative f, Monoid a) => Monoid (Ap f a) where mempty = Ap $ pure mempty mappend (Ap x) (Ap y) = Ap $ liftA2 mappend x y foldMapA :: (Foldable t, Monoid m, Applicative f) => (a -> f m) -> t a -> f m foldMapA f = getAp . foldMap (Ap . f) Of course, we can use some Data.Coerce magic to avoid silly eta expansion, as usual. The "right" way to perform the actions in the opposite order is probably just foldMapA f . Reverse and you can accumulate the other way using getDual . foldMapA (Dual . f) So I think the whole Applicative side of this proposal might be seen as further motivation for my long-ago stalled proposal to add Ap to Data.Monoid. On Wed, Dec 6, 2017 at 7:27 PM, Andrew Martin wrote:

I had not considered that. I tried it out on a gist (https://gist.github.com/andrewthad/25d1d443ec54412ae96cea3f40411e45), and you're definitely right. I don't understand right monadic folds well enough to write those out, but it would probably be worthwhile to both variants of it as well. Here's the code from the gist:

{-# LANGUAGE ScopedTypeVariables #-}

module Folds where

import Control.Applicative

-- Lazy in the monoidal accumulator. foldlMapM :: forall g b a m. (Foldable g, Monoid b, Applicative m) => (a -> m b) -> g a -> m b foldlMapM f = foldr f' (pure mempty) where f' :: a -> m b -> m b f' x y = liftA2 mappend (f x) y

-- Strict in the monoidal accumulator. For monads strict -- in the left argument of bind, this will run in constant -- space. foldlMapM' :: forall g b a m. (Foldable g, Monoid b, Monad m) => (a -> m b) -> g a -> m b foldlMapM' f xs = foldr f' pure xs mempty where f' :: a -> (b -> m b) -> b -> m b f' x k bl = do br <- f x let !b = mappend bl br k b

On Wed, Dec 6, 2017 at 6:11 PM, David Feuer wrote:

...

It seems this lazily-accumulating version should be Applicative, and a strict version Monad. Do we also need a right-to-left version of each?

On Dec 6, 2017 9:29 AM, "Andrew Martin" wrote:

Several coworkers and myself have independently reinvented this function several times:

foldMapM :: (Foldable g, Monoid b, Monad m) => (a -> m b) -> g a -> m b foldMapM f xs = foldlM (\b a -> mappend b <$> (f a)) mempty xs

I would like to propose that this be added to Data.Foldable. We have the triplet foldr,foldl,foldMap in the Foldable typeclass itself, and Data.Foldable provides foldrM and foldlM. It would be nice to provide foldMapM for symmetry and because it seems to be useful in a variety of applications.

-- -Andrew Thaddeus Martin

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-- -Andrew Thaddeus Martin