Re: Add foldMapM to Data.Foldable

6 Dec 2017


      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