[#17] Add foldMapA and foldMapM functions

CHANGELOG.md

@@ -29,6 +29,8 @@ Change log
 * [#18](https://github.com/kowainik/universum/issues/18):
   Add `LazyStrict` type class for conversions.
 * `map` is not `fmap` anymore. Reexport `map` from `Data.List`
+* [#17](https://github.com/kowainik/universum/issues/17):
+  Add `foldMapA` and `foldMapM` functions.
 
 
 `universum` uses [PVP Versioning][1].

src/Universum/Foldable/Fold.hs

@@ -11,6 +11,8 @@
 
 module Universum.Foldable.Fold
        ( flipfoldl'
+       , foldMapA
+       , foldMapM
        , safeHead
        , sum
        , product
@@ -26,13 +28,15 @@ module Universum.Foldable.Fold
 
 import GHC.TypeLits (ErrorMessage (..), TypeError)
 
-import Universum.Applicative (pure)
-import Universum.Base (Constraint, Eq, IO, Num (..), Type)
+import Universum.Applicative (Applicative (..), pure)
+import Universum.Base (Constraint, Eq, IO, Num (..), Type, ($!))
 import Universum.Bool (Bool (..))
 import Universum.Container.Reexport (HashSet, Set)
 import Universum.Foldable.Reexport (Foldable (..))
 import Universum.Function (flip, (.))
+import Universum.Functor ((<$>))
 import Universum.Monad.Reexport (Maybe (..), Monad (..))
+import Universum.Monoid (Monoid (..))
 
 import qualified Data.Foldable as F
 
@@ -59,6 +63,18 @@ flipfoldl' :: Foldable f => (a -> b -> b) -> b -> f a -> b
 flipfoldl' f = foldl' (flip f)
 {-# INLINE flipfoldl' #-}
 
+foldMapA :: (Monoid b, Applicative m, Foldable f) => (a -> m b) -> f a -> m b
+foldMapA f = foldr step (pure mempty)
+  where
+    step a mb = mappend <$> f a <*> mb
+{-# INLINE foldMapA #-}
+
+foldMapM :: (Monoid b, Monad m, Foldable f) => (a -> m b) -> f a -> m b
+foldMapM f xs = foldr step return xs mempty
+  where
+    step x r z = f x >>= \y -> r $! z `mappend` y
+{-# INLINE foldMapM #-}
+
 {- | Stricter version of 'F.sum'.
 
 >>> sum [1..10]