Proposal: Strict variant of foldMap

[Andrew Martin]

I propose adding another method to the Foldable typeclass: foldMap'

This has the same behavior as foldMap except that it is strict in the
accumulator. This can lead to considerable performance gains when the user
knows that monoidal append is strict both arguments. Consider the following
example (available as a gist at
https://gist.github.com/andrewthad/f79b7022725532baf709514cf08c3955):

  {-# LANGUAGE BangPatterns #-}
  {-# OPTIONS_GHC -O2 #-}
  import Gauge
  import Data.Foldable
  import qualified Data.Set as S

  foldMap' :: (Monoid m, Foldable f) => (a -> m) -> f a -> m
  foldMap' f = foldl' (\ !acc a -> acc <> f a) mempty

  numbers :: [Int]
  numbers = [1..4000]

  intToSet :: Int -> S.Set Int
  intToSet i = S.singleton (mod i 10)

  main :: IO ()
  main = defaultMain
    [ bench "lazy" $ whnf (foldMap intToSet) numbers
    , bench "strict" $ whnf (foldMap' intToSet) numbers
    ]

Here are the results we get from running this:

  benchmarked lazy
  time                 178.8 μs   (176.1 μs .. 183.1 μs)
                       0.996 R²   (0.993 R² .. 0.998 R²)
  mean                 180.8 μs   (179.1 μs .. 183.3 μs)
  std dev              7.242 μs   (5.856 μs .. 9.304 μs)
  variance introduced by outliers: 20% (moderately inflated)

  benchmarked strict
  time                 108.4 μs   (106.1 μs .. 111.0 μs)
                       0.997 R²   (0.996 R² .. 0.999 R²)
  mean                 107.9 μs   (107.0 μs .. 109.3 μs)
  std dev              3.672 μs   (2.451 μs .. 6.220 μs)
  variance introduced by outliers: 15% (moderately inflated)

These performance gains are considerable. It needs to be a method of
Foldable and not just a function written using foldl' for the same reason
that the lazy foldMap needs to be a method of Foldable. There are types for
which the default implementation can be improved upon. This is a
non-breaking change since there is a sensible default implementation.

-- 
-Andrew Thaddeus Martin
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