Proposal: Strict variant of foldMap

[Daniel Cartwright]

+1

On Fri, Jun 8, 2018 at 9:20 AM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> 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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