+1 On Fri, Jun 8, 2018, 7:10 AM Simon Jakobi via Libraries < libraries@haskell.org> wrote:

I'm in favor of adding this to Foldable.

2018-06-08 15:20 GMT+02:00 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

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

+1 We utilize a foldMap' in the freckle codebase. On Fri, Jun 8, 2018 at 1:10 PM, Daniel Cartwright wrote:

+1

On Fri, Jun 8, 2018 at 9:20 AM Andrew Martin 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -- Evan Borden

Is there any chance that foldMap' might still make it into base-4.12? Am Sa., 9. Juni 2018 um 10:23 Uhr schrieb Edward Kmett :

+1 from me.

-Edward

On Jun 8, 2018, at 9:11 PM, evan@evan-borden.com wrote:

+1 We utilize a foldMap' in the freckle codebase.

On Fri, Jun 8, 2018 at 1:10 PM, Daniel Cartwright wrote:

...

+1

On Fri, Jun 8, 2018 at 9:20 AM Andrew Martin 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -- Evan Borden

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

I have uploaded a patch at https://phabricator.haskell.org/D4924. Am Fr., 29. Juni 2018 um 02:11 Uhr schrieb Andrew Martin :

I don't see any reason to not do this. Someone just needs to put a differential on phabricator, adding foldMap, a default implementation, and documentation of it. It would be maybe 6 lines total. The default implementation of foldMap' would use foldl' and that would be optimal for everything in base.

On Thu, Jun 28, 2018 at 6:38 PM, Simon Jakobi via Libraries wrote:

...

Is there any chance that foldMap' might still make it into base-4.12? Am Sa., 9. Juni 2018 um 10:23 Uhr schrieb Edward Kmett :

...

+1 from me.

-Edward

On Jun 8, 2018, at 9:11 PM, evan@evan-borden.com wrote:

+1 We utilize a foldMap' in the freckle codebase.

On Fri, Jun 8, 2018 at 1:10 PM, Daniel Cartwright wrote:

...

+1

On Fri, Jun 8, 2018 at 9:20 AM Andrew Martin 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -- Evan Borden

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- -Andrew Thaddeus Martin