Re: Why are `sum` and `product` defined via foldMap' and not foldl'?

Also, note that the benchmark from the original email: 1) uses ghci, rendering it meaningless as it's not remotely representative of performance of compiled code 2) uses the list instance of Foldable which does *not* use the defaults shown in the original email, but uses explicit definitions. The foldMap' code shown (but not used!) *is* strict, so would, presumably perform comparably to foldl'. Unlike the foldl version of sum that Foldable for lists currently uses. Cheers, Merijn

On 23 Dec 2020, at 13:55, Oleg Grenrus wrote:

I stand corrected. https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4355 is merged

and seems I misread the original e-mail.

Foldable should prefer foldMap' (or foldMap) because it offers most opportunities (Monoid mappend is associative). Trees, SnocList work better by default.

For lists the implementation could use foldl', because in that special case it makes more sense.

These methods (sum, product) are part of Foldable class specifically so they can be overriden.

- Oleg

On 23.12.2020 12.44, Oleg Grenrus wrote:

...

Because the related change proposal(s) are never accepted nor implemented.

Most recent one is quite recent though. https://mail.haskell.org/pipermail/libraries/2020-October/030862.html

- Oleg

On 23.12.2020 8.32, Viktor Dukhovni wrote:

...

Is there a compelling reason for:

sum = getSum #. foldMap' Sum product = getProduct #. foldMap' Product

rather than:

sum = foldl' (+) 0 product = foldl' (*) 1

A quick ghci session with today's GHC head yields:

λ> import qualified Data.Foldable as F λ> :set +s

λ> F.sum [0..10000000] 50000005000000 (2.98 secs, 1,612,368,368 bytes)

λ> F.foldl' (+) 0 [0..10000000] 50000005000000 (0.28 secs, 880,065,112 bytes)

The `foldl'` variant looks substantially more efficient (at least for lists), is there some important context in which `foldMap'` is preferable?

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