_______________________________________________I can't think of an example where a raw lazy `sum` would be preferable to a strict version. If summing over infinite data structures, it will always be prefixed by a `take` or a `drop` prior to summing to make sure we work with finite chunks. In which case, a strict sum is still better. Maybe someone has a more complex, contrived example showing the usefulness of lazy sums.The fact that no one seems to be able to name one in this or other discussions, or come up with a contrived example, is a good indicator to me that at least the default should be strict in this case. We should offer a lazy variant as a second option, not the first.I'm in favor of Oleg's `foldMap` implementation. It's really clean. Also thank you Hécate!Cheers,EmilyOn Tue, Oct 20, 2020 at 12:40 PM, Sylvain Henry <sylvain@haskus.fr> wrote:Do we have an actual example of the use of a lazy sum in the wild?
If the most common case is the strict one, making `sum` strict would be a better default. If need be we could also provide `lazySum` or something, but is there really a need?
Sylvain
On 18/10/2020 22:16, Oleg Grenrus wrote:
Sorry I wasn't clear myself. Option C is "add sum'"
For lists:
sum = foldr (+) 0
sum' = foldl' (+) 0
For Foldable
sum = getSum #. foldMap Sum
sum' = getSum #. foldMap' Sum- Oleg
On 18.10.2020 23.11, Hécate wrote:
Indeed, and I initially went to suggest a `foldMap'`-based implementation to keep with the current implementation of many Foldable functions that are based on `foldMap` rather than a raw `fold`.
On 18/10/2020 22:04, Oleg Grenrus wrote:
For the sake of bigger audience I didn't bother mentioning #. which is a coercion helper. It's essentially better (.) when the first argument is newtype constructor (i.e. coerce).
So with Option A (strict):
sum = getSum #. foldMap' Sum
Or Option B (lazy)
sum = getSum #. foldMap Sum
---
There is also third option, Option C:
sum = foldr (+) 0
sum' = foldl' (+) 0
I don't think this is worthwhile, but it is an option.
(to rehash, I don't consider maintaining status quo to be an option at all).
- OlegOn 18.10.2020 22.54, Vanessa McHale wrote:
It's
sum = getSum #. foldMap Sum
in base.On 10/18/20 2:49 PM, Oleg Grenrus wrote:
The problem is the current definition of sum for lists which uses foldl, i.e non-strict left fold
sum = foldl (+) 0
It's utterly broken. Either we should change it to foldl' to work on some types where addition is strict, Option A:
sum = foldl' (+) 0
or alternatively (to make people using lazy accumulator types), Option B:
sum = foldr (+) 0
The current state is no good for anyone or anything.
---
Related issue which Hecate didn't clearly mention, is that Foldable class default implementation has
class Foldable f where
...
sum = getSum . foldMap Sum -- this is "good" lazy definition
If we select option A, then I argue that for consistency the default `Foldable.sum` should be
sum = getSum . foldMap' Sum -- strict foldMap'
If we select option B, Foldable definition doesn't need to be changed.
---
I repeat, using non-strict left fold, foldl, for sum and product is not good for anything.
Either foldr or foldl'.
I have no strong preference. Current state is unacceptable.
- Oleg
On 18.10.2020 22.24, Henning Thielemann wrote:
On Sun, 18 Oct 2020, Hécate wrote:
In conclusion, leaving things to the optimiser that could be trivially made fast every time seems needlessly risky.
`seq` is still a hack. A strict 'sum' and 'product' would still fail on a lazy accumulator type, say a lazy pair type. If at all, sum and product should be deepseq-strict. So currently, letting the optimiser make a lazy sum strict is still the smaller hack.
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_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries-- HécateIRC: Uniaika WWW: https://glitchbra.in RUN: BSD
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