Sylvain Henry
Do we have an actual example of the use of a lazy sum in the wild?
That’s not a very good argument, though I’ve seen it used quite a lot. The problem is that just because no-one can currently think of a “real” example it doesn’t mean that there are no cases where laziness is required. For a forced example, consider a wrapping of Double with a test for infinity on the left argument … a + b | isInfinite a = a … which doesn’t evaluate it’s second argument if the first is infinite.
If the most common case is the strict one, making `sum` strict would be a better default.
Haskell should remain a lazy language, so the defaults should be lazy unless they can be proved strict anyway.
If need be we could also provide `lazySum` or something, but is there really a need?
Who can truly say? I’d support sum' as the strict version as that is consistent with other usages. — Jón
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).
- Oleg On 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. > > _______________________________________________ > Libraries mailing list > Libraries@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
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_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries -- Hécate ✨ IRC: Uniaika WWW:https://glitchbra.in RUN: BSD
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-- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2014-04-05)