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<p>Do we have an actual example of the use of a lazy sum in the
wild?</p>
<p>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?</p>
<p>Sylvain<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 18/10/2020 22:16, Oleg Grenrus
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:2fecff7d-1a18-5c94-5e90-27c80c7fae14@iki.fi">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<p>Sorry I wasn't clear myself. Option C is "add sum'"<br>
<br>
For lists:<br>
<br>
sum = foldr (+) 0<br>
sum' = foldl' (+) 0<br>
<br>
For Foldable<br>
<br>
sum = getSum #. foldMap Sum<br>
sum' = getSum #. foldMap' Sum</p>
<p>- Oleg<br>
</p>
<div class="moz-cite-prefix">On 18.10.2020 23.11, Hécate wrote:<br>
</div>
<blockquote type="cite"
cite="mid:77399542-9ab4-381a-a8ce-3ddfe518fb9a@glitchbra.in">
<p>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`.<br>
</p>
<div class="moz-cite-prefix">On 18/10/2020 22:04, Oleg Grenrus
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:035231ea-7187-f096-d5bc-29ffe752d41d@iki.fi"> 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).<br>
<br>
So with Option A (strict):<br>
<br>
sum = getSum #. foldMap' Sum<br>
<br>
Or Option B (lazy)<br>
<br>
sum = getSum #. foldMap Sum<br>
<br>
---<br>
<br>
There is also third option, Option C:<br>
<br>
sum = foldr (+) 0<br>
sum' = foldl' (+) 0<br>
<br>
I don't think this is worthwhile, but it is an option.<br>
(to rehash, I don't consider maintaining status quo to be an
option at all).<br>
<br>
- Oleg
<div class="moz-cite-prefix">On 18.10.2020 22.54, Vanessa
McHale wrote:<br>
</div>
<blockquote type="cite"
cite="mid:ace346e5-c6d1-0cb3-7262-111d15b10b09@gmail.com">
<p>It's <br>
</p>
<p>sum = getSum #. foldMap Sum<br>
<br>
in base.</p>
<div class="moz-cite-prefix">On 10/18/20 2:49 PM, Oleg
Grenrus wrote:<br>
</div>
<blockquote type="cite"
cite="mid:cbe97c20-f365-a72e-f6e0-b00c5280f0a2@iki.fi">
<p>The problem is the current definition of sum for lists
which uses foldl, i.e non-strict left fold</p>
<p> sum = foldl (+) 0<br>
<br>
It's utterly broken. Either we should change it to
foldl' to work on some types where addition is strict,
Option A:<br>
<br>
sum = foldl' (+) 0<br>
<br>
or alternatively (to make people using lazy accumulator
types), Option B:<br>
<br>
sum = foldr (+) 0<br>
<br>
The current state is no good for anyone or anything.<br>
<br>
---<br>
<br>
Related issue which Hecate didn't clearly mention, is
that Foldable class default implementation has<br>
<br>
class Foldable f where<br>
...<br>
sum = getSum . foldMap Sum -- this is "good" lazy
definition<br>
<br>
If we select option A, then I argue that for consistency
the default `Foldable.sum` should be<br>
<br>
sum = getSum . foldMap' Sum -- strict foldMap'<br>
<br>
If we select option B, Foldable definition doesn't need
to be changed.<br>
<br>
---<br>
<br>
I repeat, using non-strict left fold, foldl, for sum and
product is not good for anything.<br>
Either foldr or foldl'.<br>
<br>
I have no strong preference. Current state is
unacceptable.<br>
<br>
- Oleg<br>
</p>
<div class="moz-cite-prefix">On 18.10.2020 22.24, Henning
Thielemann wrote:<br>
</div>
<blockquote type="cite"
cite="mid:alpine.DEB.2.20.2010182121500.5252@sputnik"> <br>
On Sun, 18 Oct 2020, Hécate wrote: <br>
<br>
<blockquote type="cite">In conclusion, leaving things to
the optimiser that could be trivially made fast every
time seems needlessly risky. <br>
</blockquote>
<br>
`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.<br>
<br>
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<pre class="moz-signature" cols="72">--
Hécate ✨
IRC: Uniaika
WWW: <a class="moz-txt-link-freetext" href="https://glitchbra.in" moz-do-not-send="true">https://glitchbra.in</a>
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