<div dir="auto">Even worse, it’s using foldl on lists, when, to the best of my knowledge, only foldr and foldl’ ever make sense for finite Haskell lists. Not sure about infinite lists ;)</div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Oct 21, 2020 at 4:11 PM chessai <<a href="mailto:chessai1996@gmail.com">chessai1996@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto">I'm pretty strongly in favour of a sum implemented using foldMap'. The status quo is atrocious (cf. Oleg's emails), and the utility of a lazy right fold is low in practise.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Oct 21, 2020, 14:38 Emily Pillmore <<a href="mailto:emilypi@cohomolo.gy" target="_blank">emilypi@cohomolo.gy</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div><div><div style="display:none;border:0px;width:0px;height:0px;overflow:hidden"><img src="https://r.superhuman.com/H6JIVSt0f7p_9Hq9EDxN8TJU814jNPk29Hh6flDILR11UjQGV_FEmd3XRsSe5h7_5d1M8swt1jQ3pyIbrHrPCwZFSfSj5zRPPpLK3H4B3N2EmBMLlbBxBNT1Dk0iy39SzTcwF5rjiu9-zJRjuJJ7u-XAXuA8_gTH_plmi2vIFgqQp-2HC0l_zV0.gif" alt=" " width="1" height="0" style="display:none;border:0px;width:0px;height:0px;overflow:hidden"></div><div><div><div>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. <br></div><div><br></div><div>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.<br></div><div><br></div><div>I'm in favor of Oleg's `foldMap` implementation. It's really clean. Also thank you Hécate!<br></div><div><br></div><div>Cheers,<br></div><div>Emily</div></div><br><div></div></div><br><div><div class="gmail_quote">On Tue, Oct 20, 2020 at 12:40 PM, Sylvain Henry <span dir="ltr"><<a href="mailto:sylvain@haskus.fr" rel="noreferrer" target="_blank">sylvain@haskus.fr</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_extra"><div class="gmail_quote" id="m_6353615111628113326m_1061459394644272651null"><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>On 18/10/2020 22:16, Oleg Grenrus
wrote:<br>
</div>
<blockquote type="cite">
<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>On 18.10.2020 23.11, Hécate wrote:<br>
</div>
<blockquote type="cite">
<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>On 18/10/2020 22:04, Oleg Grenrus
wrote:<br>
</div>
<blockquote type="cite"> 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>On 18.10.2020 22.54, Vanessa
McHale wrote:<br>
</div>
<blockquote type="cite">
<p>It's <br>
</p>
<p>sum = getSum #. foldMap Sum<br>
<br>
in base.</p>
<div>On 10/18/20 2:49 PM, Oleg
Grenrus wrote:<br>
</div>
<blockquote type="cite">
<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>On 18.10.2020 22.24, Henning
Thielemann wrote:<br>
</div>
<blockquote type="cite"> <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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