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 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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