foldl' semantic change

[David Feuer]

I would certainly have agreed back when 4.8 was in development. This was
undoubtedly a breaking change. The fact that it's now been out for some
time muddies the waters. So does the fact that the strictness has never
been documented. It seems likely that most packages relying on the old
behavior have been updated, and it's possible that some may have come to
rely on it. I see this unfortunate situation as something of an opportunity
to take a fresh look and decide what we want.

On the pro-revert side,

foldl'new f b xs = b `seq` foldl'old f b xs

which seems considerably less challenging than implementing the lazy
version from scratch with the built-in GHC magic. But there could be times
when that leads to some efficiency problem.
On Jun 10, 2016 12:10 PM, "Bart Massey" <bart.massey at gmail.com> wrote:

> -1 on retaining this. Part of the implied contract of Data.List is that
> all functions be as lazy as possible. Besides, a change that potentially
> breaks old programs that use foldl' seems like a bad idea unless there's a
> really strong reason for it. I don't have an existing example offhand, but
> it seems at least possible that something like
>
>     last = foldl' (flip const) undefined
>
> is out there...
>
> On Fri, Jun 10, 2016 at 5:29 AM David Feuer <david.feuer at gmail.com> wrote:
>
>> The semantics of foldl' for lists were changed between base 4.7 and base
>> 4.8. Specifically, foldl' became strict in the initial value of its
>> accumulator. I opened http://ghc.haskell.org/trac/ghc/ticket/12173 to
>> report this. The change was entirely accidental, according to Joachim
>> Breitner. However, Duncan Coutts indicated he is pleased with the change. I
>> don't personally have a dog in this race, but I feel very strongly about
>> three things:
>>
>> 1. The strictness should be fully documented, both in Haddock and the
>> next Haskell Report (the Haskell 2010 Report does not go into sufficient
>> detail to support either choice).
>>
>> 2. There should be *one* meaning of foldl' in base. Thus the default
>> Foldable instance should match the ones for lists and arrays.
>>
>> 3. The containers package should be consistent with base in this regard.
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20160610/0d123969/attachment.html>