What do you assume when you see fromListN in a library?

Zemyla zemyla at gmail.com
Fri Feb 28 17:54:50 UTC 2020

Honestly, we could probably have positive numbers mean "do this number
exactly" and negative numbers mean "use this as a hint". For instance:

filter p arr = fromListN (negate $ length arr) $ Prelude.filter p $ toList

This is an increase in definedness, since normally most implementations
would error when provided a negative length.

On Fri, Feb 28, 2020, 09:11 chessai . <chessai1996 at gmail.com> wrote:

> I agree with others who have stated that the interpretation as a hint is
> bad for performance. My intuition has always been that we pre-allocate an
> array of length N, fill that in with a recursive function while
> accumulating the length, and rejecting anything incorrect.
> I think the motivation behind it being a hint is still mostly a mystery to
> me, until I can find some discussion. I can see how a hint might be useful
> from the perspective of the desugarer, but for people who are calling
> fromListN in source code, this seems much less intuitive, if not
> potentially useful.
> It might be illuminating if we could find some discussion about why this
> decision was made, since there might be something more I'm not considering.
> As it stands, my intuition (and that of most library authors I know, and
> most libraries I have seen) does not align with that as a hint. E.g. if
> primitive were to switch to treating N as a hint, it would require a majour
> version bump, and also it would be a place where a size hint would be most
> unwelcome because of perf.
> On Fri, Feb 28, 2020, 7:02 AM Bardur Arantsson <spam at scientician.net>
> wrote:
>> On 28/02/2020 04.40, Zemyla wrote:
>> > I'm kind of the opposite. I think the number given to fromListN should
>> > be a "size hint", not a redundant coding of the size of the list given.
>> >
>> One option would be to have a
>>    data Size = Hint Int
>>              | Exact Int
>> as a parameter and let the user specify desired behavior. (Assuming it's
>> easy to accomodate either.)
>> Regards,
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