[Numpy-discussion] Resolving the associativity/precedence debate for @

Nathaniel Smith njs at pobox.com
Sun Apr 6 17:01:27 EDT 2014

On Tue, Apr 1, 2014 at 4:13 PM, Charles R Harris
<charlesr.harris at gmail.com> wrote:
> On Mon, Mar 24, 2014 at 6:33 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> On Mon, Mar 24, 2014 at 11:58 PM, Charles R Harris
>> <charlesr.harris at gmail.com> wrote:
>> > On Mon, Mar 24, 2014 at 5:56 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> >>
>> >> On Sat, Mar 22, 2014 at 6:13 PM, Nathaniel Smith <njs at pobox.com> wrote:
>> >> > After 88 emails we don't have a conclusion in the other thread (see
>> >> > [1] for background). But we have to come to some conclusion or
>> >> > another
>> >> > if we want @ to exist :-). So I'll summarize where the discussion
>> >> > stands and let's see if we can find some way to resolve this.
>> >>
>> >> Response in this thread so far seems (AFAICT) to have pretty much
>> >> converged on same-left.
>> >>
>> >> If you think that this would be terrible and there is some compelling
>> >> argument against it, then please speak up! Otherwise, if no-one
>> >> objects, then I'll go ahead in the next few days and put same-left
>> >> into the PEP.
>> >
>> >
>> > I think we should take a close look at broadcasting before deciding on
>> > the
>> > precedence.
>> Can you elaborate? Like what, concretely, do you think we need to do now?
> Mostly I like to think of the '@' operators like commas in a function call
> where each argument gets evaluated before the matrix multiplications take
> place, so that would put it of lower precedence than '*', but still higher
> than '+, -' . However, since most matrix expressions seem to be small it may
> not matter much and the same result could be gotten with parenthesis. But I
> do think it would make it easier to read and parse matrix expressions as the
> '@' would serve as a natural divider. So 'A @ B*v' would be equivalent to 'A
> @ (B*v)' and not '(A @B)*v'.
> Hmm, now that I stare at it, it may actually be easier to simply read left
> to right and use parenthesis when needed. So put me down as neutral at this
> point and maybe trending towards equal precedence.

Okay, I'm going to let that be the last word on this (unless someone
wants a laster word, but no-one seems too eager at this point). We
have to pick something, we seem to have mostly converged on
left-associativity, and the bulk of the opinion on left-associativity
is clearly for same-left, so I've updated the PEP to include @ parsing
as same-left:

Thanks everyone for your feedback, both here and throughout the
process in general -- I know this has been a long discussion to slog


Nathaniel J. Smith
Postdoctoral researcher - Informatics - University of Edinburgh

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