On 17 November 2015 at 14:30, Nathaniel Smith <n...@pobox.com> wrote:
> On Nov 16, 2015 12:46 PM, "Robert Collins" <robe...@robertcollins.net>
> wrote:
>>
> [...]
>> marker = wsp* marker_expr ( wsp* ('and' | 'or') wsp*
>> marker_expr)*
>
> I guess technically the spec doesn't say either way what the precedence of
> "and" versus "or" should be; it just parses unparenthesised sequences to a
> flat ast and in principle the (unspecified) ast evaluator could apply
> different precedences. The way the grammar's written though seems to suggest
> that they're equal precedence with some unspecified associativity, which is
> different from python where "and" is more tightly binding than "or" (and
> associativity doesn't matter because each operator is associative in
> isolation). Maybe this is implied here too via the language about how this
> is intended to be a python subset, but it would be good to clarify what the
> evaluation semantics should be.
e.g.
"1" < "2" and "3" < "4" or "5" < "6"
Right now we haven't used prose to define AND or OR as having
differing precedences - and PEP 345 didn't either.
I think simple left associative would be fine - right now the grammar
we give parses:
"name; os_name=='dud' and os_name=='odd' or os_name=='fred'"
as
('name', [], [], (('posix', '==', 'dud'), [('and', ('posix', '==',
'odd')), ('or', ('posix', '==', 'fred'))]))
which can just be rolled up left to right.
> Other comments:
>
> The whitespace handling looks correct to me now :-)
>
> I didn't check that the two copies of the grammar are identical (and I only
> looked at the top version, not the bottom version). Hopefully someone did?
> It'll be a headache if we discover later that there's skew between them, and
> no guidance on which is normative.
I've been pretty careful; if there's a difference we'll discuss and
figure it out :)
-Rob
--
Robert Collins <rbtcoll...@hp.com>
Distinguished Technologist
HP Converged Cloud
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