[Python-ideas] PEP 505 (None coalescing operators) thoughts

[Guido van Rossum]

Sounds like we're in violent agreement. :-)

On Monday, September 28, 2015, Random832 <random832 at fastmail.com> wrote:

> Guido van Rossum <guido at python.org <javascript:;>> writes:
> > I would at least define different classes for the uptalk versions.
> >
> > But my main complaint is using the parse tree as a spec at all
>
> Like I said, I actually came up with that structure *before* seeing that
> it mirrored a grammar element - it honestly seems like the most natural
> way to embody the fact that evaluating it requires the whole context as
> a unit and can short-circuit halfway through the list, depending on if
> the 'operator' at that position is an uptalk version.
>
> The evaluation given this structure can be described in pseudocode:
>
> def evaluate(expr):
>     value = expr.atom.evaluate()
>     for trailer in trailers:
>         if trailer.uptalk and value is None:
>             return None
>         value = trailer.evaluate_step(value)
>
> The code generation could work the same way, iterating over this and
> generating whatever instructions each trailer implies. In CPython, The
> difference between the uptalk and non-uptalk version would be that
> immediately after the left-hand value is on the stack, insert opcodes:
> DUP_TOP LOAD_CONST(None) COMPARE_OP(is) POP_JUMP_IF_TRUE(end), with the
> jump being to the location where the final value of the expression is
> expected on the stack.
>
> Assuming I'm understanding the meaning of each opcode correctly, this
> sequence would basically be equivalent to a hypothetical JUMP_IF_NONE
> opcode.
>
> I don't think a recursive definition for the structure would work,
> because evaluating / code-generating an uptalk operator needs to have
> the top-level expression in order to escape from it to yield None.
>
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-- 
--Guido van Rossum (on iPad)
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