[Python-ideas] Match statement brainstorm

[Paul Moore]

On 20 May 2016 at 17:45, Ryan Gonzalez <rymg19 at gmail.com> wrote:
> On Fri, May 20, 2016 at 5:48 AM, Franklin? Lee
> <leewangzhong+python at gmail.com> wrote:
>>
>> I think there should be different syntaxes for matching equality and
>> binding patterns, and definitely different syntax for singular and
>> plural cases.
>>
>> Let's try this:
>> - Equality:
>>     `case 5:`
>> - Conditional:
>>     `case if predicate(obj):`
>> - Pattern-matching:
>>     `case as [a, b, *_]:`
>>     `case as Point(x, y):`
>>
>> Slightly-more-explicit checks, instead of simply `case 5:`.
>> - `case == 5:`
>> - `case is _sentinel:`
>> - `case is None:`
>> - `case < 6:`
>> - `case in (1,2,3):`
>> - `case in range(2, 5):`
>> - `case in int:`
>>
>
> I personally really like the equality, conditional, and pattern-matching
> syntax you showed; it looks the most Pythonic IMO.

Agreed.

> However, the more explicit checks are a little overkill and feel like too
> much special-casing. I don't know of any languages that go so far as to
> allow stuff like `case < 6`; usually, there would instead be some syntax to
> assign the initial object to an intermediate variable, that way you can just
> use it with an `if` predicate.

Also agreed. It seems that

    switch expr as name
        case if name is _sentinel: do_stuff()

would be a reasonable syntax for naming the switch expression -
although just giving it a name before using it in the switch is
probably just as reasonable:

    name = expr
    switch name:
        case if name is _sentinel: do_stuff()

BTW, there's also a semantic question - if multiple cases match, would
only one (presumably the first) or all of them be executed? I'm sure
this was discussed to death during the discussions on PEPs 3103 and
275, but I don't have the time or inclination to re-read those now, so
I'll just leave the question open here for someone else to do the
research...

Paul