[Python-ideas] Match statement brainstorm

[Jim Baker]

Agreed, if we want to support the structural pattern matching as seen in
Haskell, Rust, or Scala, we should do the following:

   - Match in sequence against the provided cases. Cases should support
   optional guards (`if` clause)
   - Follow only one case - no fall through, no need to `break`
   - Raise a match error if the match is not exhaustive; an `else` clause
   is fine for exhaustion

Using the typical switch pattern seen in C or Java is already idiomatically
addressed in Python through dispatching through a dict. This type of
matching is different.

On Fri, May 20, 2016 at 11:47 AM, Guido van Rossum <guido at python.org> wrote:

> It's definitely going to be "try cases in order until one matches".
>
> On Fri, May 20, 2016 at 10:45 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> > 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
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>
>
> --
> --Guido van Rossum (python.org/~guido)
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