[Python-ideas] Match statement brainstorm

[Xavier Combelle]

What about a `__match__()` magic function:
so

given object:
     case Point(x,y,3):print(x,y)
     case Point(1,2,3):something_magic()
     case x,y,Point(a,b,c),1:this_is_my_tuple()


be same than writing

try:
   x,y=Point.__match__(object,("x","y",None),(None,None,3))
except MatchError:
    try:
         Point.__match__(object,(None,None,None),(1,2,3))
    except MatchError:
         try:
              x,y,var1,var2=object

              #literal testing
              if var2 != 1:
                    raise MatchError()

              a,b,c=Point.__match__(var1,("a","b","c"),(None,None,None))
              #var1 and var2 would not be really created, they are
placeholders
         except MatchError:
              #nodefault
              raise
          else:
              this_is_my_tuple
    else:
         something_magic()
else:
   print(x,y)

and let the __match__ functions know what to do to check if arguments are
correct

I purposely did let drop the dictionary and keyword arguments cases,
finding them too
much hard to understand

2016-05-30 20:35 GMT+02:00 Steven D'Aprano <steve at pearwood.info>:

> On Sat, May 28, 2016 at 02:13:33PM +1200, Greg Ewing wrote:
> [...]
> > Finally we could allow "case class" to be abbreviated to
> > just "class":
> >
> >    switch obj:
> >       class Intersection(
> >          class Line(class Point(?x1, ?y1), class Point(?x2, ?y2)),
> >          class Line(class Point(?x3, ?y3), class Point(?x4, ?y4))):
> >
> > Is that unacceptably verbose? I don't know.
>
> Verbose, not so much. Cryptic? Hell yes! Who is going to be able to
> guess what it means? We're no longer even in the same galaxy as
> executable pseudo-code.
>
> Try this as an exercise: given the above, explain in plain English what
> it does and what the result will be. What exactly is being matched?
>
> Those familiar with C-like switches are going to be totally confused.
> They'll probably wonder if you are matching "if obj == the class
> Intersection", and have no idea what's going on with the nested Line and
> Point stuff.
>
> Those familiar with classes and the class keyword are going to wonder
> what class definitions are doing inside the class declaration. Does this
> mean we can now do this?
>
> class Child(class Parent: pass):
>     def method(self): ...
>
> Obviously not, but that's what it looks like.
>
> I wonder whether the real problem here is that pattern matching as a
> concept is simply too concise for mere mortals? Once you get past the
> trivial Haskell-esque:
>
> def factorial(n):
>     switch n:
>         0: return 1
>         n: return n*fact(n-1)
>
> its just doing too much in too little code to be easily comprehensible.
>
>
>
> --
> Steve
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