[Python-ideas] Match statement brainstorm

[Nikolaus Rath]

On May 25 2016, "Franklin? Lee" <leewangzhong+python-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org> wrote:
> On Wed, May 25, 2016 at 1:52 AM, Nick Coghlan <ncoghlan-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org> wrote:
>> Using the running demo:
>>
>>     def demo(arg):
>>         given arg:
>>             case x, y, *_: # Tuple matching (implicit name binding)
>>                 ...
>>             case (.x, .y) as p, q: # Attribute matching
>>                 ...
>>             case (["x"], ["y"]) as p, q: # Item matching
>>                 ...
>>             case (.x) as p and isinstance(p, int): # Match + condition
>>                 ...
>>             case if isinstance(arg, int): # Condition only
>>                 ...
>>             else: # Default
>>                 ...
>>
>> The other key change there is introducing "as" to the individual cases
>> in order to be able to separate the match pattern definition from the
>> local name binding.
>
> I still don't like that `case THING` is a pattern, rather than a value
> to test against. Here's my modifications with "as", attributes, and
>
>      def demo(arg):
>         given arg:
>             case as x, y, *_: # Tuple matching (implicit name binding)
>                 ...
>             case as object(x=p, y=q, **_): # Attribute matching
>                 ...
>             case as {'x': p, 'y', q, **_}: # Item matching
>                 ...
>             case as object(x=p, **_) and isinstance(p, int): # Match + condition
>                 ...
>             case if isinstance(arg, int): # Condition only
>                 ...
>             else: # Default

I think all the ideas with "as" are difficult to read. I think its much
better to embed the target variables in the pattern - we just need a way
to mark them as such.

Mathematica has the same problem and solves it with a trailing _, but we
can't do that because our variables names may contain them. But maybe we
could use $? Most people already strongly associate this with variables.

Example:

given foo
   case (x,y):
       # matches if foo == (x,y)
   case (x, $y):
       # matches if len(foo) == 2 and foo[0] == x,
       # and assigns y = foo[1]
   case {'bar': $x, y: $z}:
       # matches if foo is a map that has 'bar' and y keys
       # and assigns x = foo['bar'], z = foo[y]
   case $x.bar:
       # matches if hasattr(foo, 'bar') and assigns x = foo
       

Best,
-Nikolaus

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