[Python-ideas] Match statement brainstorm

[Greg Ewing]

Nick Coghlan wrote:

>             case (.x, .y) as p, q: # Attribute matching

I don't think I like the "as". Like the "?=",
it separates the pattern from the names being bound
too much.

>             case (["x"], ["y"]) as p, q: # Item matching

That's even more confusing. The part before the "as"
looks like it should match a tuple of two one-element
lists containing the values "x" and "y".

My feeling is that the patterns should look like
constructors. The archetypal constructor for a mapping
object is the dict display, so a pattern that matches
a mapping having particular keys would be

    case {"x": p, "y": q}:
       ...

 >        case MATCH_PATTERN [as TARGET] [and CONDITION]:

That way of handling guards wouldn't allow for multiple
guards on the same case. I would suggest

    case PATTERN:
       when CONDITION:
          ...
       when CONDITION:
          ...

Note that this would be different from

    case PATTERN:
       if CONDITION:
          ...
       elif CONDITION:
          ...

because failure of all the "when" clauses should cause
the next case to be tried.

-- 
Greg