[Python-ideas] Pattern matching

[Andrew Barnert]

On Apr 7, 2015, at 07:54, Szymon Pyżalski <szymon at pythonista.net> wrote:
> 
> Hello!
> 
> I was trying to google if there were some attempts to add pattern
> matching feature (as seen in e.g. haskell) to Python. I couldn't however
> find anything. Were there any proposals for a feature like this?

http://stupidpythonideas.blogspot.com/2014/08/a-pattern-matching-case-statement-for.html

I wrote this up after getting frustrated with people suggesting Python should have "a case statement like every other language" where by "every other language" they meant C and Pascal and their descendants with the relatively useless case that's just an elif chain rather than ML and its descendants with the more useful pattern matching case statement. (Nick Coghlan pointed out that go has an interesting middle ground--it looks like a JavaScript switch, but with an as clause to optionally bind the result of each expression to a local variable.)

Once you've defined the case statement, you can add pattern-matching assignments and function definitions (especially if you know Haskell, where the equivalents are just syntactic sugar for using case).

The big problem that you have in Python that you don't have in ML, Haskell, etc. is that most values don't have a unique, evaluable representation that can be pattern-matched. I think the best solution to this is a __match__ protocol, but I went over some other ideas.

The smaller problem is that in Python, only functions (and classes and modules) have scope; the idea of a name bound "locally" is tricky. There's a bit of hackery around except clauses, and comprehensions are expressions that get compiled into calls to hidden functions in part to deal with this, but I'm not sure either of those is something you'd want to extend here.

Anyway, I think that my straw man proposal is a start toward the best design you'd be able to come up with (although it's not in itself the best design, probably), and it still doesn't fit into Python. But read it and tell me if you disagree with either half of that.


> Greetings
> zefciu
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