On Fri, May 4, 2018 at 3:18 AM, Ed Kellett <e+python-id...@kellett.im> wrote: > I believe the intention in the example you quoted is syntax something like: > > <match-case> ::= <pattern> > | <pattern> "if" <expression> > > where the expression is a guard expression evaluated in the context of > the matched pattern. > > IOW, it could be written like this, too: > > number = match x: > 1 if True => "one" > y if isinstance(y, str) => f'The string is {y}' > _ if True => "anything" > > I do see a lot of room for bikeshedding around the specific spelling. > I'm going to try to resist the temptation ;)
Okay, let me try to tease apart your example. 1) A literal matches anything that compares equal to that value. 2) A name matches anything at all, and binds it to that name. 2a) An underscore matches anything at all. It's just a name, and follows a common convention. 3) "if cond" modifies the prior match; if the condition evaluates as falsey, the match does not match. 4) As evidenced below, a comma-separated list of comparisons matches a tuple with as many elements, and each element must match. Ultimately, this has to be a series of conditions, so this is effectively a syntax for an elif tree as an expression. For another example, here's a way to use inequalities to pick a numeric formatting: display = match number: x if x < 1e3: f"{number}" x if x < 1e6: f"{number/1e3} thousand" x if x < 1e9: f"** {number/1e6} million **" x if x < 1e12: f"an incredible {number/1e9} billion" _: "way WAY too many" I guarantee you that people are going to ask for this to be spelled simply "< 1e3" instead of having the "x if x" part. :) > How about this? > > def hyperop(n, a, b): > return match (n, a, b): > (0, _, b) => b + 1 > (1, a, 0) => a > (2, _, 0) => 0 > (_, _, 0) => 1 > (n, a, b) => hyperop(n-1, a, hyperop(n, a, b-1)) > > versus: > > def hyperop(n, a, b): > if n == 0: > return b + 1 > if n == 1 and b == 0: > return a > if n == 2 and b == 0: > return 0 > if b == 0: > return 1 > return hyperop(n-1, a, hyperop(n, a, b-1)) I have no idea what this is actually doing, and it looks like a port of Haskell code. I'd want to rewrite it as a 'while' loop with maybe one level of recursion in it, instead of two. (Zero would be better, but I think that's not possible. Maybe?) Is this something that you do a lot of? Is the tuple (n, a, b) meaningful as a whole, or are the three values independently of interest? Not sure how this is useful without a lot more context. ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/