Michael Hudson wrote:
Shannon -jj Behrens email@example.com writes:
On 4/20/05, M.-A. Lemburg firstname.lastname@example.org wrote:
My use case for switch is that of a parser switching on tokens.
mxTextTools applications would greatly benefit from being able to branch on tokens quickly. Currently, there's only callbacks, dict-to-method branching or long if-elif-elif-...-elif-else.
I think "match" from Ocaml would be a much nicer addition to Python than "switch" from C.
Can you post a quick summary of how you think this would work?
Well, Python lists are used more imperatively and are not made up with cons cells, we have dictionaries which because of ordering issues are not trivial to match, and no general ordered records with labels. We have objects and not algebraic data types. Literature on the topic usually indicates the visitor pattern as the moral equivalent of pattern matching in an OO-context vs. algebraic data types/functional one. I agree with that point of view and Python has idioms for the visitor pattern.
Interestingly even in the context of objects one can leverage the infrastructure that is there for generalized copying/pickling to allow generalized pattern matching of nested object data structures. Whether it is practical I don't know.
... def __init__(self, x,y): ... self.x = x ... self.y = y ...
p(lambda _: Pt(1, _()) ).match(Pt(1,3))
p(lambda _: Pt(1, Pt(_(),_()))).match(Pt(1,Pt(Pt(5,6),3)))
(<__main__.Pt instance at 0x40200b4c>, 3)
http://codespeak.net/svn/user/pedronis/match.py is an experiment in that direction (preceding this discussion and inspired while reading a book that was using OCaml for its examples).
Notice that this is quite grossly subclassing pickling infrastracture (the innocent bystander should probably not try that), a cleaner approach redoing that logic with matching in mind is possible and would be preferable.