[Python-Dev] Re: switch statement
mwh at python.net
Thu Apr 21 19:10:05 CEST 2005
Samuele Pedroni <pedronis at strakt.com> writes:
> Michael Hudson wrote:
>>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
That's a better way of putting it than "pattern matching and python
don't really seem to fit together", for sure :)
(I'd quite like records with labels, tangentially, but am not so wild
> 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.
But the visitor pattern is pretty grim, really. It would be nice (tm)
to have something like:
match node in:
# lhs, rhs bound in here
Vyper had something like this, I think.
> 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.
> >>> class Pt:
> ... 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.
Also, the syntax is disgusting. But that's a separate issue, I guess.
/* I'd just like to take this moment to point out that C has all
the expressive power of two dixie cups and a string.
*/ -- Jamie Zawinski from the xkeycaps source
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