Hi everyone,
There has been much discussion on the syntax of pattern matching for
Python (in case you hadn't noticed ;)
Unfortunately the semantics seem to have been somewhat overlooked.
What pattern matching actually does seems at least as important as the
syntax.
I believe that a pattern matching implementation must have the following
properties:
* The semantics must be precisely defined.
* It must be implemented efficiently.
* Failed matches must not pollute the enclosing namespace.
* Objects should be able determine which patterns they match.
* It should be able to handle erroneous patterns, beyond just syntax errors.
PEP 634 and PEP 642 don't have *any* of these properties.
I've written up a document to specify a possible semantics of pattern
matching for Python that has the above properties, and includes reasons
why they are necessary.
https://github.com/markshannon/pattern-matching/blob/master/precise_semantics.rst
It's in the format of a PEP, but it isn't a complete PEP as it lacks
surface syntax.
Please, let me know what you think.
Cheers,
Mark.
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