On Fri, 19 Feb 2021 at 15:41, Tobias Kohn <kohnt@tobiaskohn.ch> wrote:
Hi Mark,
Thank you for your proposal to try and have more precise semantics for pattern matching. Of course, the proposal primarily introduces a new and extended protocol for pattern matching, upon which the 'semantics' is then based. I think it is important to recognise and discuss your work on this basis that it proposes a different protocol of how pattern matching works, before we consier whether your semantics really is 'more precise'.
...
If you go back and look at the history and development of PEP 634, you will find that we started off with a fully customisable method that we called `__match__` instead of `__deconstruct__`. The idea of more flexible and customisable pattern matching is thus certainly in the spirit of our proposal as well. However, we finally removed this part from our proposal due to strong concerns from the community that the match protocol is too complex, particularly for an initial version of pattern matching. Should the need arise, it would still be possible to later add such a customisation protocol in an upcoming version of Python.
It's not easy now to look back over the history of all of this. My recollection of the original version of PEP 622 (if that's the right one...) was that it had an overcomplicated protocol for __match__. It needed to be simplified but in the end it was dropped. The question now is if it will be straight-forward to retrofit a protocol to PEP 634 after it is released and when backward compatibility constraints kick in. PEP 653 (as discussed here) is precisely an attempt to retrofit a protocol to PEP 634. I think the difficulties involved in achieving that will become harder rather than easier in future. -- Oscar