I've built the reference implementation and I'm experimenting with the new syntax in the edgedb codebase. It seems to have plenty of places where pattern matching adds clarity. I'll see if I find particularly interesting examples of that to share. So far I'm +1 on the proposal, and I like the second iteration of it. Except that I'm really sad to see the __match__ protocol gone. Quoting the PEP:
One drawback of this protocol is that the arguments to __match__ would be expensive to construct, and could not be pre-computed due to the fact that, because of the way names are bound, there are no real constants in Python.
While it's not possible to precompute the arguments ahead of time, it certainly should be possible to cache them similarly to how I implemented global names lookup cache in CPython. That should alleviate this particular performance consideration entirely. Having __match__ would allow some really interesting use cases. For example, for binary protocol parsers it would be possible to replicate erlang approach, e.g.: match buffer: case Frame(char('X'), len := UInt32(), flags := Bits(0, 1, flag1, flag2, 1, 1)) would match a Frame of message type 'X', capture its length, and extract two bit flags. This perhaps isn't the greatest example of how a full matching protocol could be used, but it's something that I personally wanted to implement. Yury