On Fri, Jul 17, 2020 at 1:45 PM Yury Selivanov <yselivanov...@gmail.com>
wrote:
> 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.
>
It will be back, just not in 3.10. We need more experience with how
match/case are actually used to design the right `__match__` protocol.
> 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.
>
Note: That's not referring to the `__match__` protocol from version 1 of
the PEP, but to a hypothetical (and IMO sub-optimal) `__match__` protocol
that was discussed among the authors prior to settling on the protocol from
version 1.
> 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.
>
Where's that global names lookup cache? I seem to have missed its
introduction. (Unless you meant PEP 567?)
> 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.
>
I see, you'd want the *types* of the arguments to be passed into
`Frame.__match__`. That's interesting, although I have a feeling that if I
had a real use case like this I'd probably be able to come up with a better
DSL for specifying messages than this.
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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