[CogitoCogito]

I'm interested in sane semantics. In this case, that calls for block-level scoping. Those who introduced pattern matching should have understood that the lack of block-level scoping _before_ this PEP does in no way support the continuing of the status quo. The language after this PEP has changed and has turned into one where block-level scoping is appropriate in this case.

I'm honestly _not_ interested in block-level scoping in this case because I would _never_ have wanted this PEP to be accepted. This feature was quite controversial on the various python mailing lists, and yet the steering committee accepted it anyway. The steering committee might consider leading with a bit more humility and _not_ accepting such controversial PEPs. This is an example of language devolution and not evolution.

[chriswarbo]

It occurs to me that there's a nice way to understand this from what's happened in Scala.

Scala has always had built-in syntax for pattern-matching, like:

    foo match {
      case bar => ...
      case baz => ...
    }

However, Scala also has a thing called `PartialFunction[InputType, OutputType]`, which is a function defined 'case by case' (it's "partial" because we're allowed to leave out some cases). This is essentially a re-usable set of cases, which we can apply to various values just like calling a function.

For example we can write:

    val f: PartialFunction[A, B] = {
      case bar => ...
      case baz => ...
    }

    f(foo)

Scala also allows us to attach extra methods to certain types of value, via 'implicit classes' (which were added late on in Scala's history, although similar patterns were available before). As of Scala 2.13, the standard library attaches a method called `pipe` to values of every type. The `pipe` method simply takes a function and applies it to this/self. For example:

    val f: PartialFunction[A, B] = {
      case bar => ...
      case baz => ...
    }

    foo.pipe(f)

However, now that we have these two things (`PartialFunction` and `pipe`), it turns out we don't need explicit syntax for `match` at all! We can always turn:

    foo match {
      case bar => ...
      case baz => ...
    }

Into:

    foo.pipe({
      case bar => ...
      case baz => ...
    })

Hence Scala, in a round-about way, has shown us that pattern-matching is essentially a function call.

When it comes to Python, it doesn't even need to be a discussion about block scope; it's equally valid to think of this as function scope (like Python already supports), where `case` acts like `lambda`, except we can define a single function as a combination of multiple `case`s (like in the Scala above).

[pfalcon]

As said many times already, then you have the opposite problem - how to get value from "inner" to "outer" scope. If we talk about function scope, then it requires "nonlocal" declaration in the inner scope. From Python, too many declaration like that are syntactic litter. It has a scoping discipline which allows to avoid them in most cases, and that works great in 90% of cases (popularity of Python and amount of code written in it is there proof).

Yes, there're still remaining 10%, and pattern matching kinda drew attention to those 10%. I'm interested to address those, and invite other interested parties to discuss/work together on that. The meeting place is python-ideas mailing list.

[chriswarbo]

> If we talk about function scope

Note that I'm not simply saying 'match should have function scope', I'm saying that 'case' is literally a function definition. Hence functions defined using the 'case' keywork should work the same as functions defined using other keywords ('def', 'lambda' or 'class').

> you have the opposite problem - how to get value from "inner" to "outer" scope

The same way as if we defined the function using 'lambda' or 'def' or 'class'

> it requires "nonlocal" declaration in the inner scope

That's not a general solution, since it doesn't work in 'lambda'; although this exposes the existing problem that there is already a difference between functions defined using 'def'/'class' and functions defined using 'lambda'. Adding yet another way to define functions ('case') which defines functions that act in yet another different way just makes that worse.

[pfalcon]

> I'm saying that 'case' is literally a function definition

And I don't agree with saying it like that. I would agree with "a 'case' could be seen as a function definition". In other words, that's just one possible way to look at it, among others.

Note that from PoV of the functional programming, everything is a function. And block scope is actually recursively lexical lambda.

And OTOH function inlining is a baseline program transformation. Currently in Python, whether a syntactic element (not explicitly a function) gets implemented as a function is an implementation detail. For example, comprehension happen to be implemented as functions. But just as well they could be inlined.

Note that function calls are generally expensive, and even more so in Python. Thus, any optimizing Python implementation would inline whenever it makes sense (called once is obviously such a case). (CPython hardly can be called an optimizing impl, though since 3.8, there's noticeable work on that).

[pfalcon]

I mentioned that in other comments, and can repeat again, there were 2 choices: a) add initial pattern matching to reference Python implementation; b) throw all the work into /dev/null and get back to dark ages where pattern matching is implemented in hacky ways by disparate libs and macros. Common sense won, and a) was chosen. Pattern matching will be definitely elaborated further.

> This feature was quite controversial on the various python mailing lists

I'm also on various Python lists, and what I saw that various details were controversial, not pattern matching itself. Mostly, people wanted pattern matching to be better right from the start, just like many people here. Well, I also want Linux version 234536464576.3.1-final-forever, but instead run 5.4.0 currently, and install new versions from time to time. The same is essentially with Python too.

[CogitoCogito]

> throw all the work into /dev/null and get back to dark ages where pattern matching is implemented in hacky ways by disparate libs and macros.

How does not accepting this PEP throw anything away? It's literally right there. It's still hosted there on the PEP site. Those who want pattern matching can continue to refine the work. "Common sense" requires understanding the current work is a sunk cost and in no way supports its introduction into the language.

> I'm also on various Python lists, and what I saw that various details were controversial, not pattern matching itself.

The details of the PEP are the problem, not the idea. Not accepting this PEP is not the same as rejecting pattern matching. This is only one possible implementation of pattern matching. It's also a bad one and one that makes the language worse. Rejecting this PEP allows a better implementation in the future.

[pfalcon]

> It's still hosted there on the PEP site.

On the PEP site, https://www.python.org/dev/peps/ , there're a lot of deadlocked PEPs, some of them a good and better would have been within, than without.

> Rejecting this PEP allows a better implementation in the future.

Let's count - 3rd-party patmatching libs for Python exists for 10-15 years. And only now some of those people who did their work as "third parties" came to do it inside mainstream Python.

The "future" you talk about is on the order of a decade. (Decade(s) is for example a timespan between 1st attempts to add string interpolation and f-strings landing).

I myself was ardent critic of PEP622/PEP634. I find situation with requiring "case Cls.CONST:" to match against constants to be unacceptable. But I'm pragmatic guy, and had to agree that it can be resolved later. The core pattern matching support added isn't bad at all. Could have been better. Best is the enemy of good.

[CogitoCogito]

> On the PEP site, https://www.python.org/dev/peps/ , there're a lot of deadlocked PEPs, some of them a good and better would have been within, than without.

If it's deadlocked, it really _shouldn't_ be added.

> Let's count - 3rd-party patmatching libs for Python exists for 10-15 years. And only now some of those people who did their work as "third parties" came to do it inside mainstream Python.

What's wrong with multiple implementations? Maybe people want different things? Besides the implementations' existence shows that lack of language support isn't something that blocks the use of pattern matching. Also moving it into the language doesn't mean people will work on that one implementation. Haven't you heard that packages go to the standard library to die? Why would it be any different in the python language. Besides I'm sure that the 3rd party libs will continue to be used anyway.

> But I'm pragmatic guy, and had to agree that it can be resolved later. The core pattern matching support added isn't bad at all. Could have been better. Best is the enemy of good.

I'm pragmatic too. I understand that I can do everything that this PEP introduces without the change to the language. I also understand that this PEP could continue to be worked on and improved. It's true that best is the enemy of good. I (and obviously many others here) believe that this is _bad_.

[pfalcon]

> What's wrong with multiple implementations?

It's absolutely great, and I'm saying that as someone working 5+ years on an alternative Python dialect (exactly with a motto of "down with toxic lead-acid batteries").

> Also moving it into the language doesn't mean people will work on that one implementation.

Only on that one - god forbid. But gather around that particular implementation to make it better and polish rough edges - for sure. (While the rest of impls will remain niche projects unfortunately.)

> I (and obviously many others here) believe that this is _bad_.

And me, and many others, believe it's good ;).

[CogitoCogito]

Well I guess the most useful information I've gotten out of this thread is that there are many other implementations already. I'll try to remember that the next time I see someone use the PEP version in one of my python projects so I can recommend them to use one of the third-party libs. I see no reason to believe they'd be any worse than this.

[jnxx]

> Let's count - 3rd-party patmatching libs for Python exists for 10-15 years. And only now some of those people who did their work as "third parties" came to do it inside mainstream Python.

Well, somewhat tongue-in-cheek, why not introduce a macro system into Python which allows to experimentally implement such syntactic changes as a library?

[pfalcon]

First of all, macro systems for Python exist for decades (just as long as pattern matching, and indeed, many patmatching implementations are done as macros). One well-know example of both is https://macropy3.readthedocs.io/en/latest/pattern.html

Secondly, there's a PEP to embrace macros in CPython (instead of pretending they don't exist, and leaving that to external libraries): https://www.python.org/dev/peps/pep-0638/

But the point, you don't need to wait for official PEP to use macros in Python. If you wanted, you could do that yesterday (== decades ago). And I guess in absolute numbers, the same amount of people use macros in Python as in Scheme. It's just in relative figures, it's quite different, given that there're millions of Python users.

[jnxx]

So, if it isn't finished, why do people not use pattern matching implemented as a macro, until things like scoping are ironed out?

[jnxx]

> Well, I also want Linux version 234536464576.3.1-final-forever, but instead run 5.4.0 currently, and install new versions from time to time. The same is essentially with Python too.

Just one thing, if mainline Linux would work like Python in respect to stability of APIs and features, you could start to debug and re-write your system after minor kernel upgrades. Linux does not break APIs, and this is possible because people are very careful what they implement - they will need to support it for an indefinite future.

Of course you can make patched branches and experimental releases of the kernel, these exist, but few people will use them, for good reasons.

[pfalcon]

The Linux kernel has a document imaginatively called "stable API nonsense": https://www.kernel.org/doc/Documentation/process/stable-api-...

But the talk was not about that, it was about the fact that we want to get "finished software", but as soon as we ourselves deliver software, we vice-versa want to do it step by step, over long period of time. One day, we should get some reflection and self-awareness and understand that other programmers are exactly like ourselves - can't deliver everything at once.

[jnxx]

What you cite appears misleading to me - the text by Greg Kroah-Hartman talks very clearly about interfaces within the Linux kernel, not interfaces between kernel and user space, such as the syscall interface, which are stable. If you want to read the position of the lead Linux kernel developer on breaking user space APIs, here it is, in all caps:

https://linuxreviews.org/WE_DO_NOT_BREAK_USERSPACE

And here is the rationale why:

https://unix.stackexchange.com/questions/235335/why-is-there...

- you see that it makes perfect sense for a programming language like Python, too, to make only backwards-compatible changes (except perhaps if there are severe problems with a release).

In the same way, it does not matter how things are implemented within Python, but it matters a lot that the user interfaces, which includes in this case the syntax of the language, are stable.

And the fact that Python contrary to that does break backward compatibility - sometimes even in minor releases -, and continues to do so, is a reason that for my own projects I have come to the point at avoiding python for new stuff. There are other languages which are more stable and give the same flexibility, even at better runtime performance.

[pfalcon]

> for my own projects I have come to the point at avoiding python for new stuff.

But who are you, do I know you? I know some guy who said that about Python and now develops his own Python-like language. Is that you? Because if you just consumer of existing languages, it's something different, there always will be a new shiny thingy around the corner to lure you.

> There are other languages which are more stable and give the same flexibility, even at better runtime performance.

Yes, but from bird's eye view, all languages are the same, and differences only emphasize similarities. So, in a contrarian move, I decided to stay with Python, and work on adding missing things to it. Because any language has missing things, and Python isn't bad base to start from at all.

[pfalcon]

> you see that it makes perfect sense for a programming language like Python, too, to make only backwards-compatible changes

That's exactly what Python does of course (except when purposely otherwise, like 2->3 transition). And of course, that policy is implemented by humans, which are known to err.