Just my 2 cents, I find it kind of annoying that the whole structure
requires two levels of indentation to actually reach the operational
code.
This would be a first in python.
I would prefer an option akin to if elif elif else where each block is
only one level deep.
On Wed, 8 Jul 2020 at 16:10, Guido van Rossum <gu...@python.org> wrote:
>
> Today I’m happy (and a little trepidatious) to announce the next
> version of PEP 622, Pattern Matching. As authors we welcome Daniel F
> Moisset in our midst. Daniel wrote a lot of the new text in this
> version, which introduces the subject matter much more gently than the
> first version did. He also convinced us to drop the `__match__`
> protocol for now: the proposal stands quite well without that kind of
> extensibility, and postponing it will allow us to design it at a later
> time when we have more experience with how `match` is being used.
>
> That said, the new version does not differ dramatically in what we
> propose. Apart from dropping `__match__` we’re dropping the leading
> dot to mark named constants, without a replacement, and everything
> else looks like we’re digging in our heels. Why is that? Given the
> firestorm of feedback we received and the numerous proposals (still
> coming) for alternative syntax, it seems a bad tactic not to give up
> something more substantial in order to get this proposal passed. Let
> me explain.
>
> Language design is not like politics. It’s not like mathematics
> either, but I don’t think this situation is at all similar to
> negotiating a higher minimum wage in exchange for a lower pension,
> where you can definitely argue about exactly how much lower/higher
> you’re willing to go. So I don’t think it’s right to propose making
> the feature a little bit uglier just to get it accepted.
>
> Frankly, 90% of the issue is about what among the authors we’ve dubbed
> the “load/store” problem (although Tobias never tires to explain that
> the “load” part is really “load-and-compare”). There’s a considerable
> section devoted to this topic in the PEP, but I’d like to give it
> another try here.
>
> In case you’ve been avoiding python-dev lately, the problem is
> this. Pattern matching lets you capture values from the subject,
> similar to sequence unpacking, so that you can write for example
> ```
> x = range(4)
> match x:
> case (a, b, *rest):
> print(f"first={a}, second={b}, rest={rest}") # 0, 1, [2, 3]
> ```
> Here the `case` line captures the contents of the subject `x` in three
> variables named `a`, `b` and `rest`. This is easy to understand by
> pretending that a pattern (i.e., what follows `case`) is like the LHS
> of an assignment.
>
> However, in order to make pattern matching more useful and versatile,
> the pattern matching syntax also allows using literals instead of
> capture variables. This is really handy when you want to distinguish
> different cases based on some value, for example
> ```
> match t:
> case ("rect", real, imag):
> return complex(real, imag)
> case ("polar", r, phi):
> return complex(r * cos(phi), r * sin(phi))
> ```
> You might not even notice anything funny here if I didn’t point out
> that `"rect"` and `"polar"` are literals -- it’s really quite
> natural for patterns to support this once you think about it.
>
> The problem that everybody’s been concerned about is that Python
> programmers, like C programmers before them, aren’t too keen to have
> literals like this all over their code, and would rather give names to
> the literals, for example
> ```
> USE_POLAR = "polar"
> USE_RECT = "rect"
> ```
> Now we would like to be able to replace those literals with the
> corresponding names throughout our code and have everything work like
> before:
> ```
> match t:
> case (USE_RECT, real, imag):
> return complex(real, imag)
> case (USE_POLAR, r, phi):
> return complex(r * cos(phi), r * sin(phi))
> ```
> Alas, the compiler doesn’t know that we want `USE_RECT` to be a
> constant value to be matched while we intend `real` and `imag` to be
> variables to be given the corresponding values captured from the
> subject. So various clever ways have been proposed to distinguish the
> two cases.
>
> This discussion is not new to the authors: before we ever published
> the first version of the PEP we vigorously debated this (it is Issue 1
> in our tracker!), and other languages before us have also had to come
> to grips with it. Even many statically compiled languages! The reason
> is that for reasons of usability it’s usually deemed important that
> their equivalent of `case` auto-declare the captured variables, and
> variable declarations may hide (override) like-named variables in
> outer scopes.
>
> Scala, for example, uses several different rules: first, capture
> variable names must start with a lowercase letter (so it would
> handle the above example as intended); next, capture variables
> cannot be dotted names (like `mod.var`); finally, you can enclose any
> variable in backticks to force the compiler to see it as a load
> instead of a store. Elixir uses another form of markup for loads: `x`
> is a capture variable, but `^x` loads and compares the value of `x`.
>
> There are a number of dead ends when looking for a solution that works
> for Python. Checking at runtime whether a name is defined or not is
> one of these: there are numerous reasons why this could be confusing,
> not the least of which being that the `match` may be executed in a
> loop and the variable may already be bound by a previous
> iteration. (True, this has to do with the scope we’ve adopted for
> capture variables. But believe me, giving each case clause its own
> scope is quite horrible by itself, and there are other
> action-at-a-distance effects that are equally bad.)
>
> It’s been proposed to explicitly state the names of the variables
> bound in a header of the `match` statement; but this doesn’t scale
> when the number of cases becomes larger, and requires users to do
> bookkeeping the compiler should be able to do. We’re really looking
> for a solution that tells you when you’re looking at an individual
> `case` which variables are captured and which are used for
> load-and-compare.
>
> Marking up the capture variables with some sigil (e.g. `$x` or `x?`)
> or other markup (e.g. backticks or `<x>`) makes this common case ugly
> and inconsistent: it’s unpleasant to see for example
> ```
> case %x, %y:
> print(x, y)
> ```
> No other language we’ve surveyed uses special markup for capture
> variables; some use special markup for load-and-compare, so we’ve
> explored this. In fact, in version 1 of the PEP our long-debated
> solution was to use a leading dot. This was however boohed off the
> field, so for version 2 we reconsidered. In the end nothing struck our
> fancy (if `.x` is unacceptable, it’s unclear why `^x` would be any
> better), and we chose a simpler rule: named constants are only
> recognized when referenced via some namespace, such as `mod.var` or
> `Color.RED`.
>
> We believe it’s acceptable that things looking like `mod.var` are
> never considered capture variables -- the common use cases for `match`
> are such that one would almost never want to capture into a different
> namespace. (Just like you very rarely see `for self.i in …` and never
> `except E as scope.var` -- the latter is illegal syntax and sets a precedent.)
>
> One author would dearly have seen Scala’s uppercase rule adopted, but
> in the end was convinced by the others that this was a bad idea, both
> because there’s no precedent in Python’s syntax, and because many
> human languages simply don’t make the distinction between lowercase
> and uppercase in their writing systems.
>
> So what should you do if you have a local variable (say, a function
> argument) that you want to use as a value in a pattern? One solution
> is to capture the value in another variable and use a guard to compare
> that variable to the argument:
> ```
> def foo(x, spam):
> match x:
> case Point(p, q, context=c) if c == spam:
> # Match
> ```
> If this really is a deal-breaker after all other issues have been
> settled, we could go back to considering some special markup for
> load-and-compare of simple names (even though we expect this case to
> be very rare). But there’s no pressing need to decide to do this now
> -- we can always add new markup for this purpose in a future version,
> as long as we continue to support dotted names without markup,
> since that *is* a commonly needed case.
>
> There’s one other issue where in the end we could be convinced to
> compromise: whether to add an `else` clause in addition to `case
> _`. In fact, we probably would already have added it, except for one
> detail: it’s unclear whether the `else` should be aligned with `case`
> or `match`. If we are to add this we would have to ask the Steering
> Council to decide for us, as the authors deadlocked on this question.
>
> Regarding the syntax for wildcards and OR patterns, the PEP explains
> why `_` and `|` are the best choices here: no other language surveyed
> uses anything but `_` for wildcards, and the vast majority uses `|`
> for OR patterns. A similar argument applies to class patterns.
>
> If you've made it so far, here are the links to check out, with an
> open mind. As a reminder, the introductory sections (Abstract,
> Overview, and Rationale and Goals) have been entirely rewritten and
> also serve as introduction and tutorial.
>
> - PEP 622: https://www.python.org/dev/peps/pep-0622/
> - Playground:
> https://mybinder.org/v2/gh/gvanrossum/patma/master?urlpath=lab/tree/playground-622.ipynb
>
> --
> --Guido van Rossum (python.org/~guido)
> Pronouns: he/him (why is my pronoun here?)
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--
Kind regards,
Stefano Borini
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