5 ms·

Can't wait to see all the errors and questions when users start using this for the missing switch statement, and not realizing it doesn't work for non-dotted co

Can't wait to see all the errors and questions when users start using this for the missing switch statement, and not realizing it doesn't work for non-dotted constants. Because if I didn't misunderstood, this is not correct, is it? NOT_FOUND = 404 OK = 200 match getCode(): case NOT_FOUND: return default() case OK: return response()

AlexandrB 6y ago
Yup, that's what the PEP seems to suggest. This along with mutable default values is going to be one of the nastier Python warts.
- toxik 6y ago
  Not to mention that expressions after the case keyword have completely different semantics from expressions ANYWHERE else. a | b is not the bitwise or of a with b.
  - vitus 6y ago
    You can already define __or__ to do whatever you want! This is already special-cased in the standard library for, say, set union (where admittedly the concept is more similar to bitwise or).
    - toxik 6y ago
      ... except in case expressions Thanks for proving my point though!
  - amirkdv 6y ago
    This part is truly strange. This example from the tutorial drove it home for me: match event.get(): case Click(position=(x, y)): handle_click_at(x, y) case KeyPress(key_name="Q") | Quit(): game.quit() where `Click(position=(x, y))` is not at all what one would expect. What I still don't understand is: does the new syntax magically do this to a user-defined class? i.e. if I say foo = Click(position=(x, y)) vs ... case Click(position=(x, y)) those identical expressions, for the same value of Click, have widely different semantics? edit: typo fix
    - bobbylarrybobby 6y ago
      You are (sadly) correct. The latter example checks that the type of `event.get()` is a Click, then that it has a `.position` attribute that be restructured into a two-element tuple `(x,y)`. It's equivalent to `ev = event.get(); if isinstance(ev, Click): (x,y) = ev.position`
vitus 6y ago
Correct. > Patterns may use named constants. These must be dotted names to prevent them from being interpreted as capture variable: NOT_FOUND would always match as a capture variable, so you'd always return default(). I fully expect this to be covered by linters, but not with further language support.
- mvolfik 6y ago
  yeah, I would also expect the linters to catch this. has some way to allow use of non-dotted constants been considered? something like window.global_variable in JavaScript? Or would I need to do something like this:? NOT_FOUND = 404 m = {"NOT_FOUND": NOT_FOUND} match value: case m["NOT_FOUND"]: pass
  - TrianguloY 6y ago
    I may be wrong, but after reading the grammar I think your example is a syntax error. You can't access an element! A pattern syntax is completely different to a normal expression, and there is apparently no array access.
    - mvolfik 6y ago
      hmm, so I would need to use some namespace class https://docs.python.org/3.8/library/types.html#types.SimpleNamespace https://docs.python.org/3.8/library/types.html#types.SimpleN...
  - chriswarbo 6y ago
    You can do NOT_FOUND = 404 match value: case v if v == NOT_FOUND: pass On its own this is a convoluted alternative to `if`, but it can be useful when we have lots of patterns. Alternatively, we can avoid using a separate `v` variable and just use `value` directly: NOT_FOUND = 404 match value: case _ if value == NOT_FOUND: pass We can even match on a trivial value, and do all of the interesting checks using guards: NOT_FOUND = 404 match None: case _ if value == NOT_FOUND: pass This latter form acts like Lisp's `cond`, and I've used it a few times in Scala, e.g. () match { case _ if foo > bar => ... case _ if foo < bar => ... case _ if foo == bar => ... }
deleted 6y ago
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thehesiod 6y ago
plus the distinction of using dotted constants makes no sense, that can just mean a variable in another module
bjoli 6y ago
Well, of course. It is a pattern matcher. case NOT_FOUND: is a pattern that matches anything and binds it to NOT_FOUND. If you want to match the content of a variable you need to signal that in some way. In scheme that usually means unquote: (define not-found 404) (match status (,not-found (do-stuff)) (else (display "you might believe this is an else clause, but it actually just matches everything and binds it to else")))

AlexandrB 6y ago

Yup, that's what the PEP seems to suggest. This along with mutable default values is going to be one of the nastier Python warts.

toxik 6y ago
Not to mention that expressions after the case keyword have completely different semantics from expressions ANYWHERE else. a | b is not the bitwise or of a with b.
- vitus 6y ago
  You can already define __or__ to do whatever you want! This is already special-cased in the standard library for, say, set union (where admittedly the concept is more similar to bitwise or).
  - toxik 6y ago
    ... except in case expressions Thanks for proving my point though!
- amirkdv 6y ago
  This part is truly strange. This example from the tutorial drove it home for me: match event.get(): case Click(position=(x, y)): handle_click_at(x, y) case KeyPress(key_name="Q") | Quit(): game.quit() where `Click(position=(x, y))` is not at all what one would expect. What I still don't understand is: does the new syntax magically do this to a user-defined class? i.e. if I say foo = Click(position=(x, y)) vs ... case Click(position=(x, y)) those identical expressions, for the same value of Click, have widely different semantics? edit: typo fix
  - bobbylarrybobby 6y ago
    You are (sadly) correct. The latter example checks that the type of `event.get()` is a Click, then that it has a `.position` attribute that be restructured into a two-element tuple `(x,y)`. It's equivalent to `ev = event.get(); if isinstance(ev, Click): (x,y) = ev.position`

vitus 6y ago

Correct. > Patterns may use named constants. These must be dotted names to prevent them from being interpreted as capture variable: NOT_FOUND would always match as a capture variable, so you'd always return default(). I fully expect this to be covered by linters, but not with further language support.

mvolfik 6y ago
yeah, I would also expect the linters to catch this. has some way to allow use of non-dotted constants been considered? something like window.global_variable in JavaScript? Or would I need to do something like this:? NOT_FOUND = 404 m = {"NOT_FOUND": NOT_FOUND} match value: case m["NOT_FOUND"]: pass
- TrianguloY 6y ago
  I may be wrong, but after reading the grammar I think your example is a syntax error. You can't access an element! A pattern syntax is completely different to a normal expression, and there is apparently no array access.
  - mvolfik 6y ago
    hmm, so I would need to use some namespace class https://docs.python.org/3.8/library/types.html#types.SimpleNamespace https://docs.python.org/3.8/library/types.html#types.SimpleN...
- chriswarbo 6y ago
  You can do NOT_FOUND = 404 match value: case v if v == NOT_FOUND: pass On its own this is a convoluted alternative to `if`, but it can be useful when we have lots of patterns. Alternatively, we can avoid using a separate `v` variable and just use `value` directly: NOT_FOUND = 404 match value: case _ if value == NOT_FOUND: pass We can even match on a trivial value, and do all of the interesting checks using guards: NOT_FOUND = 404 match None: case _ if value == NOT_FOUND: pass This latter form acts like Lisp's `cond`, and I've used it a few times in Scala, e.g. () match { case _ if foo > bar => ... case _ if foo < bar => ... case _ if foo == bar => ... }

deleted 6y ago

[deleted]

thehesiod 6y ago

plus the distinction of using dotted constants makes no sense, that can just mean a variable in another module

bjoli 6y ago

Well, of course. It is a pattern matcher. case NOT_FOUND: is a pattern that matches anything and binds it to NOT_FOUND. If you want to match the content of a variable you need to signal that in some way. In scheme that usually means unquote: (define not-found 404) (match status (,not-found (do-stuff)) (else (display "you might believe this is an else clause, but it actually just matches everything and binds it to else")))