From: eregontp@gmail.com
To: ruby-core@ruby-lang.org
Subject: [ruby-core:92781] [Ruby trunk Feature#15865] `<expr> in <pattern>` expression
Date: Wed, 22 May 2019 10:10:38 +0000 (UTC) [thread overview]
Message-ID: <redmine.journal-78146.20190522101038.7fae2c348113472c@ruby-lang.org> (raw)
In-Reply-To: redmine.issue-15865.20190521015812@ruby-lang.org
Issue #15865 has been updated by Eregon (Benoit Daloze).
@matz Do you mean `in` rather than `if` for the keyword?
----------------------------------------
Feature #15865: `<expr> in <pattern>` expression
https://bugs.ruby-lang.org/issues/15865#change-78146
* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version:
----------------------------------------
How about adding a syntax for one-line pattern matching: `<expr> in <pattern>` ?
```
[1, 2, 3] in x, y, z #=> true (with assigning 1 to x, 2 to y, and 3 to z)
[1, 2, 3] in 1, 2, 4 #=> false
```
More realistic example:
```
json = {
name: "ko1",
age: 39,
address: { postal: 123, city: "Taito-ku" }
}
if json in { name:, age: (20..), address: { city: "Taito-ku" } }
p name #=> "ko1"
else
raise "wrong format"
end
```
It is simpler and more composable than "case...in" when only one "in" clause is needed. I think that in Ruby a pattern matching would be often used for "format-checking", to check a structure of data, and this use case would usually require only one clause. This is the main rationale for the syntax I propose.
Additional two small rationales:
* It may be used as a kind of "right assignment": `1 + 1 in x` behaves like `x = 1 + 1`. It returns true instead of 2, though.
* There are some arguments about the syntax "case...in". But if we have `<expr> in <pattern>`, "case...in" can be considered as a syntactic sugar that is useful for multiple-clause cases, and looks more natural to me.
There are two points I should note:
* `<expr> in <pattern>` is an expression like `<expr> and <expr>`, so we cannot write it as an argument: `foo(1 in 1)` causes SyntaxError. You need to write `foo((1 in 1))` as like `foo((1 and 1))`. I think it is impossible to implement.
* Incomplete pattern matching also rewrites variables: `[1, 2, 3] in x, 42, z` will write 1 to the variable "x". This behavior is the same as the current "case...in".
Nobu wrote a patch: https://github.com/nobu/ruby/pull/new/feature/expr-in-pattern
--
https://bugs.ruby-lang.org/
next prev parent reply other threads:[~2019-05-22 10:10 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <redmine.issue-15865.20190521015812@ruby-lang.org>
2019-05-21 1:58 ` [ruby-core:92733] [Ruby trunk Feature#15865] `<expr> in <pattern>` expression mame
2019-05-21 8:21 ` [ruby-core:92739] " shevegen
2019-05-21 16:22 ` [ruby-core:92746] " mame
2019-05-21 18:48 ` [ruby-core:92747] " eregontp
2019-05-22 7:38 ` [ruby-core:92769] " ko1
2019-05-22 8:04 ` [ruby-core:92773] " nobu
2019-05-22 8:54 ` [ruby-core:92775] " matz
2019-05-22 10:10 ` eregontp [this message]
2019-06-04 21:56 ` [ruby-core:92964] " eregontp
2019-06-27 0:21 ` [ruby-core:93378] " pvande
2019-06-27 2:47 ` [ruby-core:93381] " mame
2019-06-27 6:23 ` [ruby-core:93383] " pvande
2019-07-11 4:21 ` [ruby-core:93659] [Ruby master " manga.osyo
2019-07-30 4:23 ` [ruby-core:94022] " matz
2019-09-29 19:10 ` [ruby-core:95149] " jonathan
2019-10-01 6:25 ` [ruby-core:95167] " mame
2019-10-02 21:36 ` [ruby-core:95191] " jonathan
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