From: jonathan@...
Date: 2019-09-29T19:10:42+00:00
Subject: [ruby-core:95149] [Ruby master Feature#15865] `<expr> in <pattern>`	expression

Issue #15865 has been updated by jonathanhefner (Jonathan Hefner).


> We want to ask English speaker about this proposal. Do you feel natural on this syntax?

(I am a native English speaker.)  The syntax feels confusing to me.  When I read `X in Y`, I expect `Y` to be a collection of things, and `X` to be an element of that collection.  For example, `2 in [1, 2, 3]` would return true.

I understand that a pattern describes an abstract set, and a set is a collection, so I think I see the reason for choosing `in`.  But it still feels awkward.

I also agree it is too dangerous to introduce a new keyword, unfortunately.  Here are some alternatives, off the top of my head:

* `<expr> >> <pattern>` ("shoveling" the values into the captured variables)
* `<expr> ~> <pattern>`
* `<pattern> <~ <expr>`
* `<pattern> ~= <expr>` (probably too confusing with `=~`)
* `<pattern> :~ <expr>`
* `<pattern> := <expr>` (has a strong connotation of "assignment" in other languages)
* `<pattern> :=== <expr>` (as a reference to `===`)
* `<pattern> ?= <expr>` (kind of like `+=`, `*=`, etc)
* `<pattern> =? <expr>` (can be negated if desired, i.e. `!?`)


----------------------------------------
Feature #15865: `<expr> in <pattern>` expression
https://bugs.ruby-lang.org/issues/15865#change-81790

* Author: mame (Yusuke Endoh)
* Status: Closed
* 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



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