From: jonathan@...
Date: 2019-10-02T21:36:06+00:00
Subject: [ruby-core:95191] [Ruby master Feature#15865] `<expr> in <pattern>`	expression

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


mame (Yusuke Endoh) wrote:

> The difficult part is that `<pattern>` is not distinguishable from `<expr>` for a parser.  For example, `[1, 2, x, y]` is valid not only as `<pattern>` but also as `<expr>`.
> So, putting a bare `<pattern>` before `<expr>` is impossible in terms of parser implementation.  If we put a prefix before `<pattern>` (for example, `in <pattern> <~ <expr>`), it may be feasible.

Yes, I see!  I'm sorry I missed that point from your previous comment.  I do think a prefixed syntax could help it read more naturally.  If the prefixed syntax was unambiguous enough, maybe `=` could be used as `<match-op>`.  For example, could `@[1, 2, x, y] = ary` be parsed unambiguously?

> IMO it strongly resembles ... assignment rather than pattern matching

I agree, but isn't pattern matching the same as (destructuring) assignment in this context?


----------------------------------------
Feature #15865: `<expr> in <pattern>` expression
https://bugs.ruby-lang.org/issues/15865#change-81865

* 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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