Issue #14912 has been updated by shyouhei (Shyouhei Urabe).


We had some in-detail discussuion about the possibility of this issue in todays developer meeting.  Though it seemed a rough cut that needs more brush-ups, the proposal as a whole got positive reactions.  So please continue developing.

Some details the attendees did not like:

- Deconstruction seems fragile;  For instance the following case statement matches, which is very counter-intuitive.

    ```ruby
    def foo(obj)
      case obj
      in a, 1 => b, c then
        return a, b, c
      else
        abort
      end
    end

    A = Struct.new(:x, :y)
    p foo(A[1, 2]) # => [A, 1, 2]
    ```
- There is `|` operator that is good.  But why don't you have counterpart `&` operator?

- Pinning operator is necessary.  However the proposed syntax do not introduce an _operator_ rather it introduces naming convention into local variable naming.  This is no good.  We need a real operator for that purpose.

- One-liner mode seems less needed at the moment.  Is it necessary for the first version?  We can add this later if a real-world use-case is found that such shorthand is convenient, rather than cryptic.

- Some attendees do not like that arrays cannot be pattern matched as such.

    ```ruby
    case [1, 2, [3, 4]]
    in [a, b, [3, d]] # <- unable to do this
      ...
    end
    ```

- Should `#deconstruct` be called over and over again to the same case target?  Shouldn't that be cached?

But again, these points are about details.  The proposal as a whole seemed roughly okay.

----------------------------------------
Feature #14912: Introduce pattern matching syntax
https://bugs.ruby-lang.org/issues/14912#change-73001

* Author: ktsj (Kazuki Tsujimoto)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I propose new pattern matching syntax.

# Pattern syntax
Here's a summary of pattern syntax.

```
# case version
case expr
in pat [if|unless cond]
  ...
in pat [if|unless cond]
  ...
else
  ...
end

pat: var                                                   # Variable pattern. It matches any value, and binds the variable name to that value.
   | literal                                               # Value pattern. The pattern matches an object such that pattern === object.
   | Constant                                              # Ditto.
   | var_                                                  # Ditto. It is equivalent to pin operator in Elixir.
   | (pat, ..., *var, pat, ..., id:, id: pat, ..., **var)  # Deconstructing pattern. See below for more details.
   | pat(pat, ...)                                         # Ditto. Syntactic sugar of (pat, pat, ...).
   | pat, ...                                              # Ditto. You can omit the parenthesis (top-level only). 
   | pat | pat | ...                                       # Alternative pattern. The pattern matches if any of pats match.
   | pat => var                                            # As pattern. Bind the variable to the value if pat match.

# one-liner version
$(pat, ...) = expr                                         # Deconstructing pattern.
```

The patterns are run in sequence until the first one that matches.
If no pattern matches and no else clause, NoMatchingPatternError exception is raised.

## Deconstructing pattern
This is similar to Extractor in Scala.

The patten matches if:

* An object have #deconstruct method
* Return value of #deconstruct method must be Array or Hash, and it matches sub patterns of this

```
class Array
  alias deconstruct itself
end

case [1, 2, 3, d: 4, e: 5, f: 6]
in a, *b, c, d:, e: Integer | Float => i, **f
  p a #=> 1
  p b #=> [2]
  p c #=> 3
  p d #=> 4
  p i #=> 5
  p f #=> {f: 6}
  e   #=> NameError
end
```

This pattern can be used as one-liner version like destructuring assignment.

```
class Hash
  alias deconstruct itself
end

$(x:, y: (_, z)) = {x: 0, y: [1, 2]}
p x #=> 0
p z #=> 2
```

# Sample code
```
class Struct
  def deconstruct; [self] + values; end
end

A = Struct.new(:a, :b)
case A[0, 1]
in (A, 1, 1)
  :not_match
in A(x, 1) # Syntactic sugar of above
  p x #=> 0
end
```

```
require 'json'

$(x:, y: (_, z)) = JSON.parse('{"x": 0, "y": [1, 2]}', symbolize_names: true)
p x #=> 0
p z #=> 2
```

# Implementation
* https://github.com/k-tsj/ruby/tree/pm2.7-prototype
   * Test code: https://github.com/k-tsj/ruby/blob/pm2.7-prototype/test_syntax.rb

# Design policy
* Keep compatibility
   * Don't define new reserved words
   * 0 conflict in parse.y. It passes test/test-all
* Be Ruby-ish
   * Powerful Array, Hash support
   * Encourage duck typing style
   * etc
* Optimize syntax for major use case
   * You can see several real use cases of pattern matching at following links :)
      * https://github.com/k-tsj/power_assert/blob/8e9e0399a032936e3e3f3c1f06e0d038565f8044/lib/power_assert.rb#L106
      * https://github.com/k-tsj/pattern-match/network/dependents




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