Issue #14912 has been updated by mame (Yusuke Endoh).


@pitr.ch
Please briefly summarize your proposal first.  Use case is important, but explaining a proposal by use case is difficult to read (unless the use case is really simple).

I'm unsure but I guess your proposal:

1) Add a syntactic sugar: `case in <pattern>; <expr>; ...; end` вк `-> x { case x in <pattern>; <expr>; ...; end }`
2) Allow hash rocket pattern: `{ <pattern> => <pattern> }`
3) Add a syntactic sugar: `in <pattern> { <expr> }` вк `-> x { case x in <pattern>; <expr>; end }`

The following is my opinion.


I don't like (1).  It would be more readable to write it explicitly:

```
data.pop_all do |entry|
  case entry
  in ["fatal", message]
    deal_with_fatal message
  in ["error", message]
    deal_with_error message
  end
end
```


We need to be careful about (2).  If `=>` pattern is allowed, we can write a variable as a key, but it brings ambiguity.

```
h = { "foo" => 1, "bar" => 1 }

case h
in { x => 1 }
  p x #=> "foo"? "bar"?
end
```

I think the current design (allowing only symbol keys) is reasonable.


(3) will introduce syntactic ambiguity.  Consider the following example.

```
case x
in 1
  {}
in 2
  {}  # Is this the second pattern?  Or is this a lambda?  
end
```

It looks like this case statement has two clauses, but `in 2 {}` (a lambda expression you propose) can be also parsed.

----------------------------------------
Feature #14912: Introduce pattern matching syntax
https://bugs.ruby-lang.org/issues/14912#change-78403

* Author: ktsj (Kazuki Tsujimoto)
* Status: Assigned
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
* Target version: 2.7
----------------------------------------
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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