[ruby-core:98595] [Ruby master Feature#16828] Introduce find patterns

Issue #16828 has been updated by ktsj (Kazuki Tsujimoto).

Assignee set to ktsj (Kazuki Tsujimoto)
Status changed from Open to Assigned

> Accepted.

Thank you. I'll commit it.

> But we have to clarify the term "backtracking" here. It does backtrack but not with a guard, right?

Yes. This is exactly what I intend.


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Feature #16828: Introduce find patterns
https://bugs.ruby-lang.org/issues/16828#change-85912

* Author: ktsj (Kazuki Tsujimoto)
* Status: Assigned
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
I propose to add find patterns to pattern matching.

# Specification
```
find_pattern: Constant(*var, pat, ..., *var)
            | Constant[*var, pat, ..., *var]
            | [*var, pat, ..., *var]
```

This patterns are similar to array patterns,
but it finds first match values from the given object.

```ruby
case ["a", 1, "b", "c", 2, "d", "e", "f", 3]
in [*pre, String => x, String => y, *post]
  p pre  #=> ["a", 1]
  p x    #=> "b"
  p y    #=> "c"
  p post #=> [2, "d", "e", "f", 3]
end
```

Note that it doesn't support backtracking to avoid complexity.
This means that the following code raises NoMatchingPatternError.

```ruby
case [0, 1, 2]
in [*, a, *] if a == 1
end
```

# Implementation
* https://github.com/k-tsj/ruby/tree/find-pattern-prototype




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