[ruby-core:107950] [Ruby master Feature#18583] Pattern-matching: API for custom unpacking strategies?

["palkan (Vladimir Dementyev)" <noreply@ruby-lang.org>]

Issue #18583 has been updated by palkan (Vladimir Dementyev).


> This, though, raises a question of several match groups, at which point one starts to want more:
> ```ruby
> case string
> in /{{(.+?): (.+?)}}/ => [key, value]
>   # use key and value
> in /{{=(?<named>.+?)}}/ => {named:}
>   # use named
> ```
> ...so... IDK.


This one could be achieve via guards:

```ruby
case val
  in /(foo|bar)/ if $~ in [val]
    puts val
  in /(?<named>\d+)/ if $~ in {named: }
    puts named
end
```

That would require adding MatchData#{deconstruct,deconstruct_keys}, though:

```ruby
refine MatchData do
  alias deconstruct captures

  def deconstruct_keys(*)
    named_captures.transform_keys(&:to_sym)
  end
end
```

Regarding the original proposal (the unpacking API), I think, it could bring more confusion than value. Adding one more _implicit_ layer (in addition to `#deconstruct` and `#deconstruct_keys`, which could also be overridden) would make pattern matching even more _magical_ in a bad sense. 


----------------------------------------
Feature #18583: Pattern-matching: API for custom unpacking strategies?
https://bugs.ruby-lang.org/issues/18583#change-96900

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
I started to think about it when discussing https://github.com/ruby/strscan/pull/30. 
The thing is, usage of StringScanner for many complicated parsers invokes some kind of branching.

In pseudocode, the "ideal API" would allow to write something like this:
```ruby
case <what next matches>
in /regexp1/ => value_that_matched
  # use value_that_matched
in /regexp2/ => value_that_matched
  # use value_that_matched
# ...
```
This seems "intuitively" that there *should* be some way of implementing it, but we fall short. We can do some StringScanner-specific matcher object which defines its own `#===` and use it with pinning:
```ruby
case scanner
in ^(Matcher.new(/regexp1/)) => value_that_matched
# ...
```
But there is no API to tell how the match result will be unpacked, just the whole `StringScanner` will be put into `value_that_matched`.

So, I thought that maybe it would be possible to define some kind of API for pattern-like objects, the method with signature like `try_match_pattern(value)`, which by default is implemented like `return value if self === value`, but can be redefined to return something different, like part of the object, or object transformed somehow.

This will open some interesting (if maybe uncanny) possibilities: not just slicing out the necessary part, but something like
```ruby
value => ^(type_caster(Integer)) => int_value
```

So... Just a discussion topic!



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