From: eregontp@...
Date: 2020-12-19T11:31:35+00:00
Subject: [ruby-core:101538] [Ruby master Feature#17406] Add	`NoMatchingPatternError#depth`

Issue #17406 has been updated by Eregon (Benoit Daloze).


How would this work?
Mutating a field of the NoMatchingPatternError in a catch(NoMatchingPatternError) inserted in every `in` and `else`?
Seems rather hacky to me.

----------------------------------------
Feature #17406: Add `NoMatchingPatternError#depth`
https://bugs.ruby-lang.org/issues/17406#change-89320

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
----------------------------------------
Could we have `NoMatchingPatternError#depth`, returning the number of `case...in...end` an exception has traversed?

```ruby
def show_depth
  yield
rescue NoMatchingPatternError => e
  puts "Depth: #{e.depth}"
  raise
end

show_depth do
  case [1,2,3]
  in [x, y, z] then
    show_depth do
      x => [a, b] # => raises NoMatchingPatternError
    end
  end
end

# Prints "Depth: 0" then "Depth: 1"
```

This could help bring pattern match closer to a language construct people can play with.

Example usecase: implement `Ractor#receive_if` as in https://bugs.ruby-lang.org/issues/17378#note-6



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