[ruby-core:92507] [Ruby trunk Feature#15814] Capturing variable in case-when branches

[unihedron@unihedro.com]

Issue #15814 has been reported by unihedron (Unihedron 0).

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Feature #15814: Capturing variable in case-when branches
https://bugs.ruby-lang.org/issues/15814

* Author: unihedron (Unihedron 0)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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In ruby, when a case-when statement is written, the when branches accepts expressions which will be evaluated to objects, then === is called to check if any of them returns true:

```ruby
case 'a'
when 'abc'
  # not matched
when Regexp.new('[abc]')
  puts :matched # => matched
end
```

To demonstrate what is being done here, this is a mock:

```ruby
equal_all_mock = Object.new
class << equal_all_mock
  def ===(anything) true end
end
# 1
case 'a'
when equal_all_mock
  puts :matched # => matched
end
# 2
if equal_all_mock === 'a'
  puts :matched # => matched
end
```

Often times when matching for conditional statements, they have values in addition to being truthy or falsey; for example, it is very tempting to write (bugged) code like this (context: parsing 2D robot path instructions):

```ruby
case
when i = '^v<>'.index[code]
  x += [0, 0, -1, 1][i]
  y += [1, -1, 0, 0][i]
when code = '/\\'[code]
  if code == '/'
    dx, dy =  dy,  dx
  else
    dx, dy = -dy, -dx
  end
when code == '#'
  dx = -dx
  dy = -dy
end
```

This pattern has problems:

1. Using assignment to capture expressions "leaks" the local variable into the current scope, which the case block doesn't lock into a block scope, as it's not a proc

2. Even if the match fails, the expression is still written; `code = '/\\'[code]` in this case may assign nil, of which then `code == '#'` will fail

3. The alternative would be using regex, such as `/\^v<>/` and then using `$&` to fetch match data... but the global variable pattern is said to be discouraged, and while it works in this specific case it doesn't work in others, like if I want to act upon the index of an array search (but not when the search result is nil)

Thus my proposal:

```ruby
case
when '^v<>'.index[code] => i
  x += [0, 0, -1, 1][i]
  y += [1, -1, 0, 0][i]
when '/\\'[code] => code
  if code == '/'
    dx, dy =  dy,  dx
  else
    dx, dy = -dy, -dx
  end
when code == '#'
  dx = -dx
  dy = -dy
end
```

This is based on the `rescue Exception => e` syntax. The `when expression => i` format could potentially even be extended to:

```
case 'foobar'
when /fo./ => match
  p match # => foo
end
```

or with a proc that accepts 0~1 parameters (if it expects one, ruby could feed in the truthy value):
```
case
when '^v<>'.index[code] do |i|
  x += [0, 0, -1, 1][i]
  y += [1, -1, 0, 0][i]
end
when '/\\'[code] do |code|
  if code == '/'
    dx, dy =  dy,  dx
  else
    dx, dy = -dy, -dx
  end
end
when code == '#'
  dx = -dx
  dy = -dy
end
```

While some cases like these could be replaced by if-else statements I feel like this would be much better as an enhancement on the pattern-matching side. Scala, for example, does have `case x if x % 15 == 0 => { statements }` in its pattern-matching; handy when writing fizzbuzz.



-- 
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