Issue #6869 has been updated by Alexey Muranov.


It looks like the use of the underscore `_` as a "placeholder" is quite common in other languages (black hole register in Vim, pattern that matches everything in Haskell), but there it is really a placeholder and not a variable: values "assigned" to `_` cannot be retrieved.

With this in view, maybe, instead of this my proposal, the underscore can be "downgraded" to a "placeholder" (or "black hole variable")?

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Feature #6869: Do not treat `_` parameter exceptionally
https://bugs.ruby-lang.org/issues/6869#change-46191

* Author: Alexey Muranov
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: core
* Target version: Next Major
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=begin
I started by commenting on #6693, but i have realized that  this is a slightly different request.

I propose to not treat the variable name "(({_}))" exceptionally. Current behavior:

 {0=>1}.each_with_index { |_,_| p _ } # [0, 1]

prints "[0, 1]", but

 {1=>2}.each_with_index { |x,x| p x } # SyntaxError: (eval):2: duplicated argument name

raises  "SyntaxError: (eval):2: duplicated argument name".

Similarly for methods:

 def f(_, _)
   _
 end
 f(0, 1) # => 0

 def f(x, x)
   x
 end # => SyntaxError: (eval):2: duplicated argument name

Observe also that the use of repeated (({_})) parameter is not consistent between methods and blocks: for methods the value is the first assigned value, and for blocks it is the array of all the assigned values.

1. I propose to use the same rule for all variables, without distinguishing (({_})) specially.

In particular i propose to allow to repeat any variable, not only (({_})), in block or method arguments without raising an error.

There may be several solutions what the repeated argument will hold: it may hold the array of all assigned values, the first assigned value, the last assigned value, the first non-nil assigned value, or the last non-nil assigned value.

2. I propose to treat repeated arguments in methods and in blocks the same way (do not know which one).

3. For unused variables i propose to introduce a special placeholder, for example "(({-}))" not followed by anything other than a delimiter (comma or bracket):

 each_with_index { |-, value| puts value }

 -, -, suffix = parse(name)

=end




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