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Bug #16178

Numbered parameters: _1 should be the same as |x| and _0 should not exist

Added by Eregon (Benoit Daloze) about 1 year ago. Updated about 1 year ago.

Status:
Closed
Priority:
Normal
Target version:
-
ruby -v:
ruby 2.7.0dev (2019-09-24T12:57:54Z master 0e84eecc17) [x86_64-linux]
[ruby-core:95056]

Description

Currently on trunk:

array = ["string", 42, [1, 2]]

array.map { |x| x * 2 }
# => ["stringstring", 84, [1, 2, 1, 2]]

array.map { _1 * 2 }
# => ["stringstring", 84, 2]

Oops, this trivial code just lost data and completely ignored the element class!
This is clearly contrary to intuition and is very dangerous.

Using _0 instead has the correct behavior but it's clear we use 1-based indexing for numbered parameters,
and it doesn't solve that _1 has dangerous behavior.

Basically the current behavior is that _0 is the same as |x| and _1 is the same as |x,|.
|x,| is almost never used in Ruby, and for good reasons, it just throws away data/information/the class of the object.
Such a dangerous operation should only be done when it's explicit, and the trailing comma in |x,| shows that, but _1 does not.

So let's make _1 be |x| and remove _0.

I am going to be harsh, but this discussion has gone too long without any serious written argument for the current behavior:
I believe it's irrational and irresponsible to have _1 be |x,|, it's just going to lead to nasty bugs.

Try to convince me otherwise.
If not, in one week I want to apply this change.

From the discussion in https://bugs.ruby-lang.org/issues/15723#note-127
and in https://bugs.ruby-lang.org/issues/15708

Some reactions to this behavior in https://twitter.com/eregontp/status/1115318993299083265


Related issues

Related to Ruby master - Bug #15708: Implicit numbered argument decomposes an arrayRejectedmatz (Yukihiro Matsumoto)Actions
Related to Ruby master - Misc #15723: Reconsider numbered parametersFeedbackmatz (Yukihiro Matsumoto)Actions

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