Bug #16496: Numbered Parameter not parsed properly in lambda

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-darwin18]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[JustinFeng (Justin Feng)]

The space between * and 1 changes the lambda behaviour, looks like numbered parameter _1 is not parsed properly

2.7.0 :001 > l = -> { _1 * 1 }
2.7.0 :002 > l[2]
 => 2
2.7.0 :003 > l2 = -> { _1 *1 }
2.7.0 :004 > l2[2]
Traceback (most recent call last):
        5: from /Users/justin.feng/.rvm/rubies/ruby-2.7.0/bin/irb:23:in `<main>'
        4: from /Users/justin.feng/.rvm/rubies/ruby-2.7.0/bin/irb:23:in `load'
        3: from /Users/justin.feng/.rvm/rubies/ruby-2.7.0/lib/ruby/gems/2.7.0/gems/irb-1.2.1/exe/irb:11:in `<top (required)>'
        2: from (irb):4
        1: from (irb):3:in `block in irb_binding'
ArgumentError (wrong number of arguments (given 1, expected 0))
2.7.0 :005 >

[mame (Yusuke Endoh)]

• Status changed from Open to Rejected

_1 *1 is parsed as _1(*1), a call to a method _1 with a variable-length argument with 1. It is the same as p * ary and p *ary. You can see a warning under verbose mode:

$ ruby -w -e '-> { _1 *1 }.call'
-e:1: warning: `*' interpreted as argument prefix
...

It is difficult to change until a method name _1 is valid.

[JustinFeng (Justin Feng)]

mame (Yusuke Endoh) wrote:

_1 *1 is parsed as _1(*1), a call to a method _1 with a variable-length argument with 1. It is the same as p * ary and p *ary. You can see a warning under verbose mode:

$ ruby -w -e '-> { _1 *1 }.call'
-e:1: warning: `*' interpreted as argument prefix
...

It is difficult to change until a method name _1 is valid.

Hi Yusuke, thank you for the reply, good to understand what caused the issue

However, it just makes me feel the numbered parameter behaves different from traditional block parameter

2.7.0 :001 > l = lambda {|p| p *1 }
2.7.0 :002 > l[2]
 => 2 
2.7.0 :003 > l = lambda {|p| p * 1 }
2.7.0 :004 > l[2]
 => 2 

[JustinFeng (Justin Feng)]

• Status changed from Rejected to Feedback

[mame (Yusuke Endoh)]

We are now migrating. If we change it immediately, a program that uses _1 as a method name will break. So it is now warned:

warning: `_1' is reserved for numbered parameter; consider another name

After enough time, we will be able to prohibit _1 as a method name, and _1 can be always used as a numbered parameter. So please wait for a few years.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Feedback to Rejected