Why does an empty ERB file cause a warning in Ruby 1.8.7?

Question

I understand what the following warning means:

-:1: warning: useless use of a variable in void context

But I don't understand why ERB in Ruby 1.8.7 generates code that uses _erbout variable in void context:

$ rvm use ruby 1.8.7
Using /Users/radeksimko/.rvm/gems/ruby-1.8.7-head
$ touch test.erb
$ erb -x test.erb
_erbout = ''; _erbout
$ erb -x test.erb | ruby -w
-:1: warning: useless use of a variable in void context

This is not a problem in ERB / Ruby 2.0.0+ though as ERB generates code from the template differently:

$ rvm use 2.0.0
Using /Users/radeksimko/.rvm/gems/ruby-2.0.0-p598
$ erb -x test.erb
#coding:ASCII-8BIT
_erbout = ''; _erbout.force_encoding(__ENCODING__)
$ erb -x test.erb | ruby -w
$

To be clear, this has nothing to do with _ (underscores) treating in variable names in between Ruby versions:

$ rvm use 2.0.0
Using /Users/radeksimko/.rvm/gems/ruby-2.0.0-p598
$ echo "erbout = ''; erbout" | ruby -w
-:1: warning: possibly useless use of a variable in void context
$ rvm use 1.8.7
Using /Users/radeksimko/.rvm/gems/ruby-1.8.7-head
$ echo "erbout = ''; erbout" | ruby -w
-:1: warning: useless use of a variable in void context

Is this a bug that should be reported to Ruby/ERB core or am I just misunderstanding something?

@MarekLipka That depends on a library you're using I guess... — Radek Simko, Feb 25 '15 at 07:29
um, I meant project/tool/anything you're working on or contributing to that uses Ruby 1.8.7 for any reason or maintains compatibility with it. — Radek Simko, Feb 25 '15 at 07:37
So I meant Ruby 1.8.7 is no longer maintained by Ruby team. Reporting bugs to it is therefore pointless. https://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/ — Marek Lipka, Feb 25 '15 at 07:38
Well.. Now you can file a BUG(as you have reproduced it in 2.0.0).. Lets see what they say. The problem lies in ERB.. — Arup Rakshit, Feb 25 '15 at 07:51

Uri Agassi · Accepted Answer · 2015-03-05T14:50:44.097

The warning is caused by the second line:

_erbout = '';_erbout

which does nothing (_erbout is a local variable in that scope), and it is in a context where is does not return the value of the line (like in a last line of a method).

In Ruby 2.0.0 this line is replaced with

_erbout = '';_erbout.force_encoding(__ENCODING__).

Now ruby does not know for sure if the method call has any side-effects or not, so the warning is not raised.

You can reproduce this with the following code:

useless.rb

def test_me
  unused = 1
  unused
  3
end

p test_me

$ ruby -w useless.rb
useless.rb:3: warning: possibly useless use of a variable in void context
3

All this happens, because the output of erb -x is not supposed to be run on its own. When running a ruby script, the last line is not used as return value, unlike a method.

If you embed the code in a program, and actually use the _erbout, the warning will not be shown.

Great answer, thanks! This is the most important bit: "All this happens, because the output of erb -x is not supposed to be run on its own." — Radek Simko, Mar 08 '15 at 15:26

Why does an empty ERB file cause a warning in Ruby 1.8.7?

1 Answers1