From: Brian Candler <b.candler@...>
Date: 2009-11-19T23:32:08+09:00
Subject: Re: How to strip ruby comments in a ruby line of code?

Alexandre Mutel wrote:
> I could put a newline after the match like this :
>     newLabel(:my_label) { here_is_a_dsl(arg1, arg2) # my comments
>     }
> 
> Unfornutately, i'm no longer able to debug my dsl language, because the
> lines are not matching the preprocessed line.

However if you eval each line individually, then you can pass in the 
source line number.

def foo; end
src = "foo\nfoo\nbar"
src.each_with_index do |line,i|
  eval "#{line} {\n}", binding, "DSL", i+1
end

# Result:
DSL:3: undefined method `bar' for main:Object (NoMethodError)

Otherwise, if every input line maps to exactly two output lines, you can 
just patch up the line number in the exception by dividing by two.

src = "foo\nfoo\nbar\n"
begin
  eval src.gsub(/\n/, "{\n}\n"), binding, "DSL", 1
rescue => e
  if e.backtrace.first =~ /\A(.*):(\d+)\z/
    e.backtrace.first.replace "#{$1}:#{($2.to_i+1) / 2}"
  end
  raise e
end
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