From: Michael Edgar <adgar@...>
Date: 2011-08-22T08:45:41+09:00
Subject: Re: Possble bug in Ruby parser (Fixnum#times within "case" statement)

On Aug 21, 2011, at 7:12 PM, Iņaki Baz Castillo wrote:

> Hi, I cannot find an explanation for the following issue so I think it's a bug:
> 
> -----------------------
> def kk
> 
>  while case @state
> 
>    when :init
>      10.times do |index|
>        puts "index = #{index}"
>      end
>      @state = :end
>      true
> 
>     when :end
>       puts "END"
> 
>     end
>   end
> 
> end
> 
> @state = :init
> kk
> -----------------------
> 
> 
> Running the above code gives an error:
> 
> ~# ruby1.9.3 kk.rb
> 
> kk.rb:6: syntax error, unexpected keyword_do_cond, expecting keyword_end
>      10.times do |index|
>                 ^
> kk.rb:7: syntax error, unexpected tSTRING_BEG, expecting keyword_do or
> '{' or '('
>        puts "index = #{index}"
>              ^
> kk.rb:12: syntax error, unexpected keyword_when, expecting $end
>     when :end
>         ^
> 
> 
> Why???? If I replace:
> 
>      10.times do |index|
>        puts "index = #{index}"
>      end
> 
> with:
> 
>      10.times { |index|
>        puts "index = #{index}"
>      }
> 
> but of course using "do - end" is valid for Fixnum#times !!
> What is happening?

"while" cannot take braces, only do-end. Your entire `case` expression is the clause of the while loop, so the "do" is begin confused for the "do" that optionally comes between the "while"'s condition and its body. Since a brace never comes between a while condition and a while body, there's no ambiguity.

Ruby's grammar is expressive, but when you do things like put many-line case statements in a while condition, you're going to get weird corner cases in the grammar.

Michael Edgar
adgar@carboni.ca
http://carboni.ca/