From: Stefano Crocco <stefano.crocco@...>
Date: 2012-10-27T23:36:10+09:00
Subject: Re: Newbie, questions about === expression

On Saturday 27 October 2012 月忧茗 wrote
> Hi,
> 
> I'm learning ruby this days.
> And I followed this docs:  http://www.ruby-doc.org/docs/ProgrammingRuby/
> 
> 
> Now, I have some questions about ===  expression
> 
> 
> irb(main):138:0> case 1
> irb(main):139:1> when 1..10 then 'in'
> irb(main):140:1> else 'not in'
> irb(main):141:1> end
> 
> obviously, the result is 'in'
> 
> In the docs, there are an explanation:
> *case operates by comparing the target (the expression after the keyword
> case) with each of the comparison expressions after the when keywords. This
> test is done using comparison === target. As long as a class defines
> meaningful semantics for === (and all the built-in classes do), objects of
> that class can be used in case expressions. *
> 
> 
> But,  when I type this,  why throw an error ?
> 
> irb(main):149:0> 1..10 === 1
> ArgumentError: bad value for range
>     from (irb):149
>     from /opt/ruby-1.9.3/bin/irb:12:in `<main>'
> 
> 
> 
> Thanks.

You need to put parentheses around the range:

(1..10) === 1

otherwise ruby understands the expression as if it were
1..(10 === 1)

that is, as

10..false,

which, of course, is not a valid range.

I hope this helps

Stefano