Re: why is else without rescue useless with statement modifiers?

On Fri, May 18, 2012 at 06:25:19AM +0900, roob noob wrote:
> def sane?
> sane = "true" if "a" == "a" else sane = "false"
> end
> 
> puts sane?
> 
> =>warning: else without rescue is useless
> =>false

Others have touched on this, but for more detail, I'll try to explain.

You are using an inline "if" here:

    (do something) if (condition)

That usage of "if" does not accpet an "else".  If you want to have an
"else" clause with your if, you need to test the condition first.  One
form is, of course, the other example you provided:


> 
> def sane?
> if "a" == "a" then sane = true else sane = "false"
> end
> end
> 
> puts sane?
> 
> =>true

. . . except that the assignments to the "sane" variable is meaningless
here.  What you probably want is either this:

    def sane?
      if 'a' == 'a' then true else false end
    end

. . . or this (which is a bit clearer, in my opinion):

    def sane?
      if 'a' == 'a'
        true
      else
        false
      end

As mentioned by others, you could use a ternary operator:

    def sane?
      'a' == 'a' ? true : false
    end

. . . or just directly return the result of the comparison:

    def sane?
      'a' == 'a'
    end

It's also worth noting that your first example returns strings of "true"
and "false", while your next example (as well as all of my alternatives
to it) returns the true and false boolean values themselves.

Finally, the reason it is complaining about an absent "rescue" is simply
that it has no idea what your "else" is supposed to be doing in the first
example, because that form of an "if" conditional does not accept an
"else" condition.  As such, your Ruby implementation is guessing at what
you meant the "else" to do.  It happens to be guessing incorrectly in
this case.

If you do not wrap your conditional expression in a method, however, you
should get a different error message, as shown in my irb test:

    >> true if 'a' == 'a' else false
    SyntaxError: (irb):2: syntax error, unexpected keyword_else, expecting $end
    true if 'a' == 'a' else false

I hope that helps.

-- 
Chad Perrin [ original content licensed OWL: http://owl.apotheon.org ]