From: Robert Klemme <shortcutter@...>
Date: 2009-12-31T20:05:12+09:00
Subject: Re: Why can't I use "or" here?

On 12/31/2009 03:43 AM, Seebs wrote:
> On 2009-12-31, Phillip Gawlowski <pg@thimian.com> wrote:
>> Hm, on Ruby 1.9.1 I get:
>> irb(main):001:0> puts nil or 4
>>
>> => 4
> 
> Note that this is equivalent to "puts nil" followed by "4".
> 
> That's significant.
> 
> Consider:
> 	irb(main):001:0> x = nil or 4
> 	=> 4
> 	irb(main):002:0> x
> 	=> nil
> 
> The grouping is:
> 	(x = nil) or (4)
> 
> Or
> 	(puts nil) or (4)
> 
>> This seems like a bug to me, since parenthesis *should* "only" make 
>> method calls and precedent unambigious.
> 
> I believe that's precisely the problem -- the precedence of "or" is
> low enough that it can't occur inside a method argument.
> 
> Compare this with the similar situation in C, where a comma operator
> cannot occur inside an argument list for a function, because it's part
> of the function-call syntax.  So:
> 
> valid C:
> 	x = 1, y = 2;
> 
> (this performs both assignments, and returns the value of y after the
> assignment, which happens to be 2.)
> 
> printf("%d\n", x = 1, y = 2);
> 
> This probably prints 1, but I think it's undefined behavior because there's
> excess arguments to printf.  (I'm not sure whether that's permitted or not,
> but my guess would be "no, but it probably always works".)
> 
> So try:
> 
> 	irb(main):003:0> puts( (nil or 4) )
> 	4
> 	=> nil
> 
> Basically, if you really want parentheses (as in the precedence-changing
> operator), you need to include them... Not merely put something that can't
> go in a method argument list inside the confusingly-similar-looking
> () which surround method arguments.

And you can even omit the method brackets as in

irb(main):001:0> puts (nil or 4)
4
=> nil

Kind regards

	robert



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