[ruby-talk:7951] Re: defined? problem?

On Fri, 22 Dec 2000, Stephen White wrote:

> On Fri, 22 Dec 2000, Dave Thomas wrote:
> 
> > The two options above differ in that in one the condition textually
> > precedes the 'something' and in the other it follows it. Variables
> > first assigned to in the 'something' will therefore be available to
> > the condition in the modifier form, but not in the conventional form.
> 
> Simplest example:
> 
>   irb(main):007:0> b
>   NameError: undefined local variable or method `b' for #<Object:0x40181ce0>
>   (irb):7:in `irb_binding'
>   irb(main):008:0> b = 6 if false
>   nil
>   irb(main):009:0> b
>   nil
> 
> whereas I was expecting another "undefined local variable" error.

The behavior I find most surprising -- even given the underlying
principle of defining a variable upon seeing an assignment to it -- is
illustrated by this:

   irb(main):003:0> if m then m = 123 end
   NameError: undefined local variable or method `m' for #<Object:0x4017fc90>
   (irb):3:in `irb_binding'
   irb(main):004:0> if m then m = 123 end
   nil

Having rejected the first "if m", I would expect the interpreter not
to have reached the first "m = 123" -- so I don't see why the second
"if m" doesn't raise the same exception as the first.

Compare with:

   irb(main):001:0> if m then end
   NameError: undefined local variable or method `m' for #<Object:0x4017fc90>
   (irb):1:in `irb_binding'
   irb(main):002:0> if m then end
   NameError: undefined local variable or method `m' for #<Object:0x4017fc90>
   (irb):2:in `irb_binding'

where the exception is raised both times.  So it's clearly the
assignment that's defining m, even though the preceding "if" raises an
exception.   

In case this was just an irb thing, I tried:

   begin
     if g then g = 123 end
   rescue NameError
     puts "Rescuing: #{$!}"
     begin
       if g then g = 123 end
       puts "Made it here: g is #{g.inspect}"
     rescue NameError
       puts "Rescuing second level:\n\t#{$!}"
     end
   end

and got

   Rescuing: undefined local variable or method `g' for #<Object:0x4017fc90>
   Made it here: g is nil

Changing both "g = 123"s to "true" above gave me:

   Rescuing: undefined local variable or method `g' for #<Object:0x4017fc90>
   Rescuing second level:
           undefined local variable or method `g' for #<Object:0x4017fc90>

This feels like a rather dramatic (for Ruby) compile-time vs. runtime
contrast.


David

-- 
David Alan Black
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