[ruby-talk:7954] Re: defined? problem?

David Alan Black <[e-mail]> writes:

> 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

Let's play Ruby during the parsing stage (remember, we're not
executing code at this point)

   if                        <keyword, I'm looking for some
                              kind of expression>
      m                      <name. I haven't seen an assignment,
                              so it's a method call>
        then                 <keyword>
             m = 123         <assignment to m. remember the fact that
                              m is a variable for future reference> 
                     end


   if                        <keyword, I'm looking for some
                              kind of expression>
      m                      <name. I've seen 'm' before. Let me see..
                              yup, it's a variable, because it was
                              assigned to>

   etc...


Now we'll play Ruby during execution:

   if m then m = 123 end    < OK, I need to call method 'm' to
                              evaluate the condition. Send 'self'
                              the message :m. Whoops! undefined
                              method.>

   if m then m = 123 end    < OK, I need to look up the value of the
                              variable 'm'. It's nil. The if doesn't
                              get executed>


For what it's worth, I agree that it's an ugly situation, but wihout
adding declarations, or adding required parentheses to every method
call, I'm not sure how to get around it (apart possibly from getting
rid of local variables and making them all attribute methods of the
current binding).


Regards


Dave