[ruby-talk:7979] Re: Naming of "defined?"

craig duncan wrote:
> 
> Didn't i read that '?' at the end of a method name indicated a boolean
> return value?
> 
> defined? returns the symbol type, not a boolean.
>

Well, first of all, "defined?" is not a method.

cf. ruby-talk: 01637, where matz answers Quinn Dunkan:
>>
|Ah, this makes more sense, defined? isn't a method at all.  I thought
|everything was a message send in ruby? :)  It *could* be "just another message
|send" if you spelled it 'defined? :foo' or 'defined? "foo"'.  So why not?

No it isn't.  It's a control structure.  No everything is a message
send in Ruby, e.g. control structures, variables, blocks are not
objects.  `defined? ' is among these things.

`defined?' shows

  * if a global variable is defined
  * if a local variable is defined
  * if a instance variable is defined
  * if a constant is defined
  * if a method is defined
  * if a block is supplyed
  * if a method of same name in superclass is defined

all of which is not accomplished by a method.
<<

 
> Therfore, this sort of confusion:
> 
> craig@duncan:/usr/home/craig/ruby$ cat t.rb
> #!/usr/bin/ruby
> 
> def i
>   p "defining i as method"
> end
> p defined? i
> 
> i = 0 if not defined? i
> p defined? i
> 
> craig@duncan:/usr/home/craig/ruby$ t.rb
> "method"
> "local-variable"


I think your confusion is in 
1) thinking defined? is a method
2) thinking methods ending in '?' have to be boolean

Both of which are false.

I had a problem on page 221 of Programming Ruby,
where it indicates that "defined?" is a method... Dave,
could you explain? (Maybe I should have said something sooner)

Guy N. Hurst


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