From: Brian Candler <B.Candler@...>
Date: 2003-04-25T00:34:48+09:00
Subject: Re: How to call an object instance's method?

On Fri, Apr 25, 2003 at 12:10:15AM +0900, Rene Tschirley wrote:
> b.funB
> 
> funB fills the list 'aList' with some instances of A:
...
>     if @aList.empty?
>       # in fact, i check if array size is big enough and
>       # do the append if it doesn't
>       @aList = @aList + Array.new(1, A)
>     end

That's not what you've done; you have filled aList with the *class* A. Try
instead:

        @aList = @aList + [A.new]

or more Rubyesque,

        @aList << A.new

and everything is fine.

Useful debugging tip: try using 'p' in your code. e.g.

        @aList << A.new
        p @aList

This is more or less the same as puts @aList.inspect

With your version the output would be
[A]                        <<<<< element is the class A

with the modified version you will get something like:
[#<A:0x810d258>]           <<<<< an instance of A


>     puts @aList.type ? Array
> 
> I understand, so far.
> 
>     puts (@aList).type ? A
> 
> This is confusing. Why the difference? Did I miss some special meaning of 
> "()"?

Yes... it's the "poetry mode" ambiguity, and a bit nasty. Basically if you
put an open bracket after a method call, it thinks you are bracketing the
method parameters, and the rest of the expression is outside. So what you
have there is:

     ( puts (@aList) ).type

i.e. you are printing the array @aList itself, not its type. The return
value of puts is 'nil', and you are then getting the type of it (which is
NilClass), and then throwing that information away.

Use the -w flag to Ruby:

    #!/usr/local/bin/ruby -w

or 

    ruby -w <scriptname>

and a lot of this sort of bug will become apparent much more easily.

I think Matz has hinted in the past that perhaps -w should be mandatory, and
you would have a different flag to turn it off. I think I agree :-)

Regards,

Brian.