From: Bob Hutchison <hutch@...>
Date: 2001-12-08T05:40:06+09:00
Subject: [ruby-talk:27855] Method creation question

Hi,

I'm encountering a bit of difficulty and have managed to confuse myself (yet
again :-)

I'd like to write a function that behaves like 'attr' defining two accessor
functions. So, instead of:

    class Foo
        attr :bar
    End

I'd like to write:

    class Foo
        myAttr :bar
    end

and have two accessor functions added to class Foo.

Is this possible?

The trouble is that defining the method myAttr requires nesting defs, and
this isn't allowed. I thought about procs, but I cannot spot how you'd add a
proc to a module/class/object as a method.

I've looked around the ruby distribution, 'attr' is defined in one of the C
files, so it doesn't provide any clues (to me at least).

So this leaves me with eval, as far as I can tell. I've written a module (in
util.rb) with this method in it:

    module Util
        def Util.myAttr(target, name)
            doThis = <<EOS
                def (Module.const_get(:#{target})).#{name}
                    printf("HELLO #{name}\\n");
                end
EOS
            eval doThis
        end
    end

And this is used (in foo.rb) like this:

    require "util"
    
    module FooMixin
        include Util

        Util.myAttr(:FooMixin, "myMethod")
    end

    class Foo
        include FooMixin
    end

Now, if I do this in yet another file and run it:

    #!/usr/local/bin/ruby

    require "foo"

    f1 = Foo.new

    puts Module.const_get(:FooMixin).methods.inspect
    puts f1.methods.inspect

    f1.myMethod

A few things happen:
1) the Module...inspect shows myMethod as defined
2) f1.methods.inspect does *not* show myMethod as defined
3) f1.myMethod fails because ruby cannot find myMethod in f1.

So, some questions:

A) where is myMethod being defined? Where should it be defined?
B) is there a better way to do this?

Thanks,
Bob