From: Ben Tilly <ben_tilly@...>
Date: 2001-02-08T15:38:17+09:00
Subject: [ruby-talk:10549] Giving a Proc utility methods?

Here is my situation.  I have a class, call it Foo.
Foo has an attribute which is a callback.  I want to
have this callback be in the caller's context, but
there are some utility functions that I want to make
readily available.

Note that I am well aware of how to make this work
with an instance eval, but I don't want to do that
because I want to preserve the closure.

My first naive thought was to make those functions
methods in my current class.  Which doesn't work
since I have an object of another class.  (Duh, what
did I expect?)

My second, less naive, thought was to make those
functions private methods of the object.  That fails
in a way which is somewhat reasonable, I guess, but
still makes me aware that I don't understand how it
is working internally.  In particular my method is
called but generates an exception because it is a
private method.  But I can't seem to figure out how
to say that I really want it to be public after
all. :-(

I guess I could make them available in a module to
be mixed in by anyone wishing to use the class, but
I was hoping that there was a nicer approach.

Here is a sample of what I hoped would work in the
hopes that there is a simple solution I have missed:

    class Foo
      attr :err

      def initialize (&meth)
        # TRY to add the method?
        def meth.fail (msg)
          puts msg
        end
        @err = meth
      end

      def throw_err (msg)
        @err[msg]
      end
    end

    # I want to preserve this closure...
    bar = "testing..."

    # Example showing the callback works
    my_foo = Foo.new {|msg| puts msg + bar}
    my_foo.throw_err "Trial 1: "

    # Example of what I *wanted* to do...
    my_foo = Foo.new {|msg| self.fail msg + bar}
    my_foo.throw_err "Trial 2: "

(What I really wanted to do was not puts the error, but
rather throw it, and I didn't want synchronization
problems between what I label the throw and the catch
so I only wanted to define those once.)

Cheers,
Ben
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