From: Greg Weeks <greg.weeks@...>
Date: 2007-11-19T07:59:37+09:00
Subject: Re: eval, module_eval, define_method, and all that

My interest (obsession?) with methods defining methods partly derives
from the fact Ruby got a huge boost (via Rails) because of it.
Unfortunately, Ruby requires *three* special behaviors to make it work,
and these behaviors are provided nonorthogonally.  The following fails
for three reasons.  (Please forgive how silly it looks at this point.)

class A
  class <<A
    def my_attr_reader var    # 'var' will be bound to :foo.
      def var
        "@"+var.to_s
      end
    end
  end
  my_attr_reader :foo
  attr_writer :foo
end

1.  In "def foo ; @foo ; end", "foo" and "@foo" are syntax literals.
Neither can be replaced with an expression.

2.  The inner "def" is installed in <<A rather than in A.

3.  Ruby tends to scope lexically rather than dynamically.  "@foo" is
likely to be scoped as an instance variable of the A class rather than
of A instances.  Similarly, a referenced constant is likely to be looked
up in <<A rather than A.

Problem #1 can be solved either by evaluating strings or by using
"define_method", "instance_variable_get", and such.

Problem #2 can be solved using "module_eval" and/or "define_method".

Problem #3 is solved (I think) by a combination of "module_eval" and
string evaluation.

"module_eval" with a string argument seems like the best technique from
an ease-of-use point of view.  But string evaluation rubs some people
the wrong way.  On the other hand, "module_eval" (or "define_method")
with a block argument scopes constants lexically and instance variables
dynamically:

CONST = "Object"

$block = proc do
  def get_stuff
    [@foo, CONST, self]
  end
end

class A
  CONST = "A"
  class <<A
    CONST = "<<A"
    def def_get_stuff
      module_eval &$block
    end
  end
  attr_accessor :foo
  def_get_stuff
end

a = A.new
a.foo = 22
p a.get_stuff  ->  [22, "Object", #<A:0x401c2448 @foo=22>]

I don't know if this result is intrinsically messy or if I'm looking at
it from the wrong angle.
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