From: Greg Weeks <greg.weeks@...>
Date: 2007-11-19T09:22:07+09:00
Subject: Re: eval, module_eval, define_method, and all that

In this forum, I've mentioned several times that some people don't like
"eval".  Here is the end of section 11.3.5 from "The Ruby Way":

  In previous versions of Ruby, we often defined methods at runtime by
  calling "eval".  In essence, "define_method" can and should be used in
  all these circumstances.

"ALL these circumstances".  That's strong stuff.  Well, I'm ready to bow
to authority.  But how can I see "define_method" as loveable?  How about
this: "define_method" is Minimally Magical.

Here's what I mean.  Given the name "define_method", we naturally
expect "foo" and "bar" below to have the same behavior:

CONST = "Object"

class A
  CONST = "A"
  @val = "A"
  attr_accessor :val
  def foo
    [CONST, @val, self]
  end
  define_method :bar do
    [CONST, @val, self]
  end
end

a = A.new
a.val = "a"
p a.foo  ->  ["A", "a", #<A:0x401c24fc @val="a">]
p a.bar  ->  ["A", "a", #<A:0x401c24fc @val="a">]

If "define_method" was not magical, then, within its block argument, the
values of CONST and @val and self would be "A" and "A" and A (the class
object).  The minimal magic required to get the desired answer is to
make @val and self refer to the A instance, but leave CONST to scope
lexically.  And that seems to be what "define_method" does.

I personally will follow "The Ruby Way" (second edition).  Stand behind
me, *eval methods!
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