From: "Morris, Chris" <chris.morris@...>
Date: 2002-05-22T21:51:28+09:00
Subject: RE: Rewrite this without using eval...

> --
> class Foo
>   def initialize
>     @a = 1
>     @b = 2
>   end
> 
>   def inc_iv
>     [:@a, :@b].each { |iv|
>       eval "#{iv} += 1"
>     }
>   end
> end
> --
> 
> Could someone suggest a way to rewrite "inc_iv" without using 
> eval, without
> turning the guts of the each block into a method, and without 
> flattening the
> [:@a, :@b].each{} construct?

It's interesting (and presumably you're aware of all this, but I'll ramble
on cuz I've learned something here doing this...). If @a and @b reference
objects that are themselves modified, then inc_iv can become:

  def inc_iv
    [@a, @b].each { |s|
      s.do_thing! 
    }

like:

C:\temp>type rb.rb
class Foo
  def initialize
    @a = 1
    @b = 2
    @c = 'tomato'
    @d = 'cucumber'
  end

  def inc_iv
    [:@a, :@b].each { |iv|
      eval "#{iv} += 1"
    }
  end

  def rev_str
  [@c, @d].each { |s|
    s.reverse!
  }
  end

  def dump
    p @c, @d
  end
end

f = Foo.new
f.rev_str
f.dump

C:\temp>rb
"otamot"
"rebmucuc"

s in its block is pointed to the same object that @c is pointed to, and the
reverse! call does not redirect to a new object, but modifies the existing
object.

iv in its block is pointed to the same object that @a is pointed to, but the
+= operator always returns a new object and re-points the variable (iv) to
the new object, while @a stays pointed to its original object.

I hacked together MyNumeric to wrap a Numeric to allow the inc_iv to work --
it avoids eval, but requires a bit of work and most probably isn't a
sufficient wrapper. (As an aside, just having the ability to so quickly do a
hack like this is very cool -- Ruby is fun). 

C:\temp>type rb3.rb
class MyNumeric
  def initialize(numeric)
    @x = numeric
  end

  def inspect
    @x
  end

  def method_missing(methid, *args, &block)
    @x = @x.send(methid, *args, &block)
    self
  end
end

class Foo
  def initialize
    @a = MyNumeric.new(1)
    @b = MyNumeric.new(2)
  end

  def inc_iv
    [@a, @b].each { |iv|
      p iv.type.name    if ARGV.include?('-d')
      p iv.id           if ARGV.include?('-d')
      iv += 1
      p iv.type.name    if ARGV.include?('-d')
      p iv.id           if ARGV.include?('-d')
    }
  end

  def dump
    p @a, @b
  end
end

f = Foo.new
f.inc_iv
f.dump

C:\temp>rb3
2
3

C:\temp>rb3 -d
"MyNumeric"
22238768
"MyNumeric"
22238768
"MyNumeric"
22238756
"MyNumeric"
22238756
2
3

Chris