From: John Conti <john@...>
Date: 2008-05-02T14:35:26+09:00
Subject: Method lookup from a Lambda

Hi all,

I am not sure if my question is relatively obvious or instead arcane.
Being new to ruby, I am trying out some of the metaprogramming features
to generate sets of orthogonal methods for decoding various string
types. I want to provide these methods as mix-ins so many of my classes
can decode these specialized strings.  The methods have lots of
repetitive code and are easily reduced using lambdas (think DRY).

However, all has not gone as I expected.  This works:

  module Fix
    def self.manage()
      lambda do
        what = foo? ? "fix" : "leave"
        "Let's " + what + " it"
      end
    end

    module_eval %{define_method :maintenance, manage}
  end

  class X
    include Fix
    def foo?; true; end
  end

  class Y
    include Fix
    def foo?; false; end
  end

  puts X.new.maintenance
  puts Y.new.maintenance

  # Let's fix it
  # Let's leave it

This does not:

  module Fix
    def self.doer()
      lambda do
        foo? ? "fix" : "leave"
      end
    end

    def self.manage()
      grunt = doer
      lambda do
        "Let's " + grunt.call + " it"
      end
    end

    module_eval %{define_method :maintenance, manage}
  end

  class X
    include Fix
    def foo?; true; end
  end

  class Y
    include Fix
    def foo?; false; end
  end

  puts X.new.maintenance
  puts Y.new.maintenance

  # lambda2.rb:4:in `doer': undefined method `foo?' for Fix:Module
(NoMethodError)
  #      from lambda2.rb:10:in `call'
  #      from lambda2.rb:10:in `maintenance'
  #      from lambda2.rb:25

In the first example, the call to the method the mixin module relies on
gets called without problem.  I. e. the instance method of the mixin
finds the object's instance method.  However, in the second example,
methods are not looked up the same way, the module is searched and so it
fails.

What about the second example's closure on the lambda is causing
method's to be looked up differently?  I do not feel like I have a good
mental model of the method lookup and scoping rules of Ruby.  The latest
book I've looked at (Programming Ruby) doesn't seem to shed much light
on my issue.

Can anyone give me an explanation to help?

Thank you,
John
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