From: Chuck Remes <cremes.devlist@...>
Date: 2011-12-04T06:31:45+09:00
Subject: help with recursive method def'n using eval

I would like to solve a problem where a writer of a class can provide an array of fields and those fields are turned into two things.

1. A method called #each_"field_name" that iterates over all of the values in that field.

2. A method called #each_combination that defines a series of nested called to #each_"field_name" and at the inside yields a combination of the parameters.

I have the code to solve #1. That was a pretty straight-forward exercise. See the code below (save it to a file and run it).

module Parameters
  module Base
    def self.create_each(mod, fields)
      fields.each do |field_name|
        code = <<-code
        def each_#{field_name}
          @params['#{field_name}'].each do |value|
            @#{field_name}_current_value = value
            yield(value)
          end
        end
        code

        p code
        mod.class_eval(code)
      end
    end

  end # Base
end # Parameters

if __FILE__ == $0

  class Test
    include Parameters::Base

    def self.fields
      ['dollars', 'activation_time', 'personnel_count']
    end

    Parameters::Base.create_each(self, fields)
    
    def initialize
      @params = {
        'dollars' => [1_000, 5_000, 75_000],
        'activation_time' => ["06:30", "08:00", "10:00"],
        'personnel_count' => [1, 3, 7, 25]
      }
    end
  end
end

I'm stuck on solving #2. I want to nest each call to #each_'field_name' in the order given in the #fields class method. The innermost call should yield each of the block parameters.

Given the details above, I would like to produce a method like this:

def each_combination(&blk)
  each_dollars do |dollars|
    each_activation_time do |activation_time|
      each_personnel_count do |personnel_count|
        yield(dollars, activation_time, personnel_count)
      end
    end
  end
end

Any hints on how I can accomplish #2 here? I tried a few approaches similar to my #create_each metaprogramming, but I can't get the syntax right.

cr