From: Chuck Remes <cremes.devlist@...>
Date: 2011-12-04T06:44:58+09:00
Subject: Re: help with recursive method def'n using eval


On Dec 3, 2011, at 3:31 PM, Chuck Remes wrote:

> 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.
[snip]
> 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

I should have waited a few more minutes. After typing this up, I was hit by inspiration. Here is my solution.

    def self.create_combination(mod, fields)
      processed = []
      string = "def each_combination(&blk)\n"
      
      fields.each_with_index do |field_name, index|
        string << ("  " * (index + 1)) + "each_#{field_name} do |#{field_name}|\n"
        processed << field_name
      end
      
      string << ("  " * (processed.size + 1)) + "yield(#{processed.join(', ')})\n"
      processed.size.downto(1) { |i| string << ("  " * i) + "end\n" }
      puts string
    end


It's a bit messy (I do extra work so that the method is formatted correctly for the call to #puts), but it produces exactly what I need. I hope this is helpful to someone else too.

cr