From: Brian Candler <B.Candler@...>
Date: 2004-10-21T04:47:59+09:00
Subject: Re: each with separator

> Yes of course, but it's not imperative and I'm looking for an imperative
> version like the Smalltalk one.

My inclination would be to start simple:

  module Enumerable
    def each_with_separator(sepblk)
      first = true
      each do |item|
        sepblk.call unless first
        first = false
        yield item
      end
    end
  end
  
  [1,2,3].each_with_separator(proc { puts "-" }) {|obj| puts obj }

But it's admittedly a bit ugly passing two blocks in this way. If you want
you can create an intermediate object which holds the separator block and
performs the iteration:

  module Enumerable
    def separate_by(&blk)
      Separator.new(self,blk)
    end
    class Separator
      include Enumerable  # eat your own tail :-)
      def initialize(thing,sepblk)
        @thing = thing
        @sepblk = sepblk
      end
      def each
        first = true
        @thing.each do |item|
          @sepblk.call unless first
          first = false
          yield item
        end
      end
    end
  end

  [1,2,3].separate_by { puts "-" }.each {|obj| puts obj}

I'm not sure if that's a general enough pattern for you. For example, an
Enumerable::Separator is enumerable, but you can't use 'collect' to collect
the objects and the separators.

Perhaps you want to 'yield' the separator instead?

  module Enumerable
    def separate_by2(separator)
      Separator2.new(self,separator)
    end
    class Separator2
      include Enumerable
      def initialize(thing,separator)
        @thing = thing
        @separator = separator
      end
      def each
        first = true
        @thing.each do |item|
          yield @separator unless first
          first = false
          yield item   
        end
      end
    end
  end

  [1,2,3].separate_by2("-").each {|obj| puts obj}

  a = [1,2,3].separate_by2("-").collect {|obj| obj * 2}
  #=> returns [2, "--", 4, "--", 6]

Regards,

Brian.