donotation.rb

class DoNotation

  class DSL

    def initialize(&block)

      @first_lets = []
      @proc_sources = []
      @variables = Hash[]

      # evaluate DSL
      instance_eval(&block)

      bind = lambda do |head_source, tail_sources, get_symbol, lets|

        proc do |x|

          if get_symbol
            @variables[get_symbol] = x
          end

          lets.each do |let_item|
            @variables[let_item.symbol] = instance_eval(&let_item.block)
          end

          monad = instance_eval(&head_source.block)

          if tail_sources.length > 0
            inner_proc = bind.call(tail_sources.first, tail_sources[1..-1], head_source.symbol, head_source.lets)
            monad.flat_map(&inner_proc)
          else
            monad
          end

        end

      end

      @outermost_proc = bind.call(@proc_sources.first, @proc_sources[1..-1], nil, @first_lets)

    end

    def get(symbol, &block)
      if symbol
        self.class.send(:define_method, symbol) { @variables[symbol] }
      end
      @proc_sources << Struct.new(:symbol, :block, :lets).new(symbol, block, [])
    end

    def just(&block)
      self.get(nil, &block)
    end

    def let(symbol, &block)
      self.class.send(:define_method, symbol) { @variables[symbol] }
      item = Struct.new(:symbol, :block).new(symbol, block)
      if @proc_sources.length > 0
        @proc_sources.last.lets << item
      else
        @first_lets << item
      end
    end

    def get_value()
      @outermost_proc.call
    end

  end

  include Enumerable

  def initialize(&block)
    @dsl = DSL.new(&block)
  end

  def each(&block)
    value = @dsl.get_value
    value.each(&block)
  end

end

def getLine
  Enumerator.new { |y|
    y << gets
  }.lazy
end

def putStrLn(str)
  Enumerator.new { |y|
    puts str
    y << nil
  }.lazy
end

# this notation
main = DoNotation.new {
  just { putStrLn "Enter something" }
  get(:line) { getLine }
  let(:output) { "You said #{line}" }
  just { putStrLn output }
}

# equals to this
=begin
main = putStrLn("Enter something").flat_map do |x|
  getLine.flat_map do |line|
    output = "You said #{line}"
    putStrLn output
  end
end
=end

puts "Starting..."
main.force