state.rb

class State
  def initialize(&block)
    @block = block
  end

  def self.state(&block)
    State.new(&block)
  end

  def self.put(state)
    State.new do |s|
      [ nil, state ]
    end
  end

  def self.get
    State.new do |s|
      [ s, s ]
    end
  end

  def self.ret(val)
    State.new do |s|
      [ val, s ]
    end
  end

  def flat_map(&f)
    State.new do |s|
      a, new_state = @block.call(s)
      f.call(a).run(new_state)
    end
  end

  def run(initial_state)
    @block.call(initial_state)
  end
end

# 素で書いた版
#
# push = proc {|val, stack|
#   [ nil, stack + [ val ] ]
# }.curry
# 
# pop = proc {|stack|
#   [ stack[-1], stack[0..-2] ]
# }.curry
# stack = []
# _, stack = push[1, stack]
# _, stack = push[2, stack]
# _, stack = pop[stack]

push = proc {|val|
  State.new {|stack|
    [ nil, stack + [ val ] ]
  }
}.curry

pop = State.new {|stack|
  [ stack[-1], stack[0..-2] ]
}

p push[1]
  .flat_map{|_|
    push[2].flat_map{|_|
      push[3].flat_map {|_|
        pop
      }
    }
  }
  .run([]);