Customer = Struct.new(:name, :address, :city)

Customers = [
  Customer.new("Mr. Foo", "6th Ave", "New York"),
  Customer.new("Mr. Bar", "8th Ave", "Washington D.C"),
  Customer.new("Ms. Quux", "42th Ave", "New York"),
]

class Statement
  attr_reader :next

  def initialize
    @next = nil
  end

  def >>(other)
    @next = other
  end

  def call
    raise NotImplementedError
  end

  def pass_on(*args)
    @next.call(*args)
  end

  def results
    if @next
      @next.results
    else
      raise "No results available."
    end
  end
end

class Scan < Statement
  def initialize(table)
    super()
    @table = table
  end

  def scan
    @table.each { |row| pass_on row }
    self
  end
end

class Limit < Statement
  def initialize(limit)
    super()
    @limit = limit
    @count = 0
  end

  def call(row)
    if @count < @limit
      @count += 1
      pass_on row
    end
  end
end

class Where < Statement
  def initialize(&clause)
    @clause = clause
  end

  def call(row)
    if @clause.call(row)
      pass_on row
    end
  end
end

class Collect < Statement
  attr_reader :results

  def initialize(&clause)
    @clause = clause
    @results = []
  end

  def call(row)
    @results << @clause.call(row)
  end
end

(query = Scan.new(Customers)) >>
  Where.new { |c| c.city == "New York" } >>
  Limit.new(5) >>
  Collect.new { |c| [c.name, c.address] }
p query.scan.results