Lazy Seqs in Ruby

lazy.rb

# Abstract Class
# ==============
class ISeq
  def first
  end

  def rest
  end

  def [](idx)
    seq = self
    idx.times {
      seq = seq.rest
    }
    seq.first
  end

  def to_s
    ret = "["
    seq = self
    while seq != nil
      ret += seq.first.to_s + ", "
      seq = seq.rest
    end
    ret[-2..-1] = "]"
    ret
  end
end


# Linked List
# ===========
class Cell < ISeq
  def initialize(head, tail)
    @head = head
    @tail = tail
  end

  def first
    @head
  end

  def rest
    @tail
  end
end

c_list = Cell.new(1, Cell.new(2, Cell.new(3, nil)))
puts "Linked List: #{c_list}"


# Lazy List
# =========
class LazySeq < ISeq
  def initialize(head, rest_fn)
    @head = head
    @rest_fn = rest_fn
  end

  def first
    @head
  end

  def rest
    @rest_fn.call
  end
end

l_list = LazySeq.new 1, ->() { LazySeq.new 2, ->() { LazySeq.new 3, ->() { nil }} }
puts "Lazy List: #{l_list}"


# Lazy methods for ISeq
# =====================
ISeq.class_eval do
  def take(n)
    if n > 0
      LazySeq.new first, ->() do
        rest.take n-1
      end
    else
      nil
    end
  end

  def map(f)
    LazySeq.new f.call(first), ->() do
      tail = rest
      if tail
        tail.map f
      else
        ->() { nil }
      end
    end
  end
end


# Utils
# =====
def iterate(f, initial)
  LazySeq.new initial, ->() {
    iterate f, f.call(initial)
  }
end

wholes = iterate ->(x) { x+1 }, 0  # whole numbers
evens = wholes.map ->(x) { 2*x }   # even numbers
puts "Mapped Lazy List: #{evens.take 5}"