fmap.rb

module Kernel
  def Just(o)
    Maybe.of(o)
  end
end

class Maybe
  def self.of(value)
    new(value)
  end

  def to_s
    "Just #{@value}"
  end
  
  def inspect
    to_s
  end

  private
  def initialize(value)
    @value = value
  end

  public
  Nothing = new(nil)

  def map(func = nil, &blk)
    f = blk || func
    if self == Nothing
      return self
    end
    Maybe.of(f.call(@value))
  end

  class << Nothing
    def map(func = nil, &blk)
      self
    end

    def to_s
      "Nothing"
    end
  end
end

Nothing = Maybe::Nothing

module Kernel
  def map(func = nil, &blk)
    f = func || blk
    -> (e) { e.class.instance_method(:map).bind(e).call(&f) }
  end
end


double = -> (e) { e * 2 }

double‛ = map double
p double‛.([1,2,3,4])
p double‛.([])
p double‛.(Just(3))
p double‛.(Nothing)