identity.rb

##
# Identity Monad for Ruby taken from
# http://moonbase.rydia.net/mental/writings/programming/monads-in-ruby/00introduction.html
# ... but without the typos and with complete functions
##

class Identity
  attr_reader :value
  def initialize( value )
    @value = value
  end
  def self.wrap( value )
    new(value)
  end
  def pass
    yield @value
  end
end

def f(v); Identity.wrap(5*v); end
def g(v); Identity.wrap(10*v);end

# 1st law
result = Identity::wrap( 5 ).pass do |value|
  f(value)
end
puts result.value == f(5).value

# 2nd law
someidentity = Identity.new(5)
result = someidentity.pass do |value|
  Identity::wrap( value )
end
puts result.value == Identity.new(5).value

# 3rd law
r1 = someidentity.pass do |value_a|
  f( value_a )
end.pass do |value_b|
  g( value_b )
end

r2 = someidentity.pass do |value_a|
  f( value_a ).pass do |value_b|
    g( value_b )
  end
end

puts r1.value == r2.value