monkeypatched maybe monad

maybe.rb

class Object
  def maybe(&blk)
    yield self
  end
end

class NilClass
  def maybe(&blk)
    nil
  end
end

hash = {
  a: {
    b: {
      c: 1
    }
  }
}

hash.maybe { _1[:a] }
  .maybe { _1[:b] }
  .maybe { _1[:c] }

hash.maybe { _1[:a] }
  .maybe { _1[:d] }
  .maybe { _1[:c] }

def unit(a)
  a
end

def bind(m, &blk)
  m.maybe(&blk)
end

# monad law 1
raise unless bind(unit(1)) { _1 + 2 } == 3

# monad law 2
raise unless bind(unit(1)) { unit(_1) } == unit(1)

# monad law 3
raise unless bind(1.0) { |a| bind(a + 1) { |b| b / 2 }} \
  == bind(bind(1.0) { |a| a + 1}) { |b| b / 2 }

raise unless bind(1.0) { |a|
  bind(a == 1 ? nil : 1) { |b| b / 2 }
} == bind(bind(1.0) { |a|
  a == 1 ? nil : 1
}) { |b| b / 2 }