Scala monad with flatMap definition

Question

I have a construct, let's call it Container. It, as you might guess, is a container for another object, with some headers (a simple map). There are two instances of the container, a success container, or a failed container.

I have a function defined on container, called

flatMap(f: Container[T]=> Container[U]): Container[U]

Pretty familiar right, except unlike a 'real' monad, it's from M[A]->M[B], rather than A->M[B]. This is because I want the mapping functions to have access to all the fields of the container.

Now there's no real problem with this, except it doesn't follow the definitions for a monad and so it doesn't work with for comprehensions as a generator.

Any suggestions, other than changing the flatMap definition, or am I just SoL :)

Answer 1

If you're going from M[A] to M[B] then all you want is map

class Container[A](stuff: A){
  def map[B](f: A => B): Container[B] = new Container(f(stuff))
}

The function flatMap really is for functions of type A => M[B] to be able to work on a type of M[A] . Hence,

 class Container[A](stuff: A){
   def map[B](f: A => B): Container[B] = new Container(f(stuff))

   def flatMap[B](f: A => Contains[B]): Container[B] = f(stuff)
 }

so you really don't need flatMap in your definition at all.