Question

The problem is whether an instance method should in anyway alter the object that contains the method or should it return a new instance? I’m new to F# and the concept of full mmutability that is suggested for F#.

Just using psuedo code for now unless I need to be more specific.

First thought is just add the message to the message list on the object:

class Something
  ctr(messages)
     _messages.Add(messages)

  AddMessage(message)
   _messages.Add(message)

Second is to construct a new list that joins the old list and the new message. Then I would create a new instance altogther and send back.

class Something
  ctr(messages)
     _messages.Add(messages)

  AddMessage(message)
    newMessageList = _messages.Join(message)
      return new Something(newMessageList)

Am I overthinking immutability?

Answer 1

As you are doing “class based” programming which is one of the way (rather unfortunate) to do object oriented programming, you would be doing in place state modification rather than returning a new state (as that’s what would be expected when you are doing OO).

In case you really want to go towards immutability then I would suggest you need to use more FP concepts like Modules, Functions (not methods which have you have in class based programming), recursive data types etc.

My answer is way too general and the appropriate answer lies in the fact that how this class of your will fit in the big picture of your application design.