Coming from a OOP background (Java), I'm learning Scala on my own. While I can readily see the advantages of using immutable objects individually, I'm having a hard time seeing how one can design a whole application like that. I'll give an example:
Say I have objects that represents "materials" and their properties (I'm designing a game, so I actually really have that problem), like water and ice. I would have a "manager" that owns all such materials instances. One property would be the freezing and melting point, and what the material freezes or melts to.
[EDIT] All material instances are "singleton", kind of like a Java Enum.
I want "water" to say it freezes to "ice" at 0C, and "ice" to say it melts to "water" at 1C. But if water and ice are immutable, they cannot get a reference to each other as constructor parameters, because one of them has to be created first, and that one could not get a reference to the not-yet-existing other as constructor parameter. I could solve this by giving them both a reference to the manager so that they can query it to find the other material instance they need every time they are being asked for their freezing/melting properties, but then I get the same problem between the manager and the materials, that they need a reference to each other, but it can only be provided in the constructor for one of them, so either the manager or the material cannot be immutable.
Is their just no way around this problem, or do I need to use "functional" programming techniques, or some other pattern to solve it?