scala - A grasp of immutable datastructures

Question

I am learning scala and as a good student I try to obey all rules I found.

One rule is: IMMUTABILITY!!!

So I have tried to code everything with immutable data structures and vals, and sometimes this is really hard.

But today I thought to myself: the only important thing is that the object/class should have no mutable state. I am not forced to code all methods in an immutable style, because these methods don't affect each other.

My Question: Am I correct or are there any problems/disadvantages I dont see?

EDIT:

Code example for aishwarya:

def logLikelihood(seq: Iterator[T]): Double = {
  val sequence = seq.toList
  val stateSequence = (0 to order).toList.padTo(sequence.length,order)
  val seqPos = sequence.zipWithIndex

  def probOfSymbAtPos(symb: T, pos: Int) : Double = {
    val state = states(stateSequence(pos))
    M.log(state( seqPos.map( _._1 ).slice(0, pos).takeRight(order), symb))
  }

  val probs = seqPos.map( i => probOfSymbAtPos(i._1,i._2) )

  probs.sum
}  

Explanation: It is a method to calculate the log-likelihood of a homogeneous Markov model of variable order. The apply method of state takes all previous symbols and the coming symbol and returns the probability of doing so.

As you may see: the whole method is just multiplying some probabilities which would be much easier using vars.

Answer 1

The rule is not really immutability, but referential transparency. It's perfectly OK to use locally declared mutable variables and arrays, because none of the effects are observable to any other parts of the overall program.

The principle of referential transparency (RT) is this:

An expression e is referentially transparent if for all programs p every occurrence of e in p can be replaced with the result of evaluating e, without affecting the observable result of p.

Note that if e creates and mutates some local state, it doesn't violate RT since nobody can observe this happening.

That said, I very much doubt that your implementation is any more straightforward with vars.