++ operator in Scala

Question

Is there any reason for Scala not support the ++ operator to increment primitive types by default? For example, you can not write:

var i=0
i++

Thanks

Answer 1

My guess is this was omitted because it would only work for mutable variables, and it would not make sense for immutable values. Perhaps it was decided that the ++ operator doesn't scream assignment, so including it may lead to mistakes with regard to wehater or not you are mutating the variable.

I feel that something like this is safe to do (on one line):

i++

but this would be a bad practice (in any language):

var x = i++

You don't want to mix assignment statements and side effects/mutation.

Answer 2

I think the reasoning in part is that +=1 is only one more character, and ++ is used pretty heavily in the collections code for concatenation. So it keeps the code cleaner.

Also, Scala encourages immutable variables, and ++ is intrinsically a mutating operation. If you require +=, at least you can force all your mutations to go through a common assignment procedure (e.g. def a_=).

Answer 3

It isn't included because Scala developers thought it make the specification more complex while achieving only negligible benefits and because Scala doesn't have operators at all.

You could write your own one like this:

class PlusPlusInt(i: Int){
  def ++ = i+1
  }

implicit def int2PlusPlusInt(i: Int) = new PlusPlusInt(i)

val a = 5++
// a is 6

But I'm sure you will get into some trouble with precedence not working as you expect. Additionally if i++ would be added, people would ask for ++i too, which doesn't really fit into Scala's syntax.

Answer 4

In Scala, ++ is a valid method, and no method implies assignment. Only = can do that.

A longer answer is that languages like C++ and Java treat ++ specially, and Scala treats = specially, and in an inconsistent way.

In Scala when you write i += 1 the compiler first looks for a method called += on the Int. It's not there so next it does it's magic on = and tries to compile the line as if it read i = i + 1. If you write i++ then Scala will call the method ++ on i and assign the result to... nothing. Because only = means assignment. You could write i ++= 1 but that kind of defeats the purpose.

The fact that Scala supports method names like += is already controversial and some people think it's operator overloading. They could have added special behavior for ++ but then it would no longer be a valid method name (like =) and it would be one more thing to remember.

Answer 5

I like Craig's answer, but I think the point has to be more strongly made.

1. There are no "primitives" -- if Int can do it, then so can a user-made Complex (for example).

2. Basic usage of ++ would be like this:

   var x = 1 // or Complex(1, 0)

   x++

3. How do you implement ++ in class Complex? Assuming that, like Int, the object is immutable, then the ++ method needs to return a new object, but that new object has to be assigned.

It would require a new language feature. For instance, let's say we create an assign keyword. The type signature would need to be changed as well, to indicate that ++ is not returning a Complex, but assigning it to whatever field is holding the present object. In Scala spirit of not intruding in the programmers namespace, let's say we do that by prefixing the type with @.

Then it could be like this:

case class Complex(real: Double = 0, imaginary: Double = 0) {
  def ++: @Complex = {
    assign copy(real = real + 1)
    // instead of return copy(real = real + 1)
}

The next problem is that postfix operators suck with Scala rules. For instance:

def inc(x: Int) = {
  x++
  x
}

Because of Scala rules, that is the same thing as:

def inc(x: Int) = { x ++ x }

Which wasn't the intent. Now, Scala privileges a flowing style: obj method param method param method param .... That mixes well C++/Java traditional syntax of object method parameter with functional programming concept of pipelining an input through multiple functions to get the end result. This style has been recently called "fluent interfaces" as well.

The problem is that, by privileging that style, it cripples postfix operators (and prefix ones, but Scala barely has them anyway). So, in the end, Scala would have to make big changes, and it would be able to measure up to the elegance of C/Java's increment and decrement operators anyway -- unless it really departed from the kind of thing it does support.