Question

The following example is from the book ‘Programming in Scala’. Given a class ‘Rational’ and the following method definition:

def add(that: Rational): Rational =
    new Rational(
        this.numer * that.denom + that.numer * this.denom,
        this.denom * that.denom
    )

I can successfully overload the add method with a convenience version that takes an Int argument, and makes use of the definition above:

def add(that: Int): Rational =
    add(new Rational(that, 1))

No problems so far.

Now, if I change the method name to an operator style name:

def +(that: Rational): Rational =
    new Rational(
        this.numer * that.denom + that.numer * this.denom,
        this.denom * that.denom
    )

And overload like so:

def +(that: Int): Rational =
    +(new Rational(that, 1))

I get the following compile error:

(fragment of Rational.scala):19: error: value unary_+ is not a member of this.Rational
+(new Rational(that, 1))
 ^

Why is the compiler looking for a unary version of the + method?

Answer 1

In Scala, any construct of the type +x, -x, ~x and !x is transformed into a method call x.unary_+, etc. This is partially to allow Java-like syntax of having !b as the negation of the boolean b, or -x as the negation of the number x.

Therefore, the code snippet +(new Rational(that, 1)) is translated into (new Rational(that,1)).unary_+, and as Rational doesn’t have this method, you get a compile error. You will get this error only if your function is called +, -, ~ or ! as these are the only characters Scala allows as unary operators. For example, if you called your function @+, the code compiles just fine.

Though, I would suggest writing the overridden add function as:

def +(that: Int): Rational =
  this + (new Rational(that, 1))

This code shows the intent of your function better — you add a new Rational constructed from an integer as a numerator and 1 as denominator to this. This way of writing gets translated into this.+(new Rational(that, 1)), which is what you want — invoking the + function on this.

Note that you can use the infix notation however the function is called. For example, if you change the name back to add, you can still keep the definition as:

def add(that: Int): Rational =
  this add (new Rational(that, 1))