Scala trait syntax

Question

I am reading Odersky\'s book and there is a speadsheet example with the follwowing code part:

package org.stairwaybook.scells
    trait Arithmetic { this: Ev         


        

Answer 1

What you see here

this:Evaluator =>

is the usage of self-type for a trait. It basically forces class which are going to mix the trait Arithmetic to also mix the trait Evaluator.

If you try to create a class such as the following:

class ArithmeticClass extends Arithmetic

you'll get a compile time error, while if you try to do:

class ArithmeticClass extends Arithmetic with Evaluator

this is going to work. As you can see, Arithmetic class modifies add something to operations, which is probably a collection defined in the Evaluator trait.

Please note that self-types let you design a cleaner class hierarchy compared to simple inheritance:

If you use self types, you can think about something like the following:

trait Evaluator { def runEvaluation : Int }
trait Arithmetic { self: Evaluator => def evaluate: Int = runEvaluation }
trait NullEvaluator extends Evaluator { def runEvaluation: Int = 0 }

class MyClass1 extends Arithmetic with Evaluator {... concrete methods .... }
class MyClass2 extends Arithmetic with NullEvaluator { ....concrete methods ..... }

So self types let you express something different from inheritance.