Trivial OO Expression Problem Solved in TypeScript

expression-problem.mts

// See the following for a Scala example: <https://i.cs.hku.hk/~bruno/papers/Modularity2016.pdf>

/* Initial code */
abstract class Exp { abstract evaluate(): number }
class Lit extends Exp {
    constructor(readonly x: number){ super(); }
    evaluate(): number { return this.x }
}
class Add extends Exp {
    constructor(readonly left: Exp, readonly right: Exp) { super() }
    evaluate(): number { 
        return this.left.evaluate() + this.right.evaluate()
    }
}

/* Adding new operation 'print' */
abstract class ExpP extends Exp { abstract print(): string }
class LitP extends Lit implements ExpP {
    print(): string { return this.x.toString() }
}
class AddP extends Add implements ExpP {
    // Note the type refinement on the properties
    constructor(readonly left: ExpP, readonly right: ExpP) { super(left, right) }
    print(): string { 
        return this.left.print() + this.right.print()
    }
}

/* Adding new variant 'Sub' */
class Sub extends Exp {
     constructor(readonly left: Exp, readonly right: Exp) { super() }
     evaluate(): number { 
        return this.left.evaluate() - this.right.evaluate()
    }
}