Term Evaluation in Pure OO style

Term.ts

abstract class Term<T> {
    abstract eval() : T
}

class Lit extends Term<number> {
    constructor(public value: number) { super() }
    eval() : number { return this.value }
}

class Inc extends Term<number> {
    constructor(public term: Term<number>) { super() }
    eval() : number { return this.term.eval() + 1 }
}

class IsZ extends Term<boolean> {
    constructor(public term: Term<number>) { super() }
    eval() : boolean { return this.term.eval() == 0 }
}

class If<T> extends Term<T> {
    constructor(
        public pred: Term<boolean>,
        public tCond: Term<T>,
        public fCond : Term<T>
    ) { super() }
    eval(): T {
        return this.pred.eval() ? this.tCond.eval() : this.fCond.eval()
    }
}

class Pair<A, B> extends Term<[A, B]> {
    constructor(public left: Term<A>, public right: Term<B>) { super() }
    eval() : [A,B] { return [this.left.eval(), this.right.eval()] }
}

class Fst<A, B> extends Term<A> {
    constructor(public pair: Pair<A, B>) { super() }
    eval() : A { return this.pair.left.eval() }
}

class Snd<A, B> extends Term<B> {
    constructor(public pair: Pair<A, B>) { super() }
    eval() : B { return this.pair.right.eval() }
}