Expression Problem

Functional.ts

/** Number Expressions */

// data
type Exp =
    { tag: 'Lit', value: number } |
    { tag: 'Add', left: Exp, right: Exp }

// operations
function evaluate(exp: Exp): number {
    switch (exp.tag) {
        case 'Lit':
            return exp.value;
        case 'Add':
            return evaluate(exp.left) + evaluate(exp.right)
    }
}

function print(exp: Exp): string {
    switch (exp.tag) {
        case 'Lit':
            return `Lit(${exp.value})`;
        case 'Add':
            return `Add(${print(exp.left)},${print(exp.right)})`
    }
}

// 1 + 3
let exp: Exp = {
    tag: 'Add',
    left: { tag: 'Lit', value: 1 },
    right: { tag: 'Lit', value: 3 }
}

evaluate(exp) //4

print(exp) // Add(Lit(1),Lit(3))

Object-Oriented.ts

abstract class Exp {
    abstract print(): string
    abstract evaluate(): number
}

class Lit extends Exp {
    // data
    constructor(readonly value: number) { super() }

    //operations
    evaluate(): number { return this.value }

    print(): string { return `Lit(${this.value})` }
}

class Add implements Exp {
    // data
    constructor(readonly left: Exp, readonly right: Exp) { }

    // operations
    evaluate(): number {
        return this.left.evaluate() + this.right.evaluate()
    }

    print(): string {
        return `Add(${this.left.print()},${this.right.print()})`
    }
}

// 1 + 3
let exp: Exp = new Add(
    new Lit(1),
    new Lit(3)
)

exp.evaluate() // 4

exp.print() // Add(Lit(1),Lit(3))