Representing discriminated unions/ADTs in TypeScript

adt.ts

// Discriminated Union in TypeScript

interface Integer {
    tag: 'integer';
    value: number;
}

interface Sum {
    tag: 'sum';
    op1: Expression;
    op2: Expression;
}

type Expression = Integer | Sum;

function createSum(o1: Expression, o2: Expression): Sum {
    return { tag: 'sum', op1: o1, op2: o2 };
}

function createInt(n: number): Integer {
    return { tag: 'integer', value: n };
}

const e1 = createSum(createInt(3), createInt(5));

function showExpression(e: Expression): string {
    switch (e.tag) {
        case 'integer':
            return `${e.value}`;
        case 'sum':
            return `(+ ${showExpression(e.op1)} ${showExpression(e.op2)})`;
    }
}

function evalExpression(e: Expression): number {
    switch (e.tag) {
        case 'integer':
            return e.value;
        case 'sum':
            return evalExpression(e.op1) + evalExpression(e.op2);
    }
}

console.log(`e1: ${showExpression(e1)} -- value: ${evalExpression(e1)}`);