TypeScript GADT

gadt.ts

function identity<T>(t:T):T{return t};

class NumExpr<A> {
    readonly _tag = "num";
    constructor (readonly value: number, readonly proof: (v: number) => A) {}
}


class StringExpr<A> {
    readonly _tag = "str"
    constructor (readonly value: string, readonly proof: (v: string) => A) {}
}

type Expr<A> = NumExpr<A> | StringExpr<A>;

const mkNum = (n: number) => new NumExpr(n, identity);
const mkStr = (s: string) => new StringExpr(s, identity);


// We can write evaluate generically, that's the point?
function evaluate<A>(exp: Expr<A>): A {
    if(exp._tag === "num") {
        return exp.proof(exp.value);
    }
    else {
        return exp.proof(exp.value);
    }
}