Changing the type of the structure about monocle-ts HOT 7 OPEN

interface Pair<A, B> { fst: A; snd: B }
const pair = <A, B>(fst: A, snd: B): Pair<A, B> => ({ fst, snd });

type Lens<S, T, A, B> = (val: S) => Pair<A, (val: B) => T>;

const _fst = <A, C>(s: Pair<A, C>) => pair(s.fst, <B>(b: B): Pair<B, C> => pair(b, s.snd));
const _snd = <A, C>(s: Pair<C, A>) => pair(s.snd, <B>(b: B): Pair<C, B> => pair(s.fst, b));

function view<S, T, A, B>(l: Lens<S, T, A, B>, val: S): A {
    return l(val).fst;
}
function set<S, T, A, B>(l: Lens<S, T, A, B>, val: S, x: B): T {
    return l(val).snd(x);
}
function over<S, T, A, B>(l: Lens<S, T, A, B>, val: S, f: (x: A) => B): T {
    const r = l(val);
    return r.snd(f(r.fst));
}

const p = pair(1, 'a');
const stringify = (x: number) => '' + x;
const first = view<Pair<number, string>, Pair<any, string>, number, any>(_fst, p);
const stringifyFstP = over<typeof p, Pair<string, string>, number, string>(_fst, p, stringify);

interface PLens<S,T,A,B> {
    readonly get: (s:S) => A
    readonly set: (b:B) => (s:S) => T
}

interface Foo{
    x:number
}

interface Bar{
    x:string
}

const l: PLens<Foo,Bar,number,string> = {
    get: s => s.x,
    set: b => s => ({ x: b }),
}

const _ = {
    x: 5
}

const got = l.get(_) // TS infers this is type number
const sat = l.set('howdy')(_) // tS infers this is type Bar

console.log('should be 5:', got)
console.log('should be {x:"howdy"}:', sat)