Simple implementation of BatchedQueue from "Purely Functional Data Structures"

queue.ts

//BatchedQueue implementation from Chirs Okasaki's "Purely Functional Data Structures"

class Queue<A> {
    constructor(public readonly f: A[], public readonly t: A[]) {}
}

export const empty = new Queue([], []);
export const isEmpty = <A>(q: Queue<A>) => q.f.length === 0;

/**
 * Maintain the invariant: If f is empty, so is t
 */
const checkf = <A>(q: Queue<A>) => {
    if (q.f.length > 0) return q;

    const f = [...q.t].reverse();
    return new Queue(f, []);
};

export const snoc = <A>(a: A) => (q: Queue<A>) => checkf(new Queue(q.f, [a, ...q.t]));
export const head = <A>(a: A) => (q: Queue<A>) => q.f[0]; //use an option type here
export const tail = <A>(q: Queue<A>) => checkf(new Queue(q.f.slice(1), q.t));