Typed deep merge with Ramda

DeepMergeWith.ts

type Primitive = string | number | boolean | Symbol | null | undefined;
    const isObject = (x: any): x is Object => R.is(Object, x);
    const isPrimitive = (x: any): x is Primitive => !isObject(x);

    const deepMergeWith = <T>(fn: <U extends Primitive>(a: U, b: U) => U) => (a: T, b: T): T => {
        //base case
        if (isPrimitive(a) && isPrimitive(b)) {
            return fn(a, b);
        }
        //inductive step
        return R.mergeWith(deepMergeWith(fn), a, b);
    };