function f(){}
let sym = Symbol();
f[Symbol.toPrimitive] = () => sym;
let o = { [sym]: 0 };
console.log(o[f]) // 0

... hm

let functorMapSymbol = Symbol('Functor.map');
let Functor = {};
Functor.map = function(rec, ...args){ return rec[functorMapSymbol](...args); };
Functor.map[Symbol.toPrimitive] = () => functorMapSymbol;

let o = {
  [Functor.map](arg) {
    console.log(arg);
  },
};

// usage:
o[Functor.map]('hi');

// alternative usage:
Functor.map(o, 'hi');