Object map in Flow

objMap.flow.js

type ObjMapF<T> = <V:$Values<T>, K:$Keys<T>>(V, K, T)=>mixed;
type ObjMap<T:{}, F:ObjMapF<T>> = $ObjMapi<T, <K, V>(K, V)=>$Call<F, V, K, T>> ;
const objMap = <T:{}, F:ObjMapF<T>=*>(o: T, f: F): ObjMap<T, F> => {
  let keys: Array<$Keys<T>> = Object.keys(o);
  let obj = keys.map(k => {
    let v = o[k];
    type K = typeof k;
    type V = typeof v;
    let r = f<K, V>(o[k], k, o)
    return r;
  }).reduce((a, b) => Object.assign({}, a, b));
  // $FlowFixMe
  return obj;
}