//@flow
import {Map} from 'immutable';

type Key$Lower = 'application' | 'status' | 'hostname' | 'timestamp';
type Map$Lower = Map<
    Key$Lower
  , number | string
  >;

type Key$Upper = 'updates';
type Map$Upper = Map<Key$Upper, Map$Lower>;

function update_ts(timestamp: number): number
{
    console.log('timestamp=%o', timestamp);
    return timestamp;
}
function foo(current: Map$Upper)
{
    let next = current.updateIn(
        ['updates', 'timestamp']
      , 0
      , t => update_ts(t)
      );
    console.log('next=%o', next);
}

 Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/foo.js:25:32

Cannot call update_ts with t bound to timestamp because string [1] is incompatible with number [2].

    src/foo.js
[2]  15│ function update_ts(timestamp: number): number
        :
    22│     let next = current.updateIn(
    23│         ['updates', 'timestamp']
    24│       , 0
    25│       , t => update_ts(t)
    26│       );
    27│     console.log('next=%o', next);
    28│ }

    node_modules/immutable/dist/immutable.js.flow
[1] 695│   updateIn<NSV, K2: $KeyOf<V>, S: $ValOf<V, K2>>(keyPath: [K, K2], notSetValue: NSV, updater: (value: $ValOf<V, K2> | NSV) => S): this;