index.js

// @flow
import type { A } from 'my-module';

const myFunction = (foo: A): boolean => !!foo;

module-interface.js.flowtype

declare module 'my-module' {
  declare type A = {
    get: Function
  }
  
  declare function myFunction(foo: A):boolean
}

package.json

{
  "name": "my-module"
}