bind.ts

export interface Lens<T> {
  (): T
  (newValue: T): void
}

export function bind<T>(get: () => T, set: (newValue: T) => void): Lens<T>
export function bind<T, U>(target: Map<T, U>, key: T): Lens<U>
export function bind<T extends object, P extends keyof T>(
  target: T,
  key: P
): Lens<T[P]>

export function bind(get: any, set: any) {
  if (typeof get !== 'function') {
    const obj = get
    const key = set
    if (get instanceof Map) {
      get = () => obj.get(key)
      set = (value: any) => obj.set(key, value)
    } else {
      get = () => obj[key]
      set = (value: any) => (obj[key] = value)
    }
  }
  return function(value?: any) {
    return arguments.length ? void set(value) : get()
  }
}