@johngw/stream/src/transformers/stateReducer/Output.ts

Reactive programming tools using the WHATWG Streams API.

import { L, U } from 'ts-toolbelt'
import { StateReducerInit } from '#transformers/stateReducer/Reducers'

/**
 * Represents the Writable (output) types of StateReducer actions.
 *
 * @group Transformers
 * @example
 * ```
 * type T = StateReducerOutput<{ foo: string, bar: number, nothing: never }, string>
 * // | { action: 'foo', param: string, state: string }
 * // | { action: 'bar', param: number, state: string }
 * // | { action: 'nothing', state: string }
 * ```
 */
export type StateReducerOutput<
  Actions extends Record<string, unknown>,
  State
> = AccumulateStateReducerOutput<
  Actions,
  Readonly<State>,
  U.ListOf<keyof Actions>,
  { action: StateReducerInit; param: void; state: Readonly<State> }
>

/**
 * {@inheritdoc StateReducerOutput}
 */
type AccumulateStateReducerOutput<
  Actions extends Record<string, unknown>,
  State,
  ActionNames extends readonly (keyof Actions)[],
  Acc extends { action: keyof Actions; param: unknown; state: State }
> = L.Length<ActionNames> extends 0
  ? Acc
  : AccumulateStateReducerOutput<
      Actions,
      State,
      L.Tail<ActionNames>,
      | Acc
      | {
          action: L.Head<ActionNames>
          param: Actions[L.Head<ActionNames>]
          state: State
        }
    >