async-state-ts

Recipies

Combining async states

If you need to derive AsyncState from multiple given ones, you could use combine method. The resulting AsyncState will have following shape:

• Error will have collected error types from input AsyncStates (represented by union).
• Success' value type will be the type which mapping function returns.

The resulting AsyncState will be in Success state only if all input AsyncStates are succeeded.

Example: Suppose we have UserInfo and Balance holding async states with different error types, by using combine we are able to derive state with UserWithBalance by using mapping function.

import * as AsyncState from 'async-state-ts/AsyncState';

// User Info
type UserInfo = {
  name: string;
  id: string;
};
type SomeError = { type: 'SomeError' };
type UserInfoState = AsyncState.AsyncState<SomeError, UserInfo>;
declare const userInfoState: UserInfoState;

// Balance
type OtherError = { type: 'OtherError' };
type Balance = {
  amount: number;
};
type BalanceState = AsyncState.AsyncState<OtherError, Balance>;
declare const balanceState: BalanceState;

// Combination
type UserWithBalance = {
  balance: number;
  userName: string;
  userId: string;
};

// The resolved type will be AsyncState<SomeError | OtherError, UserWithBalance>
const userWithBalanceState = AsyncState.combineI(
  [userInfoState, balanceState],
  // Mapping function
  ({ id, name }, { amount }): UserWithBalance => ({
    balance: amount,
    userId: id,
    userName: name,
  })
);

Usage with React

If you need to use TaskEither returning function inside a component, you can track the lifecycle of its call by using useTaskEither and useTaskEitherImmediate hooks.

Example: Suppose you're going to fetch UserInfo when component is mounted and give client a feedback of request state (Loading, Error, Succes). You can achieve this by using useTaskEitherImmediate hook, which will call TaskEither returning function upon component mount.

import * as AsyncState from 'async-state-ts/AsyncState';
import { useTaskEitherImmediate } from 'async-state-ts-react-hooks';


type FetchError = { type: 'FetchError', message: string };
type UserInfo = {
  name: string;
  id: string;
};

declare const fetchUserInfoFromServer: () => TE.TaskEither<FetchError, UserInfo>;

const UserInfoComponent: React.FC = () => {
  const [state, retry] = useTaskEitherImmediate(fetchUserInfoFromServer);

  return pipe(
    state,
    AsyncState.match({
      Error: ({message}) => <>
        <p>Sorry, we couldn't load user info. Message: {message}</p>,
        <button onClick={retry}>Try Again</button>
      </>
      Loading: () => <p>Loading...</p>,
      Success: ({name}) => <p>Hello {name}!</p>,
    })
  )

}

But if you don't need to trigger fetchUserInfoFromServer upon mounting, but rather by effect or callback, you could make use of useTaskEither hook. Note that now state will also have NotInitiated state.

import * as AsyncStateN from 'async-state-ts/AsyncStateN';
import { useTaskEither } from 'async-state-ts-react-hooks';

declare const fetchUserInfoFromServer: () => TE.TaskEither<FetchError, UserInfo>;

const UserInfoComponent: React.FC = () => {
  const [state, execute, retry, reset] = useTaskEither(fetchUserInfoFromServer);

  return pipe(
    state,
    AsyncStateN.match({
      NotInitiated: () => <div>
        <button onClick={execute}>Load User!</button>
      </div>
      Error: ({message}) => <>
        <p>Sorry, we couldn't load user info. Message: {message}</p>,
        <button onClick={retry}>Try Again</button>
      </>
      Loading: () => <p>Loading...</p>,
      Success: ({name}) => <>
        <p>Hello {name}!</p>
        <button onClick={reset}>Reset</button>
      </>,
    })
  )

}