With algebraic data type like in Haskell, the state could be typed like this: da...

With algebraic data type like in Haskell, the state could be typed like this:

    data State a = 
      Loading
      | Failed
      | Loaded [a]

Compared to a representation like `{ loading:true, error: false, pics:[] }`, the above model has the benefit of "making illegal state not representable".

One library in js I like to use to model this type of tagged union / ADT values is single-key[1]. Each variant in a tagged union type is represented by an plain object with just one key, for example:

    const state1 = { Loading: null };
    const state2 = { Failed : <reason> };
    const state3 = { Loaded : [...pics] };

The library offers pattern matching syntax on top of this data representation:

    // render
    const reactElement = match(state, {
      Loading : () => <Loading />,
      Failed  : (reason) => <Error {...reason}/>,
      Loaded  : (pics) => <PicsList pics={pics} />
    });

    // reducer
    const reducer = (state, action) => match(state, {
        Loading : () => {
          switch (action.type) {
            case PICS_FAILURE:
              return { Failure: action.payload };
            case PICS_SUCCESS:
              return { Loaded : action.payload };
            default:
              return state
          }
        },
        Failed : () => {
          switch (action.type) {
            case PICS_REQUEST:
              return { Loading: true };
            default:
              return state;
          }
        },
        Loaded : () => {
          switch (action.type) {
            case PICS_REQUEST:
              return { Loading: true };
            default:
              return state;
          }
        }
      });

[1] https://www.npmjs.com/package/single-key