WIP - use-effect-reducer in ReScript

App.res

type effect

@module("use-effect-reducer")
external useEffectReducer: (
  @uncurry (
    'state,
    'action,
    @uncurry ((('state, unit, @uncurry ('action => unit)) => option<unit => unit>) => effect),
  ) => 'state,
  'state,
) => ('state, 'action => unit) = "useEffectReducer"

let fetchRandomDog = () =>
  Promise.make((res, rej) => {
    Js.Global.setTimeout(() => {
      let fail = Js.Math.random() < 0.1
      if fail {
        rej(. Js.Exn.anyToExnInternal("Failed"))
      } else {
        Webapi.Fetch.fetch("")
        ->Promise.then(Webapi.Fetch.Response.json)
        ->Promise.thenResolve(data => res(. data))
        ->Promise.catch(err => {
          rej(. err)
          Promise.resolve()
        })
        ->ignore
      }
    }, 1000)->ignore
  })

type event = Fetch | Resolve(Js.Json.t) | Reject(exn) | Cancel
type stateValue = Idle | Success(Js.Json.t) | Failure | Loading
type state = {promise: option<effect>, value: stateValue}

let dogReducer = (state, event, exec) =>
  switch state.value {
  | Idle
  | Success(_)
  | Failure =>
    switch event {
    | Fetch => {
        value: Loading,
        promise: Some(
          exec((_, _, dispatch) => {
            let canceled = ref(false)

            fetchRandomDog()
            ->Promise.thenResolve(data => {
              if !canceled.contents {
                dispatch(Resolve(data))
              }

              ()
            })
            ->Promise.catch(error => {
              if !canceled.contents {
                dispatch(Reject(error))
              }

              Promise.resolve()
            })
            ->ignore

            Some(() => canceled := true)
          }),
        ),
      }
    | _ => state
    }
  | Loading =>
    switch event {
    | Resolve(data) => {
        promise: state.promise,
        value: Success(data),
      }
    }
  }