AsyncData

> A ReScript variant type to represent async data

Installation

Run the following in your console:

``console
$ yarn add rescript-asyncdata
`

Then add rescript-asyncdata to your bsconfig.json's bs-dependencies:

`diff
{
"bs-dependencies": [
+ "rescript-asyncdata"
]
}
`

Basics

AsyncData provides a variant type that helps you represent the state of an async request. The type consists of three tags:

- NotAsked: the request hasn't started yet
- Loading: the request has been initialised and we're waiting for a response
- Done('a): the request has finished with 'a

Representing success & failure states

ReScript provides a result<'ok, 'error> type to represent an operation success.

You can combine AsyncData.t and result to represent a possible request failure state:

`reason
type userFromServer = AsyncData.t>
`

> Note that you can use your own error type in place of exn

Then, you can pattern match:

`reason
switch userFromServer {
| NotAsked => React.null
| Loading =>
| Done(Error(error)) =>
| Done(Ok(user)) =>
}
`

Representing reload

You can combine multiple AsyncData.t to represent more complex loading styles:

`reason
type reloadableUserFromServer = {
userFromServer: userFromServer,
userFromServerReload: userFromServer,
}

let initialState = {
userFromServer: NotAsked,
userFromServerReload: NotAsked
}

let firstLoad = {
userFromServer: Loading,
userFromServerReload: NotAsked
}

let firstLoadDone = {
userFromServer: Done(Ok(user)),
userFromServerReload: NotAsked
}

let reload = {
userFromServer: Done(Ok(user)),
userFromServerReload: Loading
}

// If you just want to replace the previous state
let reloadDone = {
userFromServer: Done(Ok(newUser)),
userFromServerReload: NotAsked
}

// If you want to compare/show a diff
let reloadDone = {
userFromServer: Done(Ok(user)),
userFromServerReload: Done(Ok(newUser))
}
`

Utility functions

This package contains a few utility functions to manipulate AsyncData.t:

- getExn: Extract the Done('a) payload or throw
- getWithDefault: Extract the Done('a) payload or return a default value
- mapWithDefault: Extract and map the Done('a) payload or return a default value
- map: Map the Done('a) payload
- flatMap: Map the Done('a) payload with a callback that returns a AsyncData.t
- isLoading
- isNotAsked
- isDone
- cmp: For sorting
- eq`: For comparison

Aknowledgments

This is heavily inspired by Elm's krisajenkins/remotedata