Vow is a tiny library which allows you to handle promises more safely in your Bucklescript application.
A Vow can be either handled and unhandled. All promises of type vow 'a handled make sure that you handled Promise rejections. Thanks to that you will avoid the Uncaught promise error.
After series of operations you usually want to "consume" a promise. Vow.sideEffect should be used for that.
It only accepts promises which are properly handled.
You can unwrap a handled promise using Vow.unwrap.
Js.Promise.t is unsafe when you nest promises. i.e. Js.Promise.t (Js.Promise.t 'a) is unsound. In the runtime it's Js.Promise.t.
This is resolved with vows. If you nest vows they behave as expected.
However if you put a Js.Promise.t inside a vow (which are boxed Js.Promise.t under the scenes) you're gonna get a vow of the following type:
/* in Reason syntax */
vow (Js.Promise.t 'a) 'statusHowever, under the scenes it'll really be
vow 'a 'statusTherefore vow is not sound.
In order to use vows you have to bind to your existing APIs using Vow.wrap/Vow.unsafeWrap.
If you unsafeWrap a promise which does throw your code will be unsound.
Let's see a real world example of vows with some comments:
let login _: Vow.Result.t authenticationState error Vow.handled =>
/* Returns a handled Vow.Result.t */
Login.logIn () |>
/* Validates the returned value. Since the vow is handled we don't need to catch*/
Vow.Result.map (
fun x =>
if x##isCancelled {
Vow.Result.fail LoginRequestCancelled
} else {
Vow.Result.return ()
}
) |>
/* Another handled Vow.Result.t */
Vow.Result.map Login.getCurrentAccessToken () |>
Vow.Result.map (
fun x => {
let token = x##accessToken;
/* This returns an unhandled Vow.Result.t.
* Note that the 'error types have to match
* Because after one error the subsequent operations
* Are not performed.
*/
Queries.login ::token
}
) |>
/* Ooops, the `Queries.login` might reject.
* We are forced to handle it in the compile time.
*/
Vow.Result.onError (fun _ => Vow.Result.fail GraphQlSignInError) |>
Vow.Result.map (
fun x =>
switch x {
| Authenticated {token, userId} =>
/* The promise we wrap is never rejected */
Vow.unsafeWrap
KeyChain.(
Js.Promise.all2 (
setGenericPassword username::"userId" password::userId service::"userId",
setGenericPassword username::"token" password::token service::"token"
)
) |>
Vow.map (fun _ => Vow.Result.return x)
| _ => Vow.Result.return x
}
);
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