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Following is a full list of supported chaining operations in Async+:


Use attempt to start a chain of commands. The provided body will begin running immediately. The return type will be a Value<T>, Result<T> (AsyncPlus.Result), Guarantee<T>, or Promise<T> depending on the async/throwing status of the body closure. If the body passed to attempt does not return a value, you will get a Value<()>, AsyncPlus.Result<()>, Guarantee<()>, or Promise<()> as a return type. You usually do not work with these return types directly, but instead use chaining operations on them.

attempt {


then takes a body closure to run that takes as input any results from earlier in the chain, and returns a value to pass along to later operations in the chain. The body closure can be async and/or throwing. If the closure does not return a value, any existing value from earlier in the chain are passed on automatically.

.then {
    upstreamResult in


.then {
    upstreamResult -> OutType in
    return <instance of OutType>

If you do not return a result from the closure, the result of the chaining operation is discardable, meaning that you will not get an unused value warning from Xcode if you choose to terminate the chain there. The return types are the same as attempt, based on the async/throwing status of the previous items in the chain, and the async/throwing status of the provided body. Again, you usually do not need to worry about the return type.


The provided closure to recover recieves any errors, and can attempt to provide a "backup" or "recovery" value to fix the error. The provided closure may be async and/or throwing.

Example using attempt, then, and recover:

let photo = try await attempt {
    return await api.getPhoto()
}.recover {
    err in
    return await cache.getPhoto()
}.then {
    photo in
    try displayPhotoToUser(photo)


Always runs the provided body in place with respect to the other chained operations, regardless of whether the chain has failed or succeeded. The body may be async but it may not throw.

.ensure {


  • Runs the provided body closure in the event that the chain fails. The error will be passed to the body closure, which may be async and/or throwing.
  • If the body closure is throwing, a PartiallyCaughtPromise or a PartiallyCaughtResult will be returned, implying that a further catch call is required or the value of the chain must be used in some other way such as through a call to asyncThrows().
  • If the body closure does not throw, the result is discardable and returns a CaughtPromise or CaughtResult type.

.catch {
    err in
    throw OtherError()
}.catch {
    err in 
    <non-throwing body>
Once you call .catch on a chain, you can continue to chain on other calls to .catch, .ensure, but you will no longer be able to call .then or .recover on the chain to continue building its return value. If you wish to be able to do this, use .recover instead of .catch.


finally may be run after a chain fully completes. The provided body closure may be async, but not throwing. finally will not be available for chains that have not fully caught any failures, or have not used results returned from the last then call. Once finally has been called, no more chained operations with body closures may follow.

.finally {