Promise[A: Any #share]

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A promise to eventually produce a result of type A. This promise can either be fulfilled or rejected.

Any number of promises can be chained after this one.

actor tag Promise[A: Any #share]

Constructors

create

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new tag create()
: Promise[A] tag^

Returns

• Promise[A] tag^

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Public Behaviours

apply

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Fulfill the promise.

be apply(
  value: A)

Parameters

• value: A

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reject

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Reject the promise.

be reject()

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Public Functions

next[B: Any #share]

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Chain a promise after this one.

When this promise is fulfilled, the result of type A is passed to the fulfill function, generating in an intermediate result of type B. This is then used to fulfill the next promise in the chain.

If there is no fulfill function, or if the fulfill function raises an error, then the next promise in the chain will be rejected.

If this promise is rejected, this step's reject function is called with no input, generating an intermediate result of type B which is used to fulfill the next promise in the chain.

If there is no reject function, of if the reject function raises an error, then the next promise in the chain will be rejected.

fun tag next[B: Any #share](
  fulfill: Fulfill[A, B] iso,
  rejected: Reject[B] iso = qualify)
: Promise[B] tag

Parameters

• fulfill: Fulfill[A, B] iso
• rejected: Reject[B] iso = qualify

Returns

• Promise[B] tag

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flatten_next[B: Any #share]

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Chain a promise after this one and unwrap the promise returned from this one.

flatten_next is a companion to next. It operates in an identical fashion except for the type of the fulfilled promise. Whereas next takes a function that returns a type B, flatten_next takes a function that returns Promise[B].

Why is flatten_next valuable given that next could take a B that is of a type like Promise[String]? Let's start with some code to demonstrate the problem that arises when returning Promise[Promise[B]] from next.

Let's say we have a library for accessing the GitHub REST API:

class GitHub
  new create(personal_access_token: String)

  fun get_repo(repo: String): Promise[Repository]

class Repository
  fun get_issue(number: I64): Promise[Issue]

class Issue
  fun title(): String

And we want to use this promise based API to look up the title of an issue. Without flatten_next, we could attempt to do the following using next:

actor Main
  new create(env: Env) =>
    let repo: Promise[Repository] =
      GitHub("my token").get_repo("ponylang/ponyc")

    //
    // do something with the repo once the promise is fulfilled
    // in our case, get the issue
    //

    let issue = Promise[Promise[Issue]] =
      repo.next[Promise[Issue]](FetchIssue~apply(1))

    // once we get the issue, print the title
    issue.next[None](PrintIssueTitle~apply(env.out))

primitive FetchIssue
  fun apply(number: I64, repo: Repository): Promise[Issue] =>
    repo.get_issue(number)

primitive PrintIssueTitle
  fun apply(out: OutStream, issue: Promise[Issue]) =>
    // O NO! We can't print the title
    // We don't have an issue, we have a promise for an issue

Take a look at what happens in the example, when we get to PrintIssueTitle, we can't print anything because we "don't have anything". In order to print the issue title, we need an Issue not a Promise[Issue].

We could solve this by doing something like this:

primitive PrintIssueTitle
  fun apply(out: OutStream, issue: Promise[Issue]) =>
    issue.next[None](ActuallyPrintIssueTitle~apply(out))

primitive ActuallyPrintIssueTitle
  fun apply(out: OutStream, issue: Issue) =>
    out.print(issue.title())

That will work, however, it is kind of awful. When looking at:

    let repo: Promise[Repoository] =
      GitHub("my token").get_repo("ponylang/ponyc")
    let issue = Promise[Promise[Issue]] =
      repo.next[Promise[Issue]](FetchIssue~apply(1))
    issue.next[None](PrintIssueTitle~apply(env.out))

it can be hard to follow what is going on. We can only tell what is happening because we gave PrintIssueTitle a very misleading name; it doesn't print an issue title.

flatten_next addresses the problem of "we want the Issue, not the intermediate Promise". flatten_next takes an intermediate promise and unwraps it into the fulfilled type. You get to write your promise chain without having to worry about intermediate promises.

Updated to use flatten_next, our API example becomes:

actor Main
  new create(env: Env) =>
    let repo: Promise[Repository] =
      GitHub("my token").get_repo("ponylang/ponyc")

    let issue = Promise[Issue] =
      repo.flatten_next[Issue](FetchIssue~apply(1))

    issue.next[None](PrintIssueTitle~apply(env.out))

primitive FetchIssue
  fun apply(number: I64, repo: Repository): Promise[Issue] =>
    repo.get_issue(number)

primitive PrintIssueTitle
  fun apply(out: OutStream, issue: Issue) =>
    out.print(issue.title())

Our promise Issue, is no longer a Promise[Promise[Issue]]. By using flatten_next, we have a much more manageable Promise[Issue] instead.

Other than unwrapping promises for you, flatten_next otherwise acts the same as next so all the same rules apply to fulfillment and rejection.

fun tag flatten_next[B: Any #share](
  fulfill: Fulfill[A, Promise[B] tag] iso,
  rejected: Reject[Promise[B] tag] iso = qualify)
: Promise[B] tag

Parameters

• fulfill: Fulfill[A, Promise[B] tag] iso
• rejected: Reject[Promise[B] tag] iso = qualify

Returns

• Promise[B] tag

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add[optional B: Any #share]

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Add two promises into one promise that returns the result of both when they are fulfilled. If either of the promises is rejected then the new promise is also rejected.

fun tag add[optional B: Any #share](
  p: Promise[B] tag)
: Promise[(A , B)] tag

Parameters

• p: Promise[B] tag

Returns

• Promise[(A , B)] tag

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join

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Create a promise that is fulfilled when the receiver and all promises in the given iterator are fulfilled. If the receiver or any promise in the sequence is rejected then the new promise is also rejected.

Join p1 and p2 with an existing promise, p3.

use "promises"

actor Main
  new create(env: Env) =>

    let p1 = Promise[String val]
    let p2 = Promise[String val]
    let p3 = Promise[String val]

    p3.join([p1; p2].values())
      .next[None]({(a: Array[String val] val) =>
        for s in a.values() do
          env.out.print(s)
        end
      })

    p2("second")
    p3("third")
    p1("first")

fun tag join(
  ps: Iterator[Promise[A] tag] ref)
: Promise[Array[A] val] tag

Parameters

• ps: Iterator[Promise[A] tag] ref

Returns

• Promise[Array[A] val] tag

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select

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Return a promise that is fulfilled when either promise is fulfilled, resulting in a tuple of its value and the other promise.

fun tag select(
  p: Promise[A] tag)
: Promise[(A , Promise[A] tag)] tag

Parameters

• p: Promise[A] tag

Returns

• Promise[(A , Promise[A] tag)] tag

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timeout

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Reject the promise after the given expiration in nanoseconds.

fun tag timeout(
  expiration: U64 val)
: None val

Parameters

• expiration: U64 val

Returns

• None val

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