ponylang/ponyc 0.43.1

on GitHub

Fixed compiler build issues on FreeBSD host

Added relevant include and lib search paths under /usr/local
prefix on FreeBSD, to satisfy build dependencies for Pony compiler.

Add FileMode.u32

Adds a public method on FileMode to get an OS-specific representation of the FileMode as a u32.

Make working with Promise[Promise[B]] chains easier

We've added a new method flatten_next to Promise in order to make working with promises of promises easier.

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[Repository] =
      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.

[0.43.1] - 2021-08-03

Fixed

• Fixed Makefile for FreeBSD (PR #3808)

Added

• Add FileMode.u32 (PR #3809)
• Add FileMode.u32 (PR #3810)
• Make working with promises of promises easier (PR #3813)