0.43.1
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)