use "time"
actor Promise[A: Any #share]
"""
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.
"""
var _value: (_Pending | _Reject | A) = _Pending
embed _list: Array[_IThen[A]] = _list.create()
be apply(value: A) =>
"""
Fulfill the promise.
"""
if _value isnt _Pending then
return
end
_value = value
for f in _list.values() do
f(value)
end
_list.clear()
be reject() =>
"""
Reject the promise.
"""
if _value isnt _Pending then
return
end
_value = _Reject
for f in _list.values() do
f.reject()
end
_list.clear()
fun tag next[B: Any #share](
fulfill: Fulfill[A, B],
rejected: Reject[B] = RejectAlways[B])
: Promise[B]
=>
"""
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.
"""
let attach = _Then[A, B](consume fulfill, consume rejected)
let promise = attach.promise()
_attach(consume attach)
promise
fun tag flatten_next[B: Any #share](
fulfill: Fulfill[A, Promise[B]],
rejected: Reject[Promise[B]] = RejectAlways[Promise[B]])
: Promise[B]
=>
"""
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:
```pony
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`:
```pony
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:
```pony
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:
```pony
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:
```pony
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.
"""
let outer = Promise[B]
next[None](object iso
var f: Fulfill[A, Promise[B]] = consume fulfill
let p: Promise[B] = outer
fun ref apply(value: A) =>
let fulfill' = f = _PromiseFulFill[A, B]
try
let inner = (consume fulfill').apply(value)?
inner.next[None](
{(fulfilled: B) => p(fulfilled)},
{()? => p.reject(); error})
else
p.reject()
end
end,
object iso
var r: Reject[Promise[B]] = consume rejected
let p: Promise[B] = outer
fun ref apply() =>
let rejected' = r = RejectAlways[Promise[B]]
try
(consume rejected').apply()?
else
p.reject()
end
end)
outer
fun tag add[B: Any #share = A](p: Promise[B]): Promise[(A, B)] =>
"""
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.
"""
let p' = Promise[(A, B)]
let c =
object
var _a: (A | _None) = _None
var _b: (B | _None) = _None
be fulfill_a(a: A) =>
match _b
| let b: B => p'((a, b))
else _a = a
end
be fulfill_b(b: B) =>
match _a
| let a: A => p'((a, b))
else _b = b
end
end
next[None](
{(a) => c.fulfill_a(a) },
{() => p'.reject() })
p.next[None](
{(b) => c.fulfill_b(b) },
{() => p'.reject() })
p'
fun tag join(ps: Iterator[Promise[A]]): Promise[Array[A] val] =>
"""
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`.
```pony
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")
```
"""
Promises[A].join(
[this]
.> concat(ps)
.values())
fun tag select(p: Promise[A]): Promise[(A, Promise[A])] =>
"""
Return a promise that is fulfilled when either promise is fulfilled,
resulting in a tuple of its value and the other promise.
"""
let p' = Promise[(A, Promise[A])]
let s =
object tag
var _complete: Bool = false
let _p: Promise[(A, Promise[A])] = p'
be apply(a: A, p: Promise[A]) =>
if not _complete then
_p((a, p))
_complete = true
end
end
next[None]({(a) => s(a, p) })
p.next[None]({(a)(p = this) => s(a, p) })
p'
fun tag timeout(expiration: U64) =>
"""
Reject the promise after the given expiration in nanoseconds.
"""
Timers.apply(Timer(
object iso is TimerNotify
let _p: Promise[A] = this
fun ref apply(timer: Timer, count: U64): Bool =>
_p.reject()
false
end,
expiration))
be _attach(attach: _IThen[A] iso) =>
"""
Attaches a step asynchronously. If this promise has already been fulfilled
or rejected, immediately fulfill or reject the incoming step. Otherwise,
keep it in a list.
"""
if _value is _Pending then
_list.push(consume attach)
elseif _value is _Reject then
attach.reject()
else
try attach(_value as A) end
end
primitive Promises[A: Any #share]
fun join(ps: Iterator[Promise[A]]): Promise[Array[A] val] =>
"""
Create a promise that is fulfilled when all promises in the given sequence
are fulfilled. If any promise in the sequence is rejected then the new
promise is also rejected. The order that values appear in the final array
is based on when each promise is fulfilled and not the order that they are
given.
Join three existing promises to make a fourth.
```pony
use "promises"
actor Main
new create(env: Env) =>
let p1 = Promise[String val]
let p2 = Promise[String val]
let p3 = Promise[String val]
Promises[String val].join([p1; p2; p3].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")
```
"""
let p' = Promise[Array[A] val]
let ps' = Array[Promise[A]] .> concat(consume ps)
if ps'.size() == 0 then
p'(recover Array[A] end)
return p'
end
let j = _Join[A](p', ps'.size())
for p in ps'.values() do
p.next[None]({(a)(j) => j(a)}, {() => p'.reject()})
end
p'
actor _Join[A: Any #share]
embed _xs: Array[A]
let _space: USize
let _p: Promise[Array[A] val]
new create(p: Promise[Array[A] val], space: USize) =>
(_xs, _space, _p) = (Array[A](space), space, p)
be apply(a: A) =>
_xs.push(a)
if _xs.size() == _space then
let len = _xs.size()
let xs = recover Array[A](len) end
for x in _xs.values() do
xs.push(x)
end
_p(consume xs)
end
primitive _None
class iso _PromiseFulFill[A: Any #share, B: Any #share] is Fulfill[A, Promise[B]]
"""
Fulfill discarding its input value of `A` and returning a promise of type `B`.
"""
new iso create() => None
fun ref apply(value: A): Promise[B] => Promise[B]