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LICENSE
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@ -1,24 +1,121 @@
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78
README.md
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@ -1,4 +1,78 @@
# Lua Promise # Roblox Lua Promise
An implementation of `Promise` similar to Promise/A+.
## Motivation
I've found that being able to yield anywhere causes lots of bugs. In [Rodux](https://github.com/Roblox/Rodux), I explicitly made it impossible to yield in a change handler because of the sheer number of bugs that occured when callbacks randomly yielded.
As such, I think that Roblox needs an object-based async primitive. It's not important to me whether these are Promises, Observables, Task objects, or Futures.
The important traits are:
* An object that represents a unit of asynchronous work
* Composability
* Predictable timing
This Promise implementation attempts to satisfy those traits.
## API
### Static Functions
* `Promise.new((resolve, reject) -> nil) -> Promise`
* Construct a new Promise that will be resolved or rejected with the given callbacks.
* `Promise.resolve(value) -> Promise`
* Creates an immediately resolved Promise with the given value.
* `Promise.reject(value) -> Promise`
* Creates an immediately rejected Promise with the given value.
* `Promise.is(object) -> bool`
* Returns whether the given object is a Promise.
### Instance Methods
* `Promise:andThen(successHandler, [failureHandler]) -> Promise`
* Chains onto an existing Promise and returns a new Promise.
* Equivalent to the Promise/A+ `then` method.
* `Promise:catch(failureHandler) -> Promise`
* Shorthand for `Promise:andThen(nil, failureHandler)`.
* `Promise:await() -> ok, value`
* Yields the current thread until the given Promise completes. Returns `ok` as a bool, followed by the value that the promise returned.
## Example
This Promise implementation finished synchronously. In order to wrap an existing async API, you should use `spawn` or `delay` in order to prevent your calling thread from accidentally yielding.
```lua
local HttpService = game:GetService("HttpService")
-- A light wrapper around HttpService
-- Ideally, you do this once per project per async method that you use.
local function httpGet(url)
return Promise.new(function(resolve, reject)
-- Spawn to prevent yielding, since GetAsync yields.
spawn(function()
local ok, result = pcall(HttpService.GetAsync, HttpService, url)
if ok then
resolve(result)
else
reject(result)
end
end)
end)
end
-- Usage
httpGet("https://google.com")
:andThen(function(body)
print("Here's the Google homepage:", body)
end)
:catch(function(err)
warn("We failed to get the Google homepage!", err)
end)
```
## Future Additions
* `Promise.all`
* Currently stubbed out, throws an error.
* `Promise.wrapAsync`
* Intended to wrap an existing Roblox API that yields, exposing a new one that returns a Promise.
## License ## License
Lua Promise is available under the terms of the Unlicense. See [LICENSE](LICENSE) for details. This project is available under the CC0 license. See [LICENSE](LICENSE) for details.

129
lib/Promise.spec.lua
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@ -1,129 +0,0 @@
return function()
local Promise = require(script.Parent.Promise)
describe("new", function()
it("should pass resolve and reject to the callback", function()
local promiseResolve
local promiseReject
local callCount = 0
local promise = Promise.new(function(resolve, reject)
callCount = callCount + 1
promiseResolve = resolve
promiseReject = reject
end)
expect(promise).to.be.ok()
expect(promiseResolve).to.be.a("function")
expect(promiseReject).to.be.a("function")
expect(callCount).to.equal(1)
end)
it("should resolve synchronously", function()
local promiseResolve
local callCount = 0
local promise = Promise.new(function(resolve)
callCount = callCount + 1
promiseResolve = resolve
end)
expect(promise._status).to.equal(Promise.Status.Started)
promiseResolve(6, 7)
expect(promise._status).to.equal(Promise.Status.Resolved)
expect(promise._value).to.be.a("table")
expect(#promise._value).to.equal(2)
expect(promise._value[1]).to.equal(6)
expect(promise._value[2]).to.equal(7)
end)
it("should reject synchronously", function()
local promiseReject
local callCount = 0
local promise = Promise.new(function(_, reject)
callCount = callCount + 1
promiseReject = reject
end)
expect(promise._status).to.equal(Promise.Status.Started)
promiseReject(6, 7)
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value).to.be.a("table")
expect(#promise._value).to.equal(2)
expect(promise._value[1]).to.equal(6)
expect(promise._value[2]).to.equal(7)
end)
end)
describe("resolve", function()
it("should be a synchronously resolved promise", function()
local promise = Promise.resolve(3)
expect(promise._status).to.equal(Promise.Status.Resolved)
expect(promise._value).to.be.a("table")
expect(#promise._value).to.equal(1)
expect(promise._value[1]).to.equal(3)
end)
end)
describe("reject", function()
it("should be a synchronously rejected promise", function()
local promise = Promise.reject(3)
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value).to.be.a("table")
expect(#promise._value).to.equal(1)
expect(promise._value[1]).to.equal(3)
end)
end)
describe("andThen", function()
it("should be chained with unresolved promises", function()
local rootResolve
local rootCallCount = 0
local childCallCount = 0
local childValues
local root = Promise.new(function(resolve)
rootCallCount = rootCallCount + 1
rootResolve = resolve
end)
local child = root:andThen(function(...)
childCallCount = childCallCount + 1
childValues = {...}
return "foo"
end)
expect(root).never.to.equal(child)
expect(rootCallCount).to.equal(1)
expect(childCallCount).to.equal(0)
expect(root._status).to.equal(Promise.Status.Started)
expect(child._status).to.equal(Promise.Status.Started)
rootResolve(16, 13)
expect(root._status).to.equal(Promise.Status.Resolved)
expect(root._value).to.be.a("table")
expect(#root._value).to.equal(2)
expect(root._value[1]).to.equal(16)
expect(root._value[2]).to.equal(13)
expect(#childValues).to.equal(2)
expect(childValues[1]).to.equal(16)
expect(childValues[2]).to.equal(13)
expect(child._status).to.equal(Promise.Status.Resolved)
expect(child._value).to.be.a("table")
expect(#child._value).to.equal(1)
expect(child._value[1]).to.equal("foo")
end)
end)
end

31
lib/Promise.lua → lib/init.lua
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@ -4,8 +4,8 @@
local PROMISE_DEBUG = false local PROMISE_DEBUG = false
-- If promise debugging is on, use a version of pcall that warns on failure. This is useful for finding errors that -- If promise debugging is on, use a version of pcall that warns on failure.
-- happen within Promise itself. -- This is useful for finding errors that happen within Promise itself.
local wpcall local wpcall
if PROMISE_DEBUG then if PROMISE_DEBUG then
wpcall = function(f, ...) wpcall = function(f, ...)
@ -71,8 +71,8 @@ Promise.Status = {
end end
get("https://google.com") get("https://google.com")
:andThen(function(body) :andThen(function(stuff)
print("Got a body:", body) print("Got some stuff!", stuff)
end) end)
]] ]]
function Promise.new(callback) function Promise.new(callback)
@ -139,23 +139,8 @@ end
* is resolved when all input promises resolve * is resolved when all input promises resolve
* is rejected if ANY input promises reject * is rejected if ANY input promises reject
]] ]]
function Promise.all(promises) function Promise.all(...)
return Promise.new(function(resolve, reject) error("unimplemented", 2)
local results = {}
local totalCount = #promises
local finishedCount = 0
for index, promise in ipairs(promises) do
promise:andThen(function(value)
results[index] = value
finishedCount = finishedCount + 1
if finishedCount == totalCount then
resolve(results)
end
end, reject)
end
end)
end end
--[[ --[[
@ -193,7 +178,7 @@ function Promise:andThen(successHandler, failureHandler)
end end
if self._status == Promise.Status.Started then if self._status == Promise.Status.Started then
-- If we haven't resolved yet, put ourself into the queue -- If we haven't resolved yet, put ourselves into the queue
table.insert(self._queuedResolve, successCallback) table.insert(self._queuedResolve, successCallback)
table.insert(self._queuedReject, failureCallback) table.insert(self._queuedReject, failureCallback)
elseif self._status == Promise.Status.Resolved then elseif self._status == Promise.Status.Resolved then
@ -323,4 +308,4 @@ function Promise:_reject(...)
end end
end end
return Promise return Promise

262
lib/init.spec.lua Normal file
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@ -0,0 +1,262 @@
return function()
local Promise = require(script.Parent)
describe("Promise.new", function()
it("should instantiate with a callback", function()
local promise = Promise.new(function() end)
expect(promise).to.be.ok()
end)
it("should invoke the given callback with resolve and reject", function()
local callCount = 0
local resolveArg
local rejectArg
local promise = Promise.new(function(resolve, reject)
callCount = callCount + 1
resolveArg = resolve
rejectArg = reject
end)
expect(promise).to.be.ok()
expect(callCount).to.equal(1)
expect(resolveArg).to.be.a("function")
expect(rejectArg).to.be.a("function")
expect(promise._status).to.equal(Promise.Status.Started)
end)
it("should resolve promises on resolve()", function()
local callCount = 0
local promise = Promise.new(function(resolve)
callCount = callCount + 1
resolve()
end)
expect(promise).to.be.ok()
expect(callCount).to.equal(1)
expect(promise._status).to.equal(Promise.Status.Resolved)
end)
it("should reject promises on reject()", function()
local callCount = 0
local promise = Promise.new(function(resolve, reject)
callCount = callCount + 1
reject()
end)
expect(promise).to.be.ok()
expect(callCount).to.equal(1)
expect(promise._status).to.equal(Promise.Status.Rejected)
end)
it("should reject on error in callback", function()
local callCount = 0
local promise = Promise.new(function()
callCount = callCount + 1
error("hahah")
end)
expect(promise).to.be.ok()
expect(callCount).to.equal(1)
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]:find("hahah")).to.be.ok()
end)
end)
describe("Promise.resolve", function()
it("should immediately resolve with a value", function()
local promise = Promise.resolve(5)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Resolved)
expect(promise._value[1]).to.equal(5)
end)
it("should chain onto passed promises", function()
local promise = Promise.resolve(Promise.new(function(_, reject)
reject(7)
end))
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]).to.equal(7)
end)
end)
describe("Promise.reject", function()
it("should immediately reject with a value", function()
local promise = Promise.reject(6)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]).to.equal(6)
end)
it("should pass a promise as-is as an error", function()
local innerPromise = Promise.new(function(resolve)
resolve(6)
end)
local promise = Promise.reject(innerPromise)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]).to.equal(innerPromise)
end)
end)
describe("Promise:andThen", function()
it("should chain onto resolved promises", function()
local args
local argsLength
local callCount = 0
local badCallCount = 0
local promise = Promise.resolve(5)
local chained = promise
:andThen(function(...)
args = {...}
argsLength = select("#", ...)
callCount = callCount + 1
end, function()
badCallCount = badCallCount + 1
end)
expect(badCallCount).to.equal(0)
expect(callCount).to.equal(1)
expect(argsLength).to.equal(1)
expect(args[1]).to.equal(5)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Resolved)
expect(promise._value[1]).to.equal(5)
expect(chained).to.be.ok()
expect(chained).never.to.equal(promise)
expect(chained._status).to.equal(Promise.Status.Resolved)
expect(#chained._value).to.equal(0)
end)
it("should chain onto rejected promises", function()
local args
local argsLength
local callCount = 0
local badCallCount = 0
local promise = Promise.reject(5)
local chained = promise
:andThen(function(...)
badCallCount = badCallCount + 1
end, function(...)
args = {...}
argsLength = select("#", ...)
callCount = callCount + 1
end)
expect(badCallCount).to.equal(0)
expect(callCount).to.equal(1)
expect(argsLength).to.equal(1)
expect(args[1]).to.equal(5)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]).to.equal(5)
expect(chained).to.be.ok()
expect(chained).never.to.equal(promise)
expect(chained._status).to.equal(Promise.Status.Resolved)
expect(#chained._value).to.equal(0)
end)
it("should chain onto asynchronously resolved promises", function()
local args
local argsLength
local callCount = 0
local badCallCount = 0
local startResolution
local promise = Promise.new(function(resolve)
startResolution = resolve
end)
local chained = promise
:andThen(function(...)
args = {...}
argsLength = select("#", ...)
callCount = callCount + 1
end, function()
badCallCount = badCallCount + 1
end)
expect(callCount).to.equal(0)
expect(badCallCount).to.equal(0)
startResolution(6)
expect(badCallCount).to.equal(0)
expect(callCount).to.equal(1)
expect(argsLength).to.equal(1)
expect(args[1]).to.equal(6)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Resolved)
expect(promise._value[1]).to.equal(6)
expect(chained).to.be.ok()
expect(chained).never.to.equal(promise)
expect(chained._status).to.equal(Promise.Status.Resolved)
expect(#chained._value).to.equal(0)
end)
it("should chain onto asynchronously rejected promises", function()
local args
local argsLength
local callCount = 0
local badCallCount = 0
local startResolution
local promise = Promise.new(function(_, reject)
startResolution = reject
end)
local chained = promise
:andThen(function()
badCallCount = badCallCount + 1
end, function(...)
args = {...}
argsLength = select("#", ...)
callCount = callCount + 1
end)
expect(callCount).to.equal(0)
expect(badCallCount).to.equal(0)
startResolution(6)
expect(badCallCount).to.equal(0)
expect(callCount).to.equal(1)
expect(argsLength).to.equal(1)
expect(args[1]).to.equal(6)
expect(promise).to.be.ok()
expect(promise._status).to.equal(Promise.Status.Rejected)
expect(promise._value[1]).to.equal(6)
expect(chained).to.be.ok()
expect(chained).never.to.equal(promise)
expect(chained._status).to.equal(Promise.Status.Resolved)
expect(#chained._value).to.equal(0)
end)
end)
end

2
rojo.json
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@ -4,7 +4,7 @@
"partitions": { "partitions": {
"lib": { "lib": {
"path": "lib", "path": "lib",
"target": "ReplicatedStorage.lua-promise" "target": "ReplicatedStorage.Promise"
} }
} }
} }