# Tour of Promises
Here's quick introduction to Promises. For more complete information, check out the API Reference.
# Creating a Promise
There are a few ways to create a Promise. The most common way is to call Promise.new:
local myFunction() return Promise.new(function(resolve, reject, onCancel) somethingThatYields() resolve("Hello world!") end) end myFunction():andThen(print)
Another example which resolves a Promise after the first time an event fires:
There's actually a built-in function called Promise.fromEvent that does exactly this!
local myFunction() return Promise.new(function(resolve, reject, onCancel) local connection connection = someEvent:Connect(function(...) connection:Disconnect() resolve(...) end) onCancel(function() connection:Disconnect() end) end) end myFunction():andThen(print)
If you just want to wrap a single value in a Promise, you can use Promise.resolve:
local myFunction() return Promise.resolve("Hello world!") end myFunction():andThen(print)
local function myYieldingFunction(waitTime, text) wait(waitTime) return text end local myFunction = Promise.promisify(myYieldingFunction) myFunction(1.2, "Hello world!"):andThen(print):catch(function() warn("Oh no... goodbye world.") end)
# Rejection and Errors
You must observe the result of a Promise, either with
finally, otherwise an unhandled Promise rejection warning will be printed to the console.
If an error occurs while executing the Promise body, the Promise will be rejected automatically with the error text and a trace back.
One of the best parts about Promises is that they are chainable.
Every time you call
finally, it returns a new Promise, which resolves with whatever value you return from the success or failure handlers, respectively.
somePromise:andThen(function(number) return number + 1 end):andThen(print)
You can also return a Promise from your handler, and it will be chained onto:
Promise.new(function(resolve) somethingThatYields() resolve(1) end):andThen(function(x) return Promise.new(function(resolve) somethingThatYields() resolve(x + 1) end) end):andThen(print) --> 2
You can also call
:andThen multiple times on a single Promise to have multiple branches off of a single Promise.
Resolving a Promise with a Promise will be chained as well:
Promise.new(function(resolve) somethingThatYields() resolve(Promise.new(function(resolve) somethingThatYields() resolve(1) end)) end):andThen(print) --> 1
However, any value that is returned from the Promise executor (the function you pass into
Promise.new) is discarded. Do not return values from the function executor.
# A Better Alternative to
delay alongside asynchronous code can be tempting, but you should never use them!
delay do not resume threads at a consistent interval. If Roblox has resumed too many threads in a single Lua step, it will begin throttling and your thread that was meant to be resumed on the next frame could actually be resumed several seconds later. The unexpected delay caused by this behavior will cause cascading timing issues in your game and could lead to some potentially ugly bugs.
You should use Promise.delay instead, which has an accurate custom scheduler.
Promise.delay(5):andThen(function() print("5 seconds have passed!") end)
For quickly launching a new thread (similar to
spawn), you can use Promise.try:
Promise.try(function() somethingThatYields() end) -- Doesn't block this someCode()
As a convenience, Promise.timeout exists, which will return a rejected Promise if the Promise you call it on doesn't resolve within the given amount of seconds:
returnsAPromise():timeout(5):andThen(function() print("This returned in at most 5 seconds") end)
Promises are cancellable, but abort semantics are optional. This means that you can cancel any Promise and it will never resolve or reject, even if the function is still working in the background. But you can optionally add a cancellation hook which allows you to abort ongoing operations with the third
onCancel parameter given to your Promise executor.
If a Promise is already cancelled at the time of calling its
onCancel hook, the hook will be called immediately.
It's good practice to add an
onCancel hook to all of your asynchronous Promises unless it's impossible to abort an operation safely.
Even if you don't plan to directly cancel a particular Promise, chaining with other Promises can cause it to become automatically cancelled if no one cares about the value anymore.
If you attach a
:catch handler to a Promise after it's been cancelled, the chained Promise will be instantly rejected with
Promise.Error(Promise.Error.Kind.AlreadyCancelled). This also applies to Promises that you pass to
finally does not have this constraint.
If you cancel a Promise immediately after creating it without yielding in between, the fate of the Promise is dependent on if the Promise handler yields or not. If the Promise handler resolves without yielding, then the Promise will already be settled by the time you are able to cancel it, thus any consumers of the Promise will have already been called and cancellation is not possible.
If the Promise does yield, then cancelling it immediately will prevent its resolution.
Attempting to cancel an already-settled Promise is ignored.
# Cancellation Propagation
When you cancel a Promise, the cancellation propagates up and down the Promise chain. Promises keep a list of other Promises that consume them (e.g.
When the upwards propagation encounters a Promise that no longer has any consumers, that Promise is cancelled as well. Note that it's impossible to cancel an already-settled Promise, so upwards propagation will stop when it reaches a settled Promise.
If a cancelled Promise has any consumers itself, those Promises are also cancelled.
Resolving a Promise with a Promise will cause the resolving Promise to become a consumer of the chained Promise, so if the chained Promise becomes cancelled then the resolving Promise will also become cancelled.
If you call
resolve with a Promise within a Promise which is already cancelled, the passed Promise will be cancelled if it has no other consumers as an optimization.