luau-promise/lib

title

docs

Promise

desc types properties functions A Promise is an object that represents a value that will exist in the future, but doesn't right now. Promises allow you to then attach callbacks that can run once the value becomes available (known as *resolving*), or if an error has occurred (known as *rejecting*). name desc kind type Status An enum value used to represent the Promise's status. enum Started Resolved Rejected Cancelled desc The Promise is executing, and not settled yet. desc The Promise finished successfully. desc The Promise was rejected. desc The Promise was cancelled before it finished. name tags type desc Status read only static enums Status A table containing all members of the `Status` enum, e.g., `Promise.Status.Resolved`. name desc static params returns new Construct a new Promise that will be resolved or rejected with the given callbacks. If you `resolve` with a Promise, it will be chained onto. You can safely yield within the executor function and it will not block the creating thread. ```lua local myFunction() return Promise.new(function(resolve, reject, onCancel) wait(1) resolve("Hello world!") end) end myFunction():andThen(print) ``` You do not need to use `pcall` within a Promise. Errors that occur during execution will be caught and turned into a rejection automatically. If `error()` is called with a table, that table will be the rejection value. Otherwise, string errors will be converted into `Promise.Error(Promise.Error.Kind.ExecutionError)` objects for tracking debug information. You may register an optional cancellation hook by using the `onCancel` argument: * This should be used to abort any ongoing operations leading up to the promise being settled. * Call the `onCancel` function with a function callback as its only argument to set a hook which will in turn be called when/if the promise is cancelled. * `onCancel` returns `true` if the Promise was already cancelled when you called `onCancel`. * Calling `onCancel` with no argument will not override a previously set cancellation hook, but it will still return `true` if the Promise is currently cancelled. * You can set the cancellation hook at any time before resolving. * When a promise is cancelled, calls to `resolve` or `reject` will be ignored, regardless of if you set a cancellation hook or not. true name type executor kind params function name type resolve kind params returns function name type ... ...any? void name type reject kind params returns function name type ... ...any? void name type onCancel kind params returns function name kind abortHandler function type desc boolean Returns `true` if the Promise was already cancelled at the time of calling `onCancel`. Promise name since desc static params returns defer 3.0.0 The same as Promise.new, except execution begins after the next `Heartbeat` event. This is a spiritual replacement for `spawn`, but it does not suffer from the same [issues](https://eryn.io/gist/3db84579866c099cdd5bb2ff37947cec) as `spawn`. ```lua local function waitForChild(instance, childName, timeout) return Promise.defer(function(resolve, reject) local child = instance:WaitForChild(childName, timeout) ;(child and resolve or reject)(child) end) end ``` true name type deferExecutor kind params function name type resolve kind params returns function name type ... ...any? void name type reject kind params returns function name type ... ...any? void name type onCancel kind params returns function name kind abortHandler function type desc boolean Returns `true` if the Promise was already cancelled at the time of calling `onCancel`. Promise name desc static params returns try Begins a Promise chain, calling a function and returning a Promise resolving with its return value. If the function errors, the returned Promise will be rejected with the error. You can safely yield within the Promise.try callback. ::: tip `Promise.try` is similar to Promise.promisify, except the callback is invoked immediately instead of returning a new function. ::: ```lua Promise.try(function() return math.random(1, 2) == 1 and "ok" or error("Oh an error!") end) :andThen(function(text) print(text) end) :catch(function(err) warn("Something went wrong") end) ``` true name type callback kind params returns function ...: ...any? ...any? name type desc ... ...any? Arguments for the callback type desc Promise<...any?> The return value of the passed callback. name desc static params returns promisify Wraps a function that yields into one that returns a Promise. Any errors that occur while executing the function will be turned into rejections. ::: tip `Promise.promisify` is similar to Promise.try, except the callback is returned as a callable function instead of being invoked immediately. ::: ```lua local sleep = Promise.promisify(wait) sleep(1):andThen(print) ``` ```lua local isPlayerInGroup = Promise.promisify(function(player, groupId) return player:IsInGroup(groupId) end) ``` true name type callback kind params function ...: ...any? desc type The function acts like the passed function but now returns a Promise of its return values. kind params returns function name type desc ... ...any? The same arguments the wrapped function usually takes. name desc * The return values from the wrapped function. name desc static params returns resolve Creates an immediately resolved Promise with the given value. true value: ...any Promise<...any> name desc static params returns reject Creates an immediately rejected Promise with the given value. ::: tip Someone needs to consume this rejection (i.e. `:catch()` it), otherwise it will emit an unhandled Promise rejection warning on the next frame. Thus, you should not create and store rejected Promises for later use. Only create them on-demand as needed. ::: ```lua -- Example using Promise.resolve to deliver cached values: function getSomething(name) if cache[name] then return Promise.resolve(cache[name]) else return Promise.new(function(resolve, reject) local thing = getTheThing() cache[name] = thing resolve(thing) end) end end ``` true value: ...any Promise<...any> name desc static params returns all Accepts an array of Promises and returns a new promise that: * is resolved after all input promises resolve. * is rejected if *any* input promises reject. Note: Only the first return value from each promise will be present in the resulting array. After any input Promise rejects, all other input Promises that are still pending will be cancelled if they have no other consumers. ```lua local promises = { returnsAPromise("example 1"), returnsAPromise("example 2"), returnsAPromise("example 3"), } return Promise.all(promises) ``` true promises: array<Promise<T>> Promise<array<T>> name desc static params returns allSettled Accepts an array of Promises and returns a new Promise that resolves with an array of in-place Statuses when all input Promises have settled. This is equivalent to mapping `promise:finally` over the array of Promises. ```lua local promises = { returnsAPromise("example 1"), returnsAPromise("example 2"), returnsAPromise("example 3"), } return Promise.allSettled(promises) ``` true promises: array<Promise<T>> Promise<array<Status>> name desc static params returns race Accepts an array of Promises and returns a new promise that is resolved or rejected as soon as any Promise in the array resolves or rejects. ::: warning If the first Promise to settle from the array settles with a rejection, the resulting Promise from `race` will reject. If you instead want to tolerate rejections, and only care about at least one Promise resolving, you should use Promise.any or Promise.some instead. ::: All other Promises that don't win the race will be cancelled if they have no other consumers. ```lua local promises = { returnsAPromise("example 1"), returnsAPromise("example 2"), returnsAPromise("example 3"), } return Promise.race(promises) -- Only returns 1st value to resolve or reject ``` true promises: array<Promise<T>> Promise<T> name desc static params returns some Accepts an array of Promises and returns a Promise that is resolved as soon as `count` Promises are resolved from the input array. The resolved array values are in the order that the Promises resolved in. When this Promise resolves, all other pending Promises are cancelled if they have no other consumers. `count` 0 results in an empty array. The resultant array will never have more than `count` elements. ```lua local promises = { returnsAPromise("example 1"), returnsAPromise("example 2"), returnsAPromise("example 3"), } return Promise.some(promises, 2) -- Only resolves with first 2 promises to resolve ``` true promises: array<Promise<T>>, count: number Promise<array<T>> name desc static params returns any Accepts an array of Promises and returns a Promise that is resolved as soon as *any* of the input Promises resolves. It will reject only if *all* input Promises reject. As soon as one Promises resolves, all other pending Promises are cancelled if they have no other consumers. Resolves directly with the value of the first resolved Promise. This is essentially Promise.some with `1` count, except the Promise resolves with the value directly instead of an array with one element. ```lua local promises = { returnsAPromise("example 1"), returnsAPromise("example 2"), returnsAPromise("example 3"), } return Promise.any(promises) -- Resolves with first value to resolve (only rejects if all 3 rejected) ``` true promises: array<Promise<T>> Promise<T> name desc params returns static delay Returns a Promise that resolves after `seconds` seconds have passed. The Promise resolves with the actual amount of time that was waited. This function is **not** a wrapper around `wait`. `Promise.delay` uses a custom scheduler which provides more accurate timing. As an optimization, cancelling this Promise instantly removes the task from the scheduler. ::: warning Passing `NaN`, infinity, or a number less than 1/60 is equivalent to passing 1/60. ::: ```lua Promise.delay(5):andThenCall(print, "This prints after 5 seconds") ``` seconds: number Promise<number> true name since desc params returns static fold unreleased Folds an array of values or promises into a single value. The array is traversed sequentially. The reducer function can return a promise or value directly. Each iteration receives the resolved value from the previous, and the first receives your defined initial value. The folding will stop at the first rejection encountered. ```lua local basket = {"blueberry", "melon", "pear", "melon"} Promise.fold(basket, function(accumulatedCost, fruit) if fruit == "blueberry" then return cost -- blueberries are free! else -- call a function that returns a promise with the fruit price return fetchPrice(fruit):andThen(function(fruitCost) return cost + fruitCost end) end end, 0) ``` name type list array<T | Promise<T>> name desc type reducer The function to call with the accumulated value, the current element from the array and its index. kind params returns function accumulator: U, value: T, index: number U | Promise<U> name type initialValue U Promise<U> true name since desc params returns static each 3.0.0 Iterates serially over the given an array of values, calling the predicate callback on each value before continuing. If the predicate returns a Promise, we wait for that Promise to resolve before moving on to the next item in the array. ::: tip `Promise.each` is similar to `Promise.all`, except the Promises are ran in order instead of all at once. But because Promises are eager, by the time they are created, they're already running. Thus, we need a way to defer creation of each Promise until a later time. The predicate function exists as a way for us to operate on our data instead of creating a new closure for each Promise. If you would prefer, you can pass in an array of functions, and in the predicate, call the function and return its return value. ::: ```lua Promise.each({ "foo", "bar", "baz", "qux" }, function(value, index) return Promise.delay(1):andThen(function() print(("%d) Got %s!"):format(index, value)) end) end) --[[ (1 second passes) > 1) Got foo! (1 second passes) > 2) Got bar! (1 second passes) > 3) Got baz! (1 second passes) > 4) Got qux! ]] ``` If the Promise a predicate returns rejects, the Promise from `Promise.each` is also rejected with the same value. If the array of values contains a Promise, when we get to that point in the list, we wait for the Promise to resolve before calling the predicate with the value. If a Promise in the array of values is already Rejected when `Promise.each` is called, `Promise.each` rejects with that value immediately (the predicate callback will never be called even once). If a Promise in the list is already Cancelled when `Promise.each` is called, `Promise.each` rejects with `Promise.Error(Promise.Error.Kind.AlreadyCancelled`). If a Promise in the array of values is Started at first, but later rejects, `Promise.each` will reject with that value and iteration will not continue once iteration encounters that value. Returns a Promise containing an array of the returned/resolved values from the predicate for each item in the array of values. If this Promise returned from `Promise.each` rejects or is cancelled for any reason, the following are true: - Iteration will not continue. - Any Promises within the array of values will now be cancelled if they have no other consumers. - The Promise returned from the currently active predicate will be cancelled if it hasn't resolved yet. name type list array<T | Promise<T>> name desc type predicate The callback to call for each value in the list. kind params returns function value: T, index: number U | Promise<U> Promise<array<U>> true name since desc params returns static retry 3.0.0 Repeatedly calls a Promise-returning function up to `times` number of times, until the returned Promise resolves. If the amount of retries is exceeded, the function will return the latest rejected Promise. ```lua local function canFail(a, b, c) return Promise.new(function(resolve, reject) -- do something that can fail local failed, thing = doSomethingThatCanFail(a, b, c) if failed then reject("it failed") else resolve(thing) end end) end local MAX_RETRIES = 10 local value = Promise.retry(canFail, MAX_RETRIES, "foo", "bar", "baz") -- args to send to canFail ``` name type callback kind params returns function ...: P Promise<T> name type times number name type optional ... P true Promise<T> true name since desc params returns static fromEvent 3.0.0 Converts an event into a Promise which resolves the next time the event fires. The optional `predicate` callback, if passed, will receive the event arguments and should return `true` or `false`, based on if this fired event should resolve the Promise or not. If `true`, the Promise resolves. If `false`, nothing happens and the predicate will be rerun the next time the event fires. The Promise will resolve with the event arguments. ::: tip This function will work given any object with a `Connect` method. This includes all Roblox events. ::: ```lua -- Creates a Promise which only resolves when `somePart` is touched by a part named `"Something specific"`. return Promise.fromEvent(somePart.Touched, function(part) return part.Name == "Something specific" end) ``` name type event kind type interface Connect type desc kind params function name type callback kind params function ...: P Any object with a `Connect` method. This includes all Roblox events. name optional type predicate true kind params returns desc function ...: P boolean A function which determines if the Promise should resolve with the given value, or wait for the next event to check again. Promise<P> true name desc static params returns is Checks whether the given object is a Promise via duck typing. This only checks if the object is a table and has an `andThen` method. true object: any type desc boolean `true` if the given `object` is a Promise. name desc params returns overloads andThen Chains onto an existing Promise and returns a new Promise. ::: warning Within the failure handler, you should never assume that the rejection value is a string. Some rejections within the Promise library are represented by Error objects. If you want to treat it as a string for debugging, you should call `tostring` on it first. ::: Return a Promise from the success or failure handler and it will be chained onto. name type successHandler kind params returns function ...: ...any? ...any? name optional type failureHandler true kind params returns function ...: ...any? ...any? Promise<...any?> params returns name type successHandler kind params returns function ...: ...any? Promise<T> name optional type failureHandler true kind params returns function ...: ...any? Promise<T> Promise<T> name desc params returns overloads catch Shorthand for `Promise:andThen(nil, failureHandler)`. Returns a Promise that resolves if the `failureHandler` worked without encountering an additional error. ::: warning Within the failure handler, you should never assume that the rejection value is a string. Some rejections within the Promise library are represented by Error objects. If you want to treat it as a string for debugging, you should call `tostring` on it first. ::: name type failureHandler kind params returns function ...: ...any? ...any? Promise<...any?> params returns name type failureHandler kind params returns function ...: ...any? Promise<T> Promise<T> name desc params returns tap Similar to Promise.andThen, except the return value is the same as the value passed to the handler. In other words, you can insert a `:tap` into a Promise chain without affecting the value that downstream Promises receive. ```lua getTheValue() :tap(print) :andThen(function(theValue) print("Got", theValue, "even though print returns nil!") end) ``` If you return a Promise from the tap handler callback, its value will be discarded but `tap` will still wait until it resolves before passing the original value through. name type tapHandler kind params returns function ...: ...any? ...any? Promise<...any?> name desc params returns overloads finally Set a handler that will be called regardless of the promise's fate. The handler is called when the promise is resolved, rejected, *or* cancelled. Returns a new promise chained from this promise. ::: warning If the Promise is cancelled, any Promises chained off of it with `andThen` won't run. Only Promises chained with `finally` or `done` will run in the case of cancellation. ::: ```lua local thing = createSomething() doSomethingWith(thing) :andThen(function() print("It worked!") -- do something.. end) :catch(function() warn("Oh no it failed!") end) :finally(function() -- either way, destroy thing thing:Destroy() end) ``` name type finallyHandler kind params returns function status: Status ...any? Promise<...any?> params returns name type finallyHandler kind params returns function status: Status Promise<T> Promise<T> name desc params returns overloads done Set a handler that will be called only if the Promise resolves or is cancelled. This method is similar to `finally`, except it doesn't catch rejections. ::: tip `done` should be reserved specifically when you want to perform some operation after the Promise is finished (like `finally`), but you don't want to consume rejections (like in <a href="/roblox-lua-promise/lib/Examples.html#cancellable-animation-sequence">this example</a>). You should use `andThen` instead if you only care about the Resolved case. ::: ::: warning Like `finally`, if the Promise is cancelled, any Promises chained off of it with `andThen` won't run. Only Promises chained with `done` and `finally` will run in the case of cancellation. ::: Returns a new promise chained from this promise. name type doneHandler kind params returns function status: Status ...any? Promise<...any?> params returns name type doneHandler kind params returns function status: Status Promise<T> Promise<T> name desc params returns andThenCall Attaches an `andThen` handler to this Promise that calls the given callback with the predefined arguments. The resolved value is discarded. ```lua promise:andThenCall(someFunction, "some", "arguments") ``` This is sugar for ```lua promise:andThen(function() return someFunction("some", "arguments") end) ``` name type callback kind params returns function ...: ...any? any name type desc ... ...any? Arguments which will be passed to the callback. Promise name desc params returns finallyCall Same as `andThenCall`, except for `finally`. Attaches a `finally` handler to this Promise that calls the given callback with the predefined arguments. name type callback kind params returns function ...: ...any? any name type desc ... ...any? Arguments which will be passed to the callback. Promise name desc params returns doneCall Same as `andThenCall`, except for `done`. Attaches a `done` handler to this Promise that calls the given callback with the predefined arguments. name type callback kind params returns function ...: ...any? any name type desc ... ...any? Arguments which will be passed to the callback. Promise name desc params returns andThenReturn Attaches an `andThen` handler to this Promise that discards the resolved value and returns the given value from it. ```lua promise:andThenReturn("some", "values") ``` This is sugar for ```lua promise:andThen(function() return "some", "values" end) ``` ::: warning Promises are eager, so if you pass a Promise to `andThenReturn`, it will begin executing before `andThenReturn` is reached in the chain. Likewise, if you pass a Promise created from Promise.reject into `andThenReturn`, it's possible that this will trigger the unhandled rejection warning. If you need to return a Promise, it's usually best practice to use Promise.andThen. ::: name type desc ... ...any? Values to return from the function. Promise<...any?> name desc params returns finallyReturn Attaches a `finally` handler to this Promise that discards the resolved value and returns the given value from it. ```lua promise:finallyReturn("some", "values") ``` This is sugar for ```lua promise:finally(function() return "some", "values" end) ``` name type desc ... ...any? Values to return from the function. Promise<...any?> name desc params returns doneReturn Attaches a `done` handler to this Promise that discards the resolved value and returns the given value from it. ```lua promise:doneReturn("some", "values") ``` This is sugar for ```lua promise:done(function() return "some", "values" end) ``` name type desc ... ...any? Values to return from the function. Promise<...any?> name params desc returns timeout seconds: number, rejectionValue: T? Returns a new Promise that resolves if the chained Promise resolves within `seconds` seconds, or rejects if execution time exceeds `seconds`. The chained Promise will be cancelled if the timeout is reached. Rejects with `rejectionValue` if it is non-nil. If a `rejectionValue` is not given, it will reject with a `Promise.Error(Promise.Error.Kind.TimedOut)`. This can be checked with Error.isKind. ```lua getSomething():timeout(5):andThen(function(something) -- got something and it only took at max 5 seconds end):catch(function(e) -- Either getting something failed or the time was exceeded. if Promise.Error.isKind(e, Promise.Error.Kind.TimedOut) then warn("Operation timed out!") else warn("Operation encountered an error!") end end) ``` Sugar for: ```lua Promise.race({ Promise.delay(seconds):andThen(function() return Promise.reject(rejectionValue == nil and Promise.Error.new({ kind = Promise.Error.Kind.TimedOut }) or rejectionValue) end), promise }) ``` Promise<T> name desc cancel Cancels this promise, preventing the promise from resolving or rejecting. Does not do anything if the promise is already settled. Cancellations will propagate upwards and downwards through chained promises. Promises will only be cancelled if all of their consumers are also cancelled. This is to say that if you call `andThen` twice on the same promise, and you cancel only one of the child promises, it will not cancel the parent promise until the other child promise is also cancelled. ```lua promise:cancel() ``` name desc params returns now Chains a Promise from this one that is resolved if this Promise is already resolved, and rejected if it is not resolved at the time of calling `:now()`. This can be used to ensure your `andThen` handler occurs on the same frame as the root Promise execution. ```lua doSomething() :now() :andThen(function(value) print("Got", value, "synchronously.") end) ``` If this Promise is still running, Rejected, or Cancelled, the Promise returned from `:now()` will reject with the `rejectionValue` if passed, otherwise with a `Promise.Error(Promise.Error.Kind.NotResolvedInTime)`. This can be checked with Error.isKind. rejectionValue: T? Promise<T> name tags desc returns await yields Yields the current thread until the given Promise completes. Returns true if the Promise resolved, followed by the values that the promise resolved or rejected with. ::: warning If the Promise gets cancelled, this function will return `false`, which is indistinguishable from a rejection. If you need to differentiate, you should use Promise.awaitStatus instead. ::: ```lua local worked, value = getTheValue():await() if worked then print("got", value) else warn("it failed") end ``` desc type `true` if the Promise successfully resolved. boolean desc type The values that the Promise resolved or rejected with. ...any? name tags desc returns awaitStatus yields Yields the current thread until the given Promise completes. Returns the Promise's status, followed by the values that the promise resolved or rejected with. type desc Status The Promise's status. type desc ...any? The values that the Promise resolved or rejected with. name tags desc returns expect yields Yields the current thread until the given Promise completes. Returns the the values that the promise resolved with. ```lua local worked = pcall(function() print("got", getTheValue():expect()) end) if not worked then warn("it failed") end ``` This is essentially sugar for: ```lua select(2, assert(promise:await())) ``` **Errors** if the Promise rejects or gets cancelled. type desc ...any? The values that the Promise resolved with. name desc returns getStatus Returns the current Promise status. Status