coroutine

Supported Library

A function that executes alongside the main thread.

Description

A coroutine is used to perform multiple tasks at the same time from within the same script. Such tasks might include producing values from inputs or performing work on a subroutine when solving a larger problem. A task doesn't even need to have a defined ending point, but it does need to define particular times at which it yields (pause) to let other things be worked on.

Using Coroutines

A new coroutine can be created by providing a function to coroutine.create(). Once created, a coroutine doesn't begin running until the first call to coroutine.resume() which passes the arguments to the function. This call returns when the function either halts or calls coroutine.yield() and, when this happens, coroutine.resume() returns either the values returned by the function, the values sent to coroutine.yield(), or an error message. If it does error, the second return value is the thrown error.

local function task(...)
    -- This function might do some work for a bit then yield some value
    coroutine.yield("first")  -- To be returned by coroutine.resume()
    -- The function continues once it is resumed again
    return "second"
end

local taskCoro = coroutine.create(task)
-- Call resume for the first time, which runs the function from the beginning
local success, result = coroutine.resume(taskCoro, ...)
print(success, result)  --> true, first (task called coroutine.yield())
-- Continue running the function until it yields or halts
success, result = coroutine.resume(taskCoro)
print(success, result)  --> true, second (task halted because it returned "second")

During the lifetime of the coroutine, you can call coroutine.status() to inspect its status:

Status	Meaning
suspended	The coroutine is waiting to be resumed. Coroutines begin in this state and enter it when their function calls coroutine.yield().
running	The coroutine is running right now.
normal	The coroutine is awaiting the yield of another coroutine; in other words, it has resumed another coroutine.
dead	The function has halted (returned or thrown an error). The coroutine cannot be used further.

Wrapping Coroutines

When working with coroutines, you can also forgo the use of the coroutine object and instead use a wrapper function. Such a wrapper function will resume a particular coroutine when it is called and will return only the yielded values. You can do this using coroutine.wrap():

-- A simple coroutine body that returns a value
local function task()
    return "done"
end

-- Create coroutine and return a wrapper function that resumes it
local f = coroutine.wrap(task)
-- Resume the coroutine as if we called coroutine.resume()
local result = f()
print(result) --> done
-- If an error occurs it will be raised here!
-- This differs from coroutine.resume() which acts similar to pcall()

The first value returned from coroutine.resume() describes whether a coroutine ran without errors. However, functions returned by coroutine.wrap() will not do this: instead they directly return the values returned or passed to coroutine.yield(), if any. Should an error have occurred while running the coroutine function, the error is raised on the call of the returned function.

Producer Pattern Example

Imagine a task that produces repetitions of a word: each time it produces a repetition, the next one will produce one more. For example, providing Hello will produce Hello, HelloHello, HelloHelloHello, etc. To do this, you can define repeatThis():

-- This function repeats a word every time its coroutine is resumed
local function repeatThis(word)
    local repetition = ""
    while true do
        -- Do one repetition then yield the result
        repetition = repetition .. word
        coroutine.yield(repetition)
    end
end

To run this function as a coroutine, you can use coroutine.create() followed by multiple calls to coroutine.resume():

local function repeatThis(word)
    local repetition = ""
    while true do
        repetition = repetition .. word
        coroutine.yield(repetition)
    end
end

local repetitionCoro = coroutine.create(repeatThis)
print(coroutine.resume(repetitionCoro, "Hello"))  -- true, Hello
print(coroutine.resume(repetitionCoro))           -- true, HelloHello
print(coroutine.resume(repetitionCoro))           -- true, HelloHelloHello

For this producer function, you can also use coroutine.wrap() to get a function that produces values:

local function repeatThis(word)
    local repetition = ""
    while true do
        repetition = repetition .. word
        coroutine.yield(repetition)
    end
end

local f = coroutine.wrap(repeatThis)
print(f("Hello"))  -- Hello
print(f())         -- HelloHello
print(f())         -- HelloHelloHello

Functions

close Supported Closes and puts the provided coroutine in a dead state.

Closes and puts the provided coroutine in a dead state. This function returns true unless the coroutine is in an error state, in which case it returns false and the error message. A coroutine that is currently running cannot be closed. A coroutine cannot be resumed after it is closed.

create Supported Creates a new coroutine, with body f. f must be a Luau function.

Creates a new coroutine, with body f. f must be a Luau function.

Creates a new coroutine with body f, which must be a Luau function, and returns it as a thread. The returned coroutine starts in the suspended state and does not begin executing until coroutine.resume() is called on it. The arguments passed to the first coroutine.resume() call are forwarded as the arguments to f.

finally Supported Registers a callback to invoke when a coroutine finishes, errors, or is closed.

Registers a callback to invoke when a coroutine finishes, errors, or is closed.

Registers callback on coroutine co that will be called when co reaches a terminal state. The callback is called with a status string and a set of values:

• callback("finished", ...results). The coroutine function returned. All result values are forwarded to the callback.
• callback("error", err). The coroutine terminated with an error. The error value is passed as the only additional argument.
• callback("cancelled"). coroutine.close() or task.cancel() was called on the coroutine. The coroutine did not complete its work. No extra values are provided.

Callbacks run at most once, in reverse registration order (LIFO).

When the coroutine reaches a terminal state during the invocation of coroutine.resume() or coroutine.close(), those functions will return only after callbacks complete their work. Callbacks are invoked on the calling thread and may yield. If a callback errors, remaining callbacks in the list are not invoked and false status with the callback error is returned.

Attempting to register a callback on a dead coroutine raises an error.

If a coroutine reaches a terminal state in a resume through coroutine.wrap() closure and has callbacks registered, it raises a "cannot run finalizers from a wrapped coroutine" error. A coroutine can also reach the terminal state when it is being resumed by the engine or closed with task.cancel(); in this case, callbacks are executed on a separate thread with errors reported as script errors.

isyieldable Supported Returns true if the coroutine this function is called within can safely yield.

Returns true if the coroutine this function is called within can safely yield. Yielding a coroutine inside metamethods or C functions is prohibited, with the exception of pcall and xpcall.

resume Supported Starts or continues the execution of coroutine co.

Starts or continues the execution of coroutine co. The first time you resume a coroutine, it starts running its body. The values ... are passed as the arguments to the body function. If the coroutine has yielded, resume restarts it; the values ... are passed as the results from the yield. If the coroutine runs without any errors, resume returns true plus any values passed to yield (if the coroutine yields) or any values returned by the body function (if the coroutine terminates). If there is any error, resume returns false plus the error message.

running Supported Returns the running coroutine.

Returns the running coroutine.

Returns the currently running coroutine as a thread value, or nil if called from the main thread. The main thread is not considered a coroutine, so only threads created by coroutine.create() or coroutine.wrap() are returned by this function.

status Supported Returns the status of coroutine co as a string.

Returns the status of coroutine co as a string.

Returns the status of coroutine co, as a string: 'running', if the coroutine is running (that is, it called status); 'suspended', if the coroutine is suspended in a call to yield, or if it has not started running yet; 'normal' if the coroutine is active but not running (that is, it has resumed another coroutine); and 'dead' if the coroutine has finished its body function, or if it has stopped with an error.

wrap Supported Creates a new coroutine and returns a function that, when called, resumes the coroutine.

Creates a new coroutine and returns a function that, when called, resumes the coroutine.

Creates a new coroutine, with body f. f must be a Luau function. Returns a function that resumes the coroutine each time it is called. Any arguments passed to the function behave as the extra arguments to resume. Returns the same values returned by resume, except the first boolean. In case of error, propagates the error.

yield Supported Suspends execution of the coroutine.

Suspends execution of the coroutine.

Suspends the execution of the calling coroutine. Any arguments to yield are passed as extra results to resume. Yielding a coroutine inside metamethods or C functions is prohibited, with the exception of pcall and xpcall.

Roblox reference: coroutine