Lua Academy · Lesson

Promise-Like Patterns

Implement deferred/promise objects using coroutines and callbacks.

Lesson 3 of 413 steps

Promise-Like Patterns is a free Lua Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

What Is a Promise?

A promise represents a value that will be available in the future. It can be pending, fulfilled (with a value), or rejected (with an error).

Lua Promise Table

Implement a minimal promise as a table with state, value, and callback lists.

local function newPromise()
  return {
    state    = "pending",
    value    = nil,
    _resolve = {},
    _reject  = {},
  }
end

resolve and reject

Fulfilling a promise calls all resolve callbacks. Rejecting it calls all reject callbacks.

local function resolve(p, value)
  if p.state ~= "pending" then return end
  p.state = "fulfilled"; p.value = value
  for _, cb in ipairs(p._resolve) do cb(value) end
end
local function reject(p, reason)
  if p.state ~= "pending" then return end
  p.state = "rejected"; p.value = reason
  for _, cb in ipairs(p._reject) do cb(reason) end
end

andThen Chaining

A :andThen(onFulfill, onReject) method returns a new promise, enabling chaining.

function promise:andThen(onFulfill, onReject)
  local next = newPromise()
  self._resolve[#self._resolve+1] = function(v)
    local ok, result = pcall(onFulfill, v)
    if ok then resolve(next, result)
    else reject(next, result) end
  end
  if onReject then
    self._reject[#self._reject+1] = function(e)
      local ok, result = pcall(onReject, e)
      if ok then resolve(next, result)
      else reject(next, result) end
    end
  end
  return next
end

Coroutine-Backed Promise

Combine promises with coroutines: a coroutine awaits a promise by yielding, and the promise's resolve callback resumes it.

Promise.all

Wait for multiple promises to all complete.

local function all(promises)
  local results = {}
  local count = #promises
  local combined = newPromise()
  for i, p in ipairs(promises) do
    p:andThen(function(v)
      results[i] = v; count = count - 1
      if count == 0 then resolve(combined, results) end
    end, function(e) reject(combined, e) end)
  end
  return combined
end

Promise.race

Resolve with the first promise that completes (fulfilled or rejected).

local function race(promises)
  local combined = newPromise()
  for _, p in ipairs(promises) do
    p:andThen(
      function(v) resolve(combined, v) end,
      function(e) reject(combined, e) end
    )
  end
  return combined
end

Error Propagation in Chains

If any step in a chain throws, the error propagates to the next rejection handler. Unhandled rejections should be logged at the end of the chain.

Cancellable Promises

Add a :cancel() method that transitions to a special cancelled state, preventing callbacks from firing.

Lazy Promises

A lazy promise only starts its work when the first andThen is attached — useful for optional async operations.

When to Use Promises vs Coroutines

Promises are callback-based and composable. Coroutines make async code look synchronous. In Lua, coroutine-backed promises give both: the composability of promises and the readability of sync code.

Promise Question

What does a promise's andThen method return?

Answer it in the interactive lesson

Recap: Promise-Like Patterns

Promises represent pending values with pending/fulfilled/rejected states. Chain operations with andThen. Combine with coroutines for async/await-style code. Use all and race for parallel coordination.

Free to start

Learn Lua with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses: 40
Lessons: 159

Start learning free View the full course

Frequently Asked Questions

Is the “Promise-Like Patterns” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Promise-Like Patterns”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Promise-Like Patterns”?

Implement deferred/promise objects using coroutines and callbacks. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Promise-Like Patterns” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Lua Academy lesson?

Yes. Every Lua Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Lua Academy