resume & yield

Part of the Logic & Flow section of Coddy's Lua journey — lesson 51 of 54.

In the previous lesson, you learned how a coroutine can pass values back to the main code using coroutine.yield(). But communication can flow in both directions. You can also send values into a coroutine when you resume it.

When you call coroutine.resume() with additional arguments after the coroutine object, those arguments are passed into the coroutine. Inside the coroutine, these values become the return values of the coroutine.yield() call that paused it.

Here's how it works:

function receiver()
    print("Coroutine started")
    local value = coroutine.yield()
    print("Received: " .. value)
end

local co = coroutine.create(receiver)
coroutine.resume(co)           -- Starts and pauses at yield
coroutine.resume(co, "Hello")  -- Passes "Hello" into the coroutine

When the coroutine resumes the second time, the string "Hello" is passed in and becomes the return value of coroutine.yield(), which gets stored in the value variable.

Challenge

Easy

Write a function createEchoCoroutine that takes initialMessage and returns a string showing the two-way communication between the main code and a coroutine.

Create a coroutine that first yields the initialMessage, then receives a response when resumed, and finally yields that response back. The function should demonstrate passing values both out of and into the coroutine.

Logic:

Create a function that yields initialMessage and stores the return value from that yield
After receiving the value, yield it back out
Wrap this function in a coroutine using coroutine.create()
Resume the coroutine the first time to get the initial message
Resume the coroutine a second time, passing in "Response"
Capture the value yielded from the second resume
Return a string in the format: First: [first_yield]\nSecond: [second_yield]

Parameters:

initialMessage (string): The message the coroutine will yield first

Returns: A string showing both yielded values separated by a newline (string). Format: First: Hello\nSecond: Response

Cheat sheet

You can send values into a coroutine when resuming it by passing additional arguments to coroutine.resume(). These arguments become the return values of the coroutine.yield() call that paused the coroutine.

function receiver()
    local value = coroutine.yield()  -- Pauses and waits for input
    print("Received: " .. value)
end

local co = coroutine.create(receiver)
coroutine.resume(co)           -- Starts coroutine, pauses at yield
coroutine.resume(co, "Hello")  -- Passes "Hello" into the coroutine

When the coroutine resumes the second time, "Hello" is passed in and becomes the return value of coroutine.yield(), which gets stored in the value variable.

Try it yourself

function createEchoCoroutine(initialMessage)
    -- Write code here
end

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Logic & Flow

1Advanced Table Iteration

Iterating with pairs()Iterating with ipairs()pairs() vs. ipairs()Recap - Character Sheet

4Introduction to Metatables

What is a Metatable?setmetatable & getmetatable The __index Metamethod The __newindex Metamethod The __tostring Metamethod Arithmetic Metamethods Part 1 Arithmetic Metamethods Part 2 Recap - Read-Only Table

7Advanced String Manipulation

string.gsub() for Substitution Intro to String Patterns string.find()string.match()Iterating with string.gmatch()Recap - Log File Parser

2More Table Library Functions

table.concat()table construction & unpack()table.sort()Custom Sorting with Functions Recap - High Score Board

5Project: Text Adventure Engine

Project Setup: The Room Linking Rooms

Handling Invalid Moves

Displaying Exits

8Project: Contact List

Project Setup The Main Loop

Adding a Contact

Listing All Contacts

Searching for a Contact

Deleting a Contact

Handling Invalid Input

3Advanced Function Concepts

Returning Multiple Values Variadic Functions (...)Functions First-Class Values Anonymous Functions What is a Closure?Recap - Simple Event Handler

6Error Handling

The error() Function Protected Calls with pcall()Recap - Safe Division

9Coroutines for Beginners

What is a Coroutine?coroutine create & resume Pausing with coroutine.yield()resume & yieldChecking Coroutine Status Recap - Number Generator Recap - Vector Math