Reading Function Signatures
Understand a function from its declaration.
Reading Function Signatures is a free Mojo Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Mojo Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Signature Is the Contract
A function's signature is its first line: the name, the parameters, and the return type. Read it well and you know how to call the function.
fn area(width: Int, height: Int) -> Int:Spotting the Name
Right after fn or def comes the name. It should hint at what the function does, like area, parse, or connect.
fn parse(text: String) -> Int:Reading the Parameters
Inside the parentheses are the parameters the function expects. Each pairs a name with a type, telling you what to pass in.
fn area(width: Int, height: Int) -> Int:Parameter Types Guide You
The type after each colon tells you the kind of value to send. width: Int means area expects a whole number, not text.
fn greet(name: String, times: Int): ...The Return Type Arrow
The arrow points to what the function gives back. Here -> Int promises an integer result you can store or print.
fn area(width: Int, height: Int) -> Int:No Arrow Means No Value
When there is no arrow, the function returns None. It does its work for the side effect, like printing, not for a result.
fn log(msg: String):
print(msg)Defaults in the Signature
An equals sign in a parameter marks a default. It signals the value is optional and shows the fallback Mojo will use.
fn open(path: String, mode: String = "r"): ...raises Warns of Failure
A raises keyword on the signature means the function can fail. That tells you to wrap your call in error handling.
fn read(path: String) raises -> String:fn vs def at a Glance
The opening word matters. fn means strict and typed; def means flexible and Python-like. The signature tells you which contract you are under.
def loose(x): ...
fn strict(x: Int) -> Int: ...A Signature as Mini-Docs
Before reading any body, the signature already answers what goes in and what comes out. Treat it as the function's summary.
Putting It All Together
Name, parameters, defaults, return type, and raises: each part of the signature is a clue. Together they tell you exactly how to use the function.
fn fetch(url: String, retries: Int = 3) raises -> String:Quick Check
In a Mojo signature, what does the part after the arrow tell you?
Recap: Read Before You Call
A signature shows the name, parameters with types, any defaults, the return type, and raises. Read it first and calling correctly becomes easy.
Frequently asked questions
Is the “Reading Function Signatures” lesson free?
Yes — the full text of “Reading Function Signatures” is free to read here on the web, and the Mojo Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Mojo Academy course, upgrade to CoddyKit PRO.
What will I learn in “Reading Function Signatures”?
Understand a function from its declaration. You practise Mojo 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 Mojo Academy?
No prior experience is required. Mojo Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Reading Function Signatures” 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 Mojo Academy lesson?
Yes. Every Mojo 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
- Positional and Keyword Arguments
- Default Argument Values
- Returning Multiple Values
- Reading Function Signatures