Lesson 5 — Functions

You are here

1. Your first program ✓
2. Values and variables ✓
3. Making decisions ✓
4. Repeating things ✓
5. Functions ←
6. Lists and hashes
7. Strings
8. Organizing programs
9. Classes

What are we learning?

A function is a named group of instructions you can run whenever you need them.

Smallest useful example

fn greet() {
    say("Hello!");
}

greet();

Output

Hello!

Line by line

1. fn greet() — define a function named greet with no inputs.
2. { ... } — the instructions that belong to greet.
3. greet(); — run those instructions.

Defining a function does not run it. Calling greet(); does.

Inputs (parameters)

A parameter is a name for a value you pass in when you call the function.

fn greet(name: str) {
    say("Hello,", name);
}

greet("Echo");

Output

Hello, Echo

• name: str — this function expects text.
• greet("Echo"); — pass the text Echo in.

Answers back (return)

Some functions give a value back. You mark the kind of answer with ->, and use return:

fn add(a: int, b: int) -> int {
    return a + b;
}

say(add(2, 3));

Output

5

• -> int — this function’s answer is a whole number.
• return a + b; — send that answer back to the caller.
• say(add(2, 3)); — run add, then print the answer.

Try it

Write a function shout that takes a str and prints it. Call it once.

Predict it

fn twice(n: int) -> int {
    return n * 2;
}

say(twice(4));

Show answer

8

Fix it — return needs a return type

This fails:

fn f() {
    return;
}

If you use return, write a return type. For “no value,” use void:

fn f() -> void {
    return;
}

Deeper (later)

Reading and changing names from outside a function (use / use mut), lambdas, and advanced shapes are in Functions (full) and Scope, use, and watch. You do not need them yet.

Next

→ Lesson 6 — Lists and hashes