Defining with fn

Define functions with the fn keyword.

Defining functions

Functions are defined with fn; names are written in snake_case. Definition order doesn't matter; a function defined after main can be called inside main.

fn main() {
    selam_ver();
}

fn selam_ver() {
    println!("Merhaba!");
}

Functions that return no value actually return () (unit).

Why functions?

A function binds a repeated job to a name: the code is written once, called from many places and fixed in one place. A good function does one job and its name says so (kdv_ekle, gecerli_mi). Rust has no default parameters or overloading; different behaviors get different names.

Exercise Frame

Let fn cizgi() print "----------". In main, call cizgi(), then println!("Rapor");, then cizgi() again.

Show example solution

fn cizgi() {
    println!("----------");
}

fn main() {
    cizgi();
    println!("Rapor");
    cizgi();
}

Test yourself

In which style are function names written in Rust?
- SCREAMING_CASE
- snake_case
- camelCase
- PascalCase
Show answer

snake_case — Functions and variables use snake_case, types PascalCase, constants SCREAMING_SNAKE_CASE; the compiler warns about names that break this.

Function keyword

____ cizgi() {
    println!("----------");
}

Show answer

fn cizgi() {
    println!("----------");
}

Name and style

hesapla_toplam

Function (snake_case)
HesapMakinesi

Type (PascalCase)
MAKS_DEGER

Constant
()

Return of a function returning nothing

Next lesson: Parameters

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