Basics

basics/main.xo

const GREETING = "hello"

fn greet(name: Str) -> Str {
  "${GREETING}, ${name}"
}

fn sign(n: Int) -> Str {
  if n < 0 { "negative" } else if n == 0 { "zero" } else { "positive" }
}

fn main(os: Os) {
  let who = "Ada"
  var count = 0
  for i in 0..3 {
    count += i
  }
  os.stdio.println(greet(who))
  os.stdio.println("count = ${count}, sign = ${sign(count - 5)}")
}

Output:

hello, Ada
count = 3, sign = negative

• let binds a value that never changes; var binds one that can. Mutating a let is a compile error, and so is a binding you never use.
• The last expression of a block is its value, so greet needs no return. if is an expression too.
• "${expr}" interpolates any value that implements Display.
• main receives the operating system as os. Printing goes through os.stdio; step 8 explains why.
• 0..3 is a half open range (0, 1, 2); 0..=3 includes 3.