Booleans

bool holds true or false, and nothing else.

use std;
  
let ready = true;
let done = false;
  
println($"{ready} {done}");

It is what the comparison operators answer with, and what if and while require. Nothing converts to a bool, so a number is never a condition on its own.

Operators

&& is and, || is or, and ! negates.

use std;
  
let ready = true;
let done = false;
  
println($"{ready && done}");
println($"{ready || done}");
println($"{!ready}");

== and != compare two of them.

use std;
  
let ready = true;
  
println($"{ready == true}");
println($"{ready != false}");

Short circuiting

&& and || evaluate their right operand only when the left has not already settled the answer. false && x never looks at x, and neither does true || x.

That is what lets a cheap test guard an expensive or unsafe one, because the guard is evaluated first and the rest is skipped when it fails.

use std;
  
pub fn at(values: i32[], index: i32) -> i32 {
    if index < values.count && values[index] > 0 {
        return values[index];
    }
  
    return -1;
}
  
println($"{at(new i32[] { 5, 6 }, 0)}");
println($"{at(new i32[] { 5, 6 }, 9)}");

The index check runs first, so the read never happens when the index is out of range. Every other binary operator evaluates both sides, left before right, before it is applied.

use std;
  
pub fn noisy(answer: bool) -> bool {
    println("evaluated");
    return answer;
}
  
let skipped = false && noisy(true);
  
println($"{skipped}");

That program prints false and nothing else. noisy is never called.