Conditions

if runs a block when a condition holds, and else says what to do when it does not.

use std;
  
let value = 3;
  
if value == 0 {
    println("zero");
} else if value < 0 {
    println("negative");
} else {
    println("positive");
}

There are no parentheses around the condition, and the branches are always blocks. There is no single statement form, so a body is written in braces however short it is.

The condition is a bool

A condition has to be a bool and nothing else. Mew has no notion of a truthy value, so a number is not a condition, and neither is a value that might be null.

let count = 1;
  
if count {
    let unreachable = 0;
}

Compare it instead, and the comparison is the bool.

use std;
  
let count = 1;
  
if count != 0 {
    println("not zero");
}

The operators that produce one are the comparisons and the logical operators, and && and || short circuit, so a cheap test can guard an expensive one.

use std;
  
pub fn at(values: i32[], index: i32) -> i32 {
    if index >= 0 && index < values.count {
        return values[index];
    }
  
    return -1;
}
  
println($"{at(new i32[] { 1, 2, 3 }, 1)}");
println($"{at(new i32[] { 1, 2, 3 }, 9)}");

An if produces no value

if is a statement. It does not produce a value, so it cannot be assigned from, and Mew has no conditional expression to reach for instead.

Write the branch as an assignment to a mut local.

use std;
  
let value = 3;
let mut label = "";
  
if value < 0 {
    label = "negative";
} else {
    label = "positive";
}
  
println(label);

Or return from each branch, which is usually clearer.

use std;
  
pub fn label(value: i32) -> string {
    if value < 0 {
        return "negative";
    }
  
    return "positive";
}
  
println(label(3));

match is the one conditional that does read as a value, and it works on a union rather than on a bool.