Variables and values

Use let to give a value a name. Use let mut if you will change it later. An annotation such as : Int specifies a type; omit it when the value makes the type clear.

Save this as variables.xen:

fn main() {
    let price: Int = 12;
    let mut quantity = 2;
    quantity = quantity + 1;
    let total = price * quantity;
    println(total);
    println(float(total) / 2.0);
}

$ xen run variables.xen
36
18.0

Changing quantity does not automatically update a previously calculated total. Calculate again to get a new total. Assigning a new value to a variable declared with let is an error.

Everyday types

Type	Values
Int	Integers such as 42 and -7; the same type as I64
Float	Numbers such as 2.5; the same type as F64
Bool	true and false
String	Strings such as "hello"
Unit	The return type of a function with no result; its value is ()

Integer literals default to Int, and floating literals to Float. A known type at the point of use can determine their type. Other integer choices are I8, I16, I32, I64 and unsigned U8, U16, U32, U64. Floating types are F32 and F64.

Arithmetic and conversion

Use +, -, *, /, and integer remainder %. Dividing two Int values produces an integer. Convert with float(...) first if you need a fractional result. Functions such as i32(...), u8(...), and f32(...) select other numeric types explicitly. Values outside the destination type’s range cause an error.

Different numeric types do not mix automatically. This deliberately incorrect program demonstrates the rule:

fn main() {
    let wide: I64 = 7;
    let narrow: I32 = wide;
    println(narrow);
}

xen check numeric_error.xen

The error includes expected I32. Change the declaration to let narrow: I32 = i32(wide);.

Integer +, -, and * wrap when a result exceeds the type’s range. Consider the range in large calculations. Division by zero and dividing the minimum signed value by -1 cause runtime errors.

Comparisons and comments

==, !=, <, <=, >, and >= produce a Bool. Combine conditions with &&, ||, and !. && and || skip the right operand when the left operand determines the answer. Write a line comment after //.

You cannot declare the same name twice in one block. A nested { ... } block may declare a name used outside it; the outer name becomes visible again after leaving the block.

The next chapter, Data types and conversions, develops type selection, conversion failures, and the boundaries of inference through worked examples.