Void and never types

Tolk supports the types void and never, similar to TypeScript. Both represent the absence of a value, but in different ways.

A function that returns nothing is void

Both functions below return void:

fun demo1() {
    debug.print(globalVar);
}

fun demo2() {
    if (random.uint256() == 0) {
        return;
    }
    globalVar = 123;
}

When the return type of a function is not specified, it is inferred from the return statements. A return without a value means “void”.

Explicitly specifying fun(...): void behaves identically:

fun demo1(): void {
    // ...
}

Special fields with void type

If a structure field is void, it indicates that the field does not exist at all. This technique is used in combination with generics <T = void>, allowing structures to express optional fields.

struct WithOptionalField<T> {
    a: int
    b: T
    c: slice
}

type OnlyAC = WithOptionalField<void>

fun demo() {
    // field `b` may be omitted
    val v: OnlyAC = { a: 10, c: "" };
}

A function that never returns is never

An always-throwing function is an example:

fun accessDenied(): never {
    throw 403
}

fun validate(userId: int) {
    if (userId > 0) {
        return userId;
    }
    accessDenied();
    // no `return` statement is needed
}

In a match expression, branches that never return are also of type never:

val result = match (ownerKey) {
    ADMIN_KEY => true,     // `bool`
    else => throw 403,     // `never`
};
// result is `bool`

Implicit never in unreachable conditions

The never type occurs implicitly when a condition is impossible to satisfy. More precisely, it means that no value of such a type can exist; no possible data can produce such a value:

fun demo(v: int | slice) {
    if (v is builder) {
        // v is `never`
    }
}

Another case arises when a non-nullable type is checked against null:

fun demo(v: int) {
    if (v == null) {
        // v is `never`
        v + 10;   // error, can not apply `+` `never` and `int`
    }
    // v is `int` again
}

Encountering never in a compilation error typically indicates an issue in the preceding code.

Stack layout and serialization

Both mean “absence of a value” and occupy zero stack slots and zero bits.

For example, a void function does not place any value onto the stack.

For details, follow TVM representation and Serialization.