Structures

Tolk supports structures — similar to TypeScript classes, but designed to operate on TVM.

struct Point {
    x: int
    y: int
}

fun calcMaxCoord(p: Point) {
    return p.x > p.y ? p.x : p.y
}

fun demo() {
    // declared using object-literal syntax
    var p: Point = { x: 10, y: 20 };
    calcMaxCoord(p);

    // constructed using object-literal syntax
    calcMaxCoord({ x: 10, y: 20 });
}

Two identical structures are not assignable

struct SomeA { v: int }
struct SomeB { v: int }

fun acceptA(a: SomeA) {}

fun demo(a: SomeA, b: SomeB) {
    b = a;      // error, can not assign `SomeA` to `SomeB`
    acceptA(b); // error, can not pass `SomeB` to `SomeA`
}

Even though A and B have identical body, they represent distinct types. Analogy: a struct behaves like a TypeScript class, not interface. Typing is nominal, not structural.

The compiler infers the type from context

In a snippet below, the compiler understands that { ... } is StoredInfo because of parameter’s type:

fun store(info: StoredInfo) {
    // ...
}

fun demo() {
    store({
        counterValue: ...,
        ownerAddress: ...,
    });
}

The same applies to return values and assignments:

fun loadData(): StoredInfo {
    return {
        counterValue: ...,
        ownerAddress: ...,
    }
}

fun demo() {
    var s: StoredInfo = { counterValue, ... };
    var s: (int, StoredInfo) = (0, { counterValue, ... });
}

Explicit type hints are also available

Besides the plain { ... } syntax, the form StructName { ... } may be used, similar to Rust. The snippet below is equivalent to the above:

fun loadData() {
    return StoredInfo {
        counterValue: ...,
        ownerAddress: ...,
    }
}

fun demo() {
    var s = StoredInfo { counterValue, ... };
    var s = (0, StoredInfo { counterValue, ... });
}

When neither contextual information nor an explicit type hint is available, the type cannot be inferred and an error is produced.

val o = { x: 10, y: 20 };    // error, what type is it?

Methods for structures

Methods are declared as extension functions, similar to Kotlin:

fun Point.calcSquare(self) {
    return self.x * self.y
}

Notice the first self parameter. Without it, a method will be static.

By default, self is immutable. The form mutate self enables mutation.

Read Functions and methods.

Prefixes do not affect typing or layout

Syntax struct (PREFIX) Name { ... } specifies a serialization prefix. It affects binary representation only, nothing else changes:

struct (0x12345678) CounterIncrement {
    byValue: uint32
}

fun demo(inc: CounterIncrement) {
    // `inc` has one field; the prefix is not a property
    inc.byValue
    // `inc` is still one TVM `INT` on the stack
}

Syntax of structures

• Shorthand syntax { x, y } is available
• Default values for fields: a: int32 = 10
• private and readonly fields
• Serialization prefixes (opcodes)

Read Syntax of structures and fields.

Stack layout and serialization

Fields are placed on the stack sequentially and are serialized in the same order. If a struct has a prefix, it is written first.

For details, follow TVM representation and Serialization.