Language guide

Basic syntax

Comments

Tolk uses traditional C-style syntax for comments:

// Single-line comment

/* 
   Multi-line comment
   can span multiple lines
*/

Identifiers

Identifiers must start with [a-zA-Z$_] — that is, a letter, underscore, or dollar sign — and may continue with characters from [a-zA-Z0-9$_].

As in most programming languages, camelCase is the preferred naming convention.

var justSomeVariable = 123;

Functions

Function declaration

Use the fun keyword with TypeScript-like syntax:

fun functionName(param1: type1, param2: type2): returnType {
    // function body
}

Examples:

fun parseData(cs: slice): cell { }
fun loadStorage(): (cell, int) { }
fun main() { ... }

If the return type is omitted, it’s auto-inferred.

Generic functions

Tolk supports generic functions:

fun swap<T1, T2>(a: T1, b: T2): (T2, T1) {
    return (b, a);
}

Default parameters

Function parameters can have default values:

fun increment(x: int, by: int = 1): int {
    return x + by;
}

GET methods

Contract getters use the get fun syntax:

get fun seqno(): int {
    return 1;
}

Variables and types

Variable declaration

Declare variables with var for mutable variables and val for immutable ones:

var mutableVar: int = 10;
val immutableVar: int = 20;

Type annotations are optional for local variables:

var i = 10; // int inferred
var b = beginCell(); // builder inferred

Variable scope

Variables cannot be redeclared within the same scope:

var a = 10;
var a = 20; // Error! Use: a = 20;

if (true) {
    var a = 30; // OK, different scope
}

Control flow

Conditional statements

if (condition) {
    // code
} else if (anotherCondition) {
    // code
} else {
    // code
}

Loops

While loop

while (condition) {
    // code
}

Do-while loop

do {
    // code
} while (condition);

Repeat loop

repeat (10) {
    // body
}

Type system

Basic types

• int - integer; fixed-width variants like int32 and uint64 are also supported
• bool - boolean (true/false). Note: in TVM, true is represented as -1, not 1
• cell - a TVM cell
• slice - a TVM slice; you can also use bitsN. (e.g. bits512 for storing a signature)
• builder - a TVM builder
• address - blockchain address
• coins - Toncoin or coin amounts
• void - no return value
• never - function never returns (always throws an exception)

Fixed-width integers

var smallInt: int32 = 42;
var bigInt: uint64 = 1000000;

Boolean type

The Boolean type is distinct from integers:

var valid: bool = true;
var result: bool = (x > 0);

if (valid) { // accepts bool
    // code
}

// Cast to int if needed
var intValue = valid as int; // -1 for true, 0 for false

Nullable types

Nullable types are denoted by ?:

var maybeInt: int? = null;
var maybeCell: cell? = someCell;

if (maybeInt != null) {
    // Smart cast: maybeInt is now int
    var result = maybeInt + 5;
}

Union types

Union types allow a value to have multiple possible types. Typically, you handle them using match by type:

fun processValue(value: int | slice) {
    match (value) {
        int => {
            // Got integer
        }
        slice => {
            // Got slice
        }
    }
}

Alternatively, you can test a union value using the is or !is operators:

fun processValue(value: int | slice) {
    if (value is slice) {
        // call methods for slice
        return;
    }
    // value is int
    return value * 2;
}

Tuple types

Tuples with explicit types:

var data: [int, slice, bool] = [42, mySlice, true];

Tensor types

Tensors are similar to tuples but stored on the stack:

var coords: (int, int) = (10, 20);
var x = coords.0; // Access first element
var y = coords.1; // Access second element

Type aliases

Create aliases to improve code clarity:

type UserId = int32
type MaybeOwnerHash = bits256?

fun calcHash(id: UserId): MaybeOwnerHash { ... }

Address type

A dedicated type for blockchain addresses:

val addr = address("EQDKbjIcfM6ezt8KjKJJLshZJJSqX7XOA4ff-W72r5gqPrHF");

if (addr.isInternal()) {
    var workchain = addr.getWorkchain();
}

Tensors

Indexed access

Access elements using dot . notation:

var t = (5, someSlice, someBuilder);
t.0 = 10; // Modify first element
t.1; // Access second element

Tuples

var t = [5, someSlice, someBuilder];
t.0 = 10; // asm "SETINDEX"
var first = t.0; // asm "INDEX"

Error handling

Throw and assert

Simplified error handling:

throw 404; // Throw exception with code
throw (404, "Not found"); // Throw with data

assert (condition) throw 404; 
assert (!condition) throw 404;

Try-catch

try {
    riskyOperation();
} catch (excNo, arg) {
    // Handle exception
}

Structures

Struct declaration

struct Point {
    x: int
    y: int
}

Creating objects

var p: Point = { x: 10, y: 20 };
var p2 = Point { x: 5, y: 15 };

Default values

struct Config {
    timeout: int = 3600
    enabled: bool = true
}

var config: Config = {}; // Uses defaults

Generic structures

struct Container<T> {
    value: T
    isEmpty: bool
}

var intContainer: Container<int> = { value: 42, isEmpty: false };

Field modifiers

• private field — accessible only within methods
• readonly field — immutable after object creation

struct PosInTuple {
    private readonly t: tuple
    curIndex: int
}

fun PosInTuple.last(mutate self) {
    // `t` is visible only in methods
    // and can not be modified
    self.curIndex = self.t.size() - 1;
}

Methods

Instance methods

Methods are implemented as extension functions that accept self:

fun Point.distanceFromOrigin(self): int {
    return sqrt(self.x * self.x + self.y * self.y);
}

Mutating methods

Use the mutate keyword for methods that modify the receiver:

fun Point.moveBy(mutate self, dx: int, dy: int) {
    self.x += dx;
    self.y += dy;
}

Methods for any type

fun int.isZero(self) {
    return self == 0;
}

fun T.copy(self): T {
    return self;
}

Chaining methods

Methods can return self to enable method chaining:

fun builder.storeInt32(mutate self, value: int32): self {
    return self.storeInt(value, 32);
}

Enums

// will be 0 1 2
enum Color {
    Red
    Green
    Blue
}

They are:

• similar to TypeScript/C++ enums
• distinct type, not just int
• checked on deserialization
• allowed in throw and assert

Enums syntax

Enum members can be separated by , or by ; or by a newline — like struct fields.

Like in TypeScript and C++, you can manually specify a value, the following will be auto-calculated.

enum Mode {
    Foo = 256,
    Bar,        // implicitly 257
}

Enums usage: they are distinct types

Since enums are types, you can

• declare variables and parameters
• declare methods for an enum
• use them in struct fields, in unions, in generics, etc.

struct Gradient {
    from: Color
    to: Color? = null
}

fun Color.isRed(self) {
    return self == Color.Red
}

var g: Gradient = { from: Color.Blue };
g.from.isRed();       // false

Enums are integers under the hood. They can be cast to int and back with as operator.

Serialization of enums

You can manually specify :intN. Otherwise, the compiler will auto-calculate it.

// will be (un)packed as `int8`
enum Role1: int8 { Admin, User, Guest }

// will (un)packed as `uint2` (auto-calculated to fit all values)
enum Color { Red, Green, Blue }

On deserialization, an input value is checked for correctness. E.g., for Role, if input<0 or input>2, an exception “5 (integer out of range)” will be thrown.

Imports and modules

Import syntax

import "another"
import "@stdlib/gas-payments"

Advanced features

TVM assembler functions

You can implement functions directly in TVM assembler:

@pure
fun third<X>(t: tuple): X 
    asm "THIRD"

Function attributes

Functions can have attributes using the @ syntax:

@inline
fun fastFunction() {}

@inline_ref
fun load_data() {}

@deprecated
fun oldFunction() {}

@method_id(1666)
fun afterCodeUpgrade(oldCode: continuation) {}

Trailing commas

Tolk supports trailing commas in tensors, tuples, function calls, and parameters:

var items = (
    totalSupply,
    verifiedCode,
    validatorsList,
);

Optional semicolons

Semicolons are optional for the last statement in a block:

fun f() {
    doSomething();
    return result // Valid without semicolon
}

Semicolons are also optional for top-level declarations such as constants, type aliases, and struct fields.

Compile-time functions

String-processing functions execute at compile time:

const BASIC_ADDR = address("EQDKbjIcfM6ezt8KjKJJLshZJJSqX7XOA4ff-W72r5gqPrHF");
const HASH = stringSha256_32("transfer(slice, int)");

Available compile-time functions include: stringCrc32, stringCrc16, stringSha256, stringSha256_32, stringHexToSlice, and stringToBase256.

Toncoin amounts

Use human-readable Toncoin amounts:

val cost = ton("0.05"); // 50,000,000 nanotons
const ONE_TON = ton("1");

Smart casts

Automatic type narrowing:

if (value != null) {
    // value is automatically cast from T? to T
    value.someMethod();
}

Non-null assertion

Use ! when you are sure a value is not null:

fun processCell(maybeCell: cell?) {
    if (hasCell) {
        processCell(maybeCell!); // bypass nullability check
    }
}

Auto-packing

Structures can be automatically packed to and unpacked from cells:

struct Point {
    x: int8
    y: int8
}

var point: Point = { x: 10, y: 20 };
var cell = point.toCell(); // Auto-pack
var restored = Point.fromCell(cell); // Auto-unpack

Deep dive: Auto-packing.

”Lazy loading” from cells: unpack only requested fields

val st = lazy Storage.load();
// the compiler skips everything and loads only what you access
return st.publicKey;

Deep dive: Lazy loading.

Universal message composition

Create messages using a high-level syntax:

val reply = createMessage({
    bounce: false,
    value: ton("0.05"),
    dest: senderAddress,
    body: RequestedInfo { ... }
});
reply.send(SEND_MODE_REGULAR);

Deep dive: Sending messages.

Standard library

Common functions are available by default:

// Common functions always available
var time = blockchain.logicalTime();

While more specific ones require importing (IDE suggests you):

import "@stdlib/gas-payments"

var fee = calculateStorageFee(...);