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Language guide

Table of contents​

  1. Basic syntax
  2. Functions
  3. Variables and types
  4. Control flow
  5. Type system
  6. Collections
  7. Error handling
  8. Structures
  9. Methods
  10. Imports and modules
  11. Advanced features
  12. Standard library

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 };

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);
}

Imports and modules​

Import syntax​

import "another";
import "@stdlib/tvm-dicts";

Standard library​

Common functions are available by default:

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

Some functions require importing specific modules (your IDE will suggest them):

import "@stdlib/tvm-dicts";

var dict = createEmptyDict();

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.

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