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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 comments:

// Single-line comment

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

Identifiers​

Identifiers can start with [a-zA-Z$_] and continue with [a-zA-Z0-9$_], like in most languages, camelCase preferred.

var justSomeVariable = 123;

Functions​

Function declaration​

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() { ... }

When return type 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​

Parameters can have default values:

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

GET methods​

Contract getters — with get fun:

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

Variables and types​

Variable declaration​

Variables are declared with var (mutable) or val (immutable):

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 in 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, you can also use fixed-width integers e.g. int32, uint64
  • bool - boolean (true/false) (caution! TRUE in TVM is -1, not 1)
  • cell - TVM cell
  • slice - TVM slice, you can also use bitsN. (e.g. bits512 for storing signature)
  • builder - TVM builder
  • address - blockchain address
  • coins - Toncoin/coins amounts
  • void - no return value
  • never - never returns (always throws)

Fixed-width integers​

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

Boolean type​

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 with ?:

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 multiple possible types. They are typically handled with match by type:

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

Alternatively, a union can be tested via 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​

Typed tuples:

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

Tensor types​

Tensors (like tuples but on stack):

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

Type aliases​

Create type aliases for clarity:

type UserId = int32
type MaybeOwnerHash = bits256?

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

Address type​

Dedicated type for blockchain addresses:

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

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

Tensors​

Indexed access​

Access elements with 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 extension functions accepting self:

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

Mutating methods​

Use mutate 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 for 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();

While specific require importing some module (IDE suggests you):

import "@stdlib/tvm-dicts";

var dict = createEmptyDict();

Advanced features​

TVM assembler functions​

You can implement functions in TVM Assembler:

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

Function attributes​

Functions can have attributes using @ 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
}

Also, semicolons are optional for top-level declarations: constants, type aliases, struct fields.

Compile-time functions​

String processing functions work at compile time:

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

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

Toncoin amounts​

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're certain a value isn't null:

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

Auto-packing​

Structures can be automatically packed to/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 with 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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