Contracts

Contracts in Tact are similar to classes in popular object-oriented languages, except that their instances are deployed on the blockchain and they can't be passed around like Structs and Messages.

Self-references

Contracts and traits have a built-in identifier self, which is used for referring to their fields (persistent state variables and constants) and methods (internal functions):

contract Example {
    // persistent state variables
    foo: Int;
 
    init() {
        self.foo = 42; // <- referencing variable foo through self.
    }
}

Structure

Each contract can contain:

• Supported interfaces
• Persistent state variables
• Constructor function init()
• Contract constants
• Getter functions
• Receiver functions
• Internal functions

Furthermore, contracts can inherit all the declarations and definitions from traits and override some of their default behaviours. If declared or defined in a trait, internal functions and constants can be marked as virtual or abstract and overridden in contracts inheriting from the trait.

Supported interfaces, @interface(…)

It's hard to figure out what a contract does and what receivers and getters it has without looking at its source code. Sometimes the source is unavailable or inaccessible, and all that's left is to try to disassemble the contract and introspect it that way, which is a very messy and error-prone approach with diminishing returns and no real reproducibility.

In order to resolve this issue, an OTP-001: Supported Interfaces was created. In accordance to it, Tact contracts can report the list of supported interfaces as a return value of a special supported_interfaces getter. That getter is accessible off-chain using any TON Blockchain explorer — one just needs to specify supported_interfaces as a method to execute and get a list of hexadecimal values in return.

These hexadecimal values are truncated to the first 128 bit of SHA-256 (opens in a new tab) hashes of the original String values of the supported interfaces. The first value in this list must be equal to $\mathrm{0x5cec3d5d2cae7b1e84ec39d64a851b66}$ in hexadecimal notation, which is the first half of the SHA-256 hash for "org.ton.introspection.v0". If the first value is wrong, you must stop trying to introspect the contract, as it doesn't conform to the Supported Interfaces proposal.

To declare support of a certain interface, add one or more @interface("…") attributes right before contract and trait declarations:

@interface("His name is")
@interface("John")
contract SeeNah with Misc {
    // ...
}
 
@interface("name_of_your_org - miscellaneous")
trait Misc {
    // ...
}

Tact has a small set of interfaces provided under specific conditions:

• "org.ton.abi.ipfs.v0", in accordance to OTP-003: Self-ABI Reporting — opt-in via ipfsAbiGetter config property
• "org.ton.deploy.lazy.v0", in accordance to OTP-005: Argument-addressable contracts
• "org.ton.debug.v0", but only if debug mode is enabled
• "org.ton.chain.any.v0" if masterchain support is enabled, and "org.ton.chain.workchain.v0" otherwise

Some traits in standard libraries define their interfaces too:

• Ownable trait specifies "org.ton.ownable"
• OwnableTransferable trait specifies "org.ton.ownable.transferable.v2"
• Stoppable trait specifies "org.ton.stoppable"
• Resumable trait specifies "org.ton.resumable"

To enable supported_interfaces getter generation and use @interface() attribute in your Tact contracts, modify a tact.config.json file in the root of your project (or create it if it didn't exist yet), and set the interfacesGetter property to true.

If you're working on a Blueprint (opens in a new tab)-based project, you can enable supported_interfaces in the compilation configs of your contracts, which are located in a directory named wrappers/:

import { CompilerConfig } from '@ton/blueprint';
 
export const compile: CompilerConfig = {
  lang: 'tact',
  target: 'contracts/your_contract_name.tact',
  options: {
    interfacesGetter: true, // ← that's the stuff!
  }
};

In addition to that, tact.config.json may still be used in Blueprint (opens in a new tab) projects. In such cases values specified in tact.config.json act as default unless modified in the wrappers/.

Persistent state variables

Contracts can define state variables that persist between contract calls. Contracts in TON pay rent (opens in a new tab) in proportion to the amount of persistent space they consume, so compact representations via serialization are encouraged.

contract Example {
    // persistent state variables
    val: Int;              // Int
    val32: Int as uint32;  // Int serialized to an 32-bit unsigned
    mapVal: map<Int, Int>; // Int keys to Int values
    optVal: Int?;          // Int or null
}

State variables must have a default value or initialized in init() function, that runs on deployment of the contract. The only exception is persistent state variables of type map<K, V> since they are initialized empty by default.

Contract constants

Unlike variables, constants cannot change. Their values are calculated in compile-time and cannot change during execution.

There isn't much difference between constants defined outside of a contract (global constants) and inside the contract (contract constants). Those defined outside can be used by other contracts in your project.

Constant initializations must be relatively simple and only rely on values known during compilation. If you add two numbers for example, the compiler will calculate the result during build and put the result in your compiled code.

You can read constants both in receivers and in getters.

Unlike contract variables, contract constants don't consume space in persistent state. Their values are stored directly in the code Cell of the contract.

// global constants are calculated in compile-time and cannot change
const GlobalConst1: Int = 1000 + ton("42") + pow(10, 9);
 
contract Example {
    // contract constants are also calculated in compile-time and cannot change
    const ContractConst1: Int = 2000 + ton("43") + pow(10, 9);
 
    // contract constants can be an easy alternative to enums
    const StateUnpaid: Int = 0;
    const StatePaid: Int = 1;
    const StateDelivered: Int = 2;
    const StateDisputed: Int = 3;
 
    get fun sum(): Int {
        // access constants from anywhere
        return GlobalConst1 + self.ContractConst1 + self.StatePaid;
    }
}

Read more about constants on their dedicated page: Constants.

Constructor function init()

On deployment of the contract, the constructor function init() is run.

If a contract has any persistent state variables without default values specified, it must initialize them in this function.

contract Example {
    // persistent state variables
    var1: Int = 0; // initialized with default value 0
    var2: Int;     // must be initialized in the init() function
 
    // constructor function
    init() {
        self.var2 = 42;
    }
}

If a contract doesn't have any persistent state variables, or they all have their default value specified, it may omit the init() function declaration altogether. That's because unless explicitly declared, the empty init() function is present by default in all contracts.

The following is an example of a valid empty contract:

contract IamEmptyAndIKnowIt {}

For your convenience, parameter list of init() can have a trailing comma:

contract TheySeeMeTrailing {
    init(
        param1: Int,
        param2: Int, // trailing comma is allowed
    ) {
        // ...
    }
}

Getter functions

Getter functions are not accessible from other contracts and exported only to off-chain world.

Additionally, getters cannot modify the contract's state variables, only read their values and use them in expressions.

contract HelloWorld {
    foo: Int;
 
    init() {
        self.foo = 0;
    }
 
    // getter function with return type Int
    get fun foo(): Int {
        return self.foo; // can't change self.foo here
    }
}

Read more about them in their dedicated section: Getter functions

Receiver functions

Receiver functions in Tact can be one of the following three kinds:

• receive(), which receive internal messages (from other contracts).
• bounced(), which are called when outgoing message from this contract has bounced back.
• external(), which don't have a sender and can be sent by anyone in the world.

message CanBounce {
    counter: Int;
}
 
contract HelloWorld {
    counter: Int;
 
    init() {
        self.counter = 0;
    }
 
    get fun counter(): Int {
        return self.counter;
    }
 
    // internal message receiver, which responds to a string message "increment"
    receive("increment") {
        self.counter += 1;
 
        // sending the message back to the sender
        send(SendParameters{
            to: sender(),
            value: 0,
            mode: SendRemainingValue | SendIgnoreErrors,
            body: CanBounce{counter: self.counter}.toCell(),
        });
    }
 
    // bounced message receiver, which is called when the message bounces back to this contract
    bounced(src: bounced<MsBounced>) {
        self.counter = 0; // reset the counter in case message bounced
    }
 
    // external message receiver, which responds to off-chain message "hello, it's me"
    external("hello, it's me") {
        // can't be replied to as there's no sender!
        self.counter = 0;
    }
}

Internal functions

These functions behave similarly to private methods in popular object-oriented languages — they're internal to contracts and can be called by prefixing them with a special identifier self. That's why internal functions can sometimes be referred to as "contract methods".

Internal functions can access the contract's persistent state variables and constants.

They can only be called from receivers, getters and other internal functions, but not from other contracts or init().

contract Functions {
    val: Int = 0;
 
    // this contract method can only be called from within this contract and access its variables
    fun onlyZeros() {
        require(self.val == 0, "Only zeros are permitted!");
    }
 
    // receiver function, which calls the internal function onlyZeros
    receive("only zeros") {
        self.onlyZeros();
    }
}