Learn Tact in Y minutes
Tact is a fresh programming language for TON Blockchain that is focused on efficiency and ease of development. It is a good fit for complex smart contracts, quick onboarding, and rapid prototyping.
You can try Tact without installing anything locally using the Web IDE. In addition, most examples below have an “Open in Web IDE” button for your convenience.
Comments
// Single-line (//) comments for occasional and casual annotations
/// Documentation comments that support Markdown”Hello, World!”
// Defining a contractcontract HelloWorld { // Listens to incoming Ping messages receive(msg: Ping) { // Sends a Pong reply message reply(Pong {}.toCell()); }
// Listens to incoming Hello messages receive(msg: Hello) { // Replies with the received Hello message reply(msg.toCell()); }
// Listens to incoming empty messages, // which are very handy and cheap for the deployments. receive() { // Forward the remaining value in the // incoming message back to the sender. cashback(sender()); }}
// A helper inlined function to send binary messages.// See the "Primitive types" section below for more info about cells.inline fun reply(msgBody: Cell) { message(MessageParameters { to: sender(), value: 0, mode: SendRemainingValue | SendIgnoreErrors, body: msgBody, });}
// Empty message structs with specified 32-bit integer prefix.// See the "Structs and message structs" section below for more info.message(1) Ping {}message(2) Pong {}message(3) Hello {}Primitive types
fun showcase() { // There are two main groups of primitive types in Tact: integers and cells. // All other primitive types are derivatives of those two.
// --- // Integers, // always 257-bit signed in runtime operations, // but may have different lengths in persistent contract's state (storage) // ---
let one_plus_one: Int = 1 + 1; // 2 let two_by_two: Int = 2 / 2; // 1 let three_by_two: Int = 3 / 2; // 1, because the division operator rounds // toward -∞, which is identical to // operator // from Python
let one_billion = 1_000_000_000; // decimal let binary_mask = 0b1_1111_1111; // binary let permissions = 0o7_5_5; // octal let heHex = 0xFF80_0000_0000; // hexadecimal
let nanoToncoin: Int = 1; // 1 nanoToncoin = 0.000,000,001 Toncoin let toncoin: Int = ton("1"); // 1 Toncoin = 1,000,000,000 nanoToncoin
// --- // Booleans: true and false. // They take only 1 bit in persistent storage. // ---
let factual: Bool = !!(true || false); let fax: Bool = true && factual;
// --- // Addresses of smart contracts, // deterministically obtained by combining the initial code and initial data. // ---
// Address of the current contract let myAddr: Address = myAddress();
// You can parse the Address to view components of the standard address: // * a workchain ID: 8-bit signed Int // * and an account ID: 256-bit unsigned Int let addrComponents: StdAddress = parseStdAddress(myAddr.asSlice()); addrComponents.workchain; // 0, basechain: the most commonly used workchain on TON addrComponents.address; // ...lots of digits...
// --- // Cells, Builders, Slices. // ---
// Cell is an immutable data structure that can contain up to 1023 bits // with up to 4 reference to other cells. Cyclic references are prohibited. let emptyC: Cell = emptyCell();
// Cells are a fundamental primitive and data structure on TON Blockchain: // contracts communicate and interact by sending and receiving cells while // their code and data are themselves stored as cells on the blockchain // the code and the data of each contract are cells and contracts // communicate and interact by sending and receiving cells. // // Furthermore, all data layouts are also expressed in terms of cells and // cell (de)serialization primitives. That said, Tact provides declarative means // to express (de)serialization to and from cells conveniently — // see the "Structs and message structs" subsection below for more info.
// Builder is an immutable primitive to construct (compose) cells. let bb: Builder = beginCell() .storeUint(42, 6) // storing 42 using 6 bits .storeInt(42, 7) // storing 42 using 7 bits (signed Int) .storeBool(true) // writing 1 as a single bit .storeBit(true) // alias to storeBool() .storeCoins(40) // common way of storing nanoToncoins .storeAddress(myAddress()) .storeRef(emptyC); // storing a reference let composed: Cell = bb.endCell();
// Slice is a mutable primitive to deconstruct (parse) cells. let target: Slice = composed.asSlice(); // let's start parsing `composed` Cell
// The type ascription is optional for most cases except for maps // and optional types, but we'll discuss those in the // "Composite types" section below. let fortyTwo = target.loadUint(6); // taking 6 bits out of the `target` Slice, // mutating it in the process
// If you don't want the result, you can ignore it with a wildcard. let _ = target.loadInt(7);
// Finally, there are methods to skip the value, i.e., to discard it. target.skipBool();
// Manual composition and parsing of Cells is tedious, // error-prone and is generally not recommended. // Instead, prefer using structures: struct and message struct types. // See the "Composite types" section below for more info.
// --- // Strings are immutable sequences of characters, // which are used mainly to send and receive text message bodies. // ---
// String literals are wrapped in double-quotes and can contain escape sequences, // but they intentionally cannot be concatenated via any operators. let str: String = "I am a string literal, 👻!"; // see the "Expressions" section for more
// Strings are useful for storing text, // so they can be converted to a Cell type to be used as message bodies. let noComments: Cell = "yes comments".asComment(); // prefixes a string with 32 zero bits}
// Finally, under the hood, Address and String types are a Slice,// although with a well-defined distinct data layout for each.//// While implicit type conversions aren't allowed in Tact,// there are extension functions that can be used for those purposes,// such as String.asSlice() or Address.asSlice().//// Advanced users can introduce their own casts by using assembly functions.// See the "Functions" section below for more info.
// An empty contract needed for the showcase above to work.contract MyContract() {}Read more: Primitive types.
Composite types
Optionals
fun showcase() { // An optional is a value than can be of any type or null. // Null is a special value that represents the intentional // absence of any other value.
// Int keys to Int values. // Type ascription of optionals is mandatory. let optionalVal: Int? = null; optionalVal = 255;
// If you're certain that the value isn't null at a given moment, // use the non-null assertion operator !! to access it. dump(optionalVal!!);
// If you are not certain, then it is better to explicitly compare // the value to null to avoid errors at runtime. if (optionalVal != null) { // here we go! } else { // not happening right now }}
// You can make almost any variable or field optional by adding// a question mark (?) after its type declaration.// The only exceptions are map<K, V> and bounced<Message>,// in which you cannot make the inner key/value type (in the case of a map)// or the inner message struct (in the case of a bounced) optional.Read more: Optionals.
Maps
fun showcase() { // The composite type map<K, V> is used to associate // keys of type K with corresponding values of type V.
// A map of Int keys to Int values. // Type ascription is mandatory. let myMap: map<Int, Int> = emptyMap();
// Maps have a number of built-in methods. myMap.set(0, 10); // key 0 now points to value 10 myMap.set(0, 42); // overriding the value under key 0 with 42 myMap.get(0)!!; // 42, because get can return null if the key doesn't exist myMap.replace(1, 55); // false, because there was no key 1 and map didn't change myMap.replaceGet(0, 10)!!; // 42, because the key 0 exists and the old value there was 42 myMap.get(0)!!; // 10, since we've just replaced the value with .replaceGet myMap.del(0); // true, because the map contained an entry under key 0 myMap.del(0); // false and not an error, because deletion is idempotent myMap.exists(0); // false, there is no entry under key 0 myMap.isEmpty(); // true, there is no other entries
// Statically known `.set`s can be replaced by map literals. // That way, map entries will be defined at compile-time and consume much less gas. let myMap2: map<Int as uint8, Int as int13> = map<Int as uint8, Int as int13> { // Key expression: Value expression 1 + 2: 10 * pow2(3), // key 3, value 80 1 + 3: 20 * pow2(4), // key 4, value 320 };
// In most cases, to compare two maps it's sufficient to use the shallow // comparison via the equality == and inequality != operators. myMap == emptyMap(); // true
// To traverse maps, the foreach statement is used. // See the "Statements" section below for more info. foreach (k, v in myMap) { // ...do something for each entry, if any }
// There are many other allowed kinds of map value types for Int keys let _: map<Int, Bool> = emptyMap(); // Int keys to Bool values let _: map<Int, Cell> = emptyMap(); // Ints to Cells let _: map<Int, Address> = emptyMap(); // Ints to Addresses let _: map<Int, AnyStruct> = emptyMap(); // Ints to some structs let _: map<Int, AnyMessage> = emptyMap(); // Ints to some message structs
// And all the same value types for maps with Address keys are also allowed. let _: map<Address, Int> = emptyMap(); // Address keys to Int values let _: map<Address, Bool> = emptyMap(); // Addresses to Bools let _: map<Address, Cell> = emptyMap(); // Addresses to Cells let _: map<Address, Address> = emptyMap(); // Addresses to Addresses let _: map<Address, AnyStruct> = emptyMap(); // Addresses to some structs let _: map<Address, AnyMessage> = emptyMap(); // Addresses to some message structs
// Under the hood, empty maps are nulls, which is why it's important to provide a type ascription. let _: map<Int, Int> = null; // like emptyMap(), but less descriptive and generally discouraged
// Furthermore, as with many other types, maps are just Cells with a distinct data layout. // Therefore, you can type cast any map back to its underlying Cell type. myMap.asCell();}
// Serialization of integer keys or values is possible but only meaningful// for maps as fields of structures and maps in the contract's persistent state.// See the "Structs and message structs" and "Persistent state"// sections below for more info.
// Finally, mind the limits — maps are quite gas-expensive// and have an upper limit of around 32k entries for the whole contract.//// On TON, contracts are very limited in their state, and for large// or unbounded (infinitely large) maps, it is better to use contract sharding// and essentially make the entire blockchain part of your maps.//// See this approach in action for the Jetton (token)// contract system by the end of this tour.
// The following are dummy structures needed for the showcase above to work.struct AnyStruct { field: Int }message AnyMessage { field: Int }Read more:
Structs and message structs
// Structs and message structs allow multiple values to be packed together// in a single type. They are very useful for (de)serialization of Cells// and for usage as parameters or return types in functions.
// Struct containing a single valuestruct One { number: Int; }
// Struct with default fields, fields of optional types, and nested structsstruct Params { name: String = "Satoshi"; // default value
age: Int?; // field with an optional type Int? // and an implicit default value of null
val: One; // nested struct One}
// You can instruct how to (de)compose the Cells to and from structs// by specifying certain serialization options after the `as` keyword.struct SeriesXX { i64: Int as int64; // signed 64-bit integer u32: Int as uint32; // unsigned 32-bit integer ufo51: Int as uint51; // uneven formats are allowed too, // so this is an unsigned 51-bit integer
// In general, uint1 through uint256 and int1 through int257 // are valid serialization formats for integer values. maxi: Int as int257; // Int is serialized as int257 by default, // but now it is explicitly specified
// If this struct will be obtained from some Slice, // you can instruct the compiler to place the remainder of that Slice // as the last field of the struct, and even type cast the value // of that field to Cell, Builder or Slice at runtime. lastFieldName: Cell as remaining; // there can only be a single `remaining` field, // and it must be the last one in the struct}
// The order of fields matters, as it corresponds to the resulting// memory layout when the struct will be used to compose a Cell// or to parse a Slice back to the struct.struct Order { first: Int; // 257 continuously laid out bits second: Cell; // up to 1023 bits, // which will be placed in a separate ref // when composing a Cell third: Address; // 267 bits}
// Message structs are almost the same as regular structs,// but they have a 32-bit integer header in their serialization.// This unique numeric ID is commonly referred to as an opcode (operation code),// and it allows message structs to be used with special receiver functions// that distinguish incoming messages based on this ID.message ImplicitlyAssignedId {} // no fields, // but not empty because of the automatically // generated and implicitly set 32-bit Int opcode
// You can manually override an opcode with any compile-time expression// that evaluates to a non-negative 32-bit integer.
// This message has an opcode of 898001897, which is the evaluated// integer value of the specified compile-time expression.message((crc32("Tact") + 42) & 0xFFFF_FFFF) MsgWithExprOpcode { // All the contents are defined identical to regular structs. field1: Int as uint4; // serialization field2: Bool?; // optionals field3: One; // nested structs field4: ImplicitlyAssignedId; // nested message structs}
// Some usage examples.fun usage() { // Instantiation of a struct. // Notice the lack of the "new" keyword used for this in many // other traditional languages. let val: One = One { number: 50 };
// You can omit the fields with default values. let _ = Params { val }; // the field punning works — // instead of `val: val` you could write just `val`
// Convert a struct to a Cell or a Slice. let valCell = val.toCell(); let valSlice = val.toSlice();
// Obtain a struct from a Cell or a Slice. let _ = One.fromCell(valCell); let _ = One.fromSlice(valSlice);
// Conversion works both ways. One.fromCell(val.toCell()).toCell() == valCell; One.fromSlice(val.toSlice()).toSlice() == valSlice;}Read more: Structs and Messages.
Operators
fun showcase() { // Let's omit the type ascriptions and let the compiler infer the types. let five = 5; // = is an assignment operator, // but it can be a part of the assignment statement only, // because there is no assignment expression let four = 4;
// Most operators below have augmented assignment versions, like +=, -=, etc. // See the "Statements" section below for more info.
// Common arithmetic operators have predictable precedences. five + four - five * four / five % four; // 9
// You can change order of operations with parentheses. (five + (four - five)) * four / (five % four); // 16
// The % is the modulo, not the remainder operator. 1 % five; // 1 1 % -five; // -4
// Negation and bitwise NOT. -five; // -5: negation of 5 ~five; // -6: bitwise NOT of 5 -(~five); // 6: bitwise NOT, then negation ~(-five); // 4: negation, then bitwise NOT
// Bitwise shifts. five << 2; // 20 four >> 2; // 1 -four >> 2; // -1, because negation is applied first // and >> performs arithmetic or sign-propagating right shift
// Other common bitwise operators. five & four; // 4, due to bitwise AND five | four; // 5, due to bitwise OR five ^ four; // 1, due to bitwise XOR
// Relations. five == four; // false five != four; // true five > four; // true five < four; // false five - 1 >= four; // true five - 1 <= four; // true
// Logical checks. !(five == 5); // false, because of the inverse ! operator false && five == 5; // false, because && is short-circuited true || five != 5; // true, because || is also short-circuited
// The non-null assertion operator raises a compilation error if the value // is null or if the type of the value is not optional, // i.e., it can never be null. let maybeFive: Int? = five; maybeFive!!; // 5
// Ternary operator ?: is right-associative. false ? 1 : (false ? 2 : 3); // 3 false ? 1 : true ? 2 : 3; // 2}Read more: Operators.
Expressions
contract MyContract() { fun showcase() { // Integer literals. 0; 42; 1_000; 020; // decimal, base 10 0xABC; 0xf; 0x001; // hexadecimal, base 16 0o777; 0o00000001; // octal, base 8 0b111010101111010; // binary, base 2
// Boolean literals. true; false;
// String literals. "You can be The Good Guy or the guy who saves the world... You can't be both."; "1234"; // a string, not a number "👻"; // strings support Unicode "\\ \" \n \r \t \v \b \f \x00 through \xFF"; // common escape sequences "\u0000 through \uFFFF and \u{0} through \u{10FFFF}"; // unicode escape sequences
// `null` and `self` literals. null; // not an instance of a primitive type, but // a special value that represents the intentional absence // of any other value
self; // used to reference the current contract from within // and the value of the currently extended type inside // the extension function. See the "Functions" section below for more.
// Map literals. map<Int, Int as coins> { 11: 11 };
// Identifiers, with usual naming conventions: // They may contain Latin lowercase letters `a-z`, // Latin uppercase letters `A-Z`, underscores `_`, // and digits 0 - 9, but may not start with a digit. // No other symbols are allowed, and Unicode identifiers are prohibited. // They also cannot start with __gen or __tact since those prefixes // are reserved by the Tact compiler. let azAZ09_ = 5; azAZ09_;
// Instantiations or instance expressions of structs and message structs. let addr = BasechainAddress { hash: null, };
// Field access. addr.hash; // null self.MOON_RADIUS_KM; // 1738, a contract-level constant // defined below this function
// Extension function calls (methods). self.MOON_RADIUS_KM.toString(); // "1738" self.notify("Cashback".asComment()); // rather expensive, // use cashback() instead "hey".asComment(); // allowed on literals
// Global function calls. now(); // UNIX timestamp in seconds cashback(sender());
// Some of the functions can be computed at compile-time given enough data. sha256("hey, I'll produce the SHA256 number at compile-time");
// But there are special, compile-time-only functions. let _: Address = address("EQDtFpEwcFAEcRe5mLVh2N6C0x-_hJEM7W61_JLnSF74p4q2"); let _: Cell = cell("te6cckEBAQEAAgAAAEysuc0="); // an empty Cell let _: Slice = slice("te6cckEBAQEADgAAGEhlbGxvIHdvcmxkIXgtxbw="); // a Slice with Hello world! let _: Slice = rawSlice("000DEADBEEF000"); // CS{Cell{03f...430} bits: 588..644; refs: 1..1} let _: Int = ascii("⚡"); // 14850721 or 0xE29AA1, 3 bytes in total let _: Int = crc32("000DEADBEEF000"); // 1821923098 let _: Int = ton("1"); // 10^9 nanoToncoin = one Toncoin, // the main currency of TON Blockchain
// initOf, which obtains the initial code and initial data // of the given contract, i.e., it's initial state. initOf MyContract(); // StateInit { code, data }
// codeOf, which only obtains the code. codeOf MyContract; }
// Constants support compile-time expressions const MOON_RADIUS_KM: Int = 1730 + (8 | 8);}Read more: Expressions.
Statements and control flow
fun showcase() { // As we've seen above, the let statement defines new variables. // You must always provide an initial value, but type ascriptions // aren't mandatory except for maps and null values. let theAnswer = 42; // type ascription is not required here, let m: map<Int, Int> = emptyMap(); // but we must specify it for maps let opt: Int? = null; // and when assigning a null value.
// Block statement creates an enclosed scope. { // theAnswer is accessible here let privateVal = theAnswer + 27; // but privateVal is no longer visible after this block ends. }
// Assignment statement allows reassigning variables. theAnswer = -(~theAnswer + 1);
// Almost every binary operator can form an augmented assignment, // except for relational and equality ones, // and excluding the assignment operator itself. theAnswer += 5; // equivalent to: theAnswer = theAnswer + 5; theAnswer -= 5; // equivalent to: theAnswer = theAnswer - 5; theAnswer *= 5; // and so on, see the Operators page for more.
// Destructuring assignment is a concise way to // unpack structures into distinct variables. let st = StdAddress { workchain: 0, address: 0 }; // let statement let StdAddress { address, .. } = st; // destructuring statement // ------- -- // ↑ ↑ // | ignores all unspecified fields // Int as uint256, // a variable out of the second field of StdAddress struct address; // 0
// You can also define new names for variables // derived from the struct fields. let StdAddress { address: someNewName, .. } = st; someNewName; // 0
// Conditional branching with if...else. if (false) { // curly brackets (code blocks) are required! // ...then branch } else if (false) { // ...else branch } else { // ...last else }
// Try and try...catch, with partial rollback. try { throw(777); } catch (exitCode) { // 777 // An exit code is an integer that indicates whether the transaction // was successful, and if not — holds the code of the exception that occurred. // // The catch block that can catch run-time (compute phase) exit codes // will roll back almost all changes made in the try block, // except for: codepage changes, gas usage counters, etc. // // See the "Testing and debugging" section below for more info. }
// Repeat something N times. repeat (2003) { dump("mine"); // greet the Nemo }
// Loop with a pre-condition: while. while (theAnswer > 42) { theAnswer /= 5; }
// Loop with a post-condition: do...until. do { // This block will be executed at least once, // because the condition in the until close // is checked after each iteration. m = emptyMap(); } until (false);
// Traverse over all map entries with foreach. m.set(100, 456); m.set(23, 500); foreach (key, value in m) { // or just k, v: naming is up to you // Goes from smaller to bigger keys: // first iteration key = 23 // second iteration key = 100 }
// If you don't want key, value, or both, then use a wildcard. let len = 0; foreach (_, _ in m) { len += 1; // don't mind me, just counting the size of the map }
// Finally, return statement works as usual. return; // implicitly produces nothing (named "void" in the compiler) // return 5; // would explicitly produce 5}Read more: Statements.
Functions
// Global function with parameters and return type.fun add(a: Int, b: Int): Int { return a + b;}
// Global function have a set of optional attributes that can change their demeanor.// For example, inline attribute will make the body of this// function inlined in all places where this function is called,// increasing the total code size and possibly reducing computational fees.inline fun reply(str: String) { message(MessageParameters { to: sender(), value: 0, mode: SendRemainingValue | SendIgnoreErrors, body: str.asComment(), });}
// The extends attribute allows to implement extension functions for any type.// Its first parameter in the signature must be named self,// and its type is the type this function is extending.// Think of extension functions as very flexible method definitions// in popular programming languages.extends fun toCoinsString2(self: Int): String { return self.toFloatString(9);}
/// On top of the extends attribute, you may add the mutates attribute,/// which would allow mutating the value of the currently extended type.extends mutates fun hippityHoppity(self: Int) { // ...something that would mutate `self` self += 1;}
/// Tact allows you to import Tact and FunC files./// To bind to or wrap the respective functions in FunC,/// the so-called native functions are used.////// Prior to defining them, make sure to add the/// required `import "./path/to/file.fc";` on top of the file.@name(get_data) // here, import is not needed, // because the stdlib.fc is always implicitly importednative getData(): Cell;
/// Finally, there are advanced module-level functions that allow you/// to write Tat assembly. Unlike all other functions, their bodies consist/// only of TVM instructions and some other primitives as arguments to instructions.asm fun rawReserveExtra(amount: Int, extraAmount: Cell, mode: Int) { RAWRESERVEX }
// Examples of calling the functions defined abovefun showcase() { // Global function add(1, 2); // 3
// Inlined global function reply("Viltrum Empire");
// Extension function 5.toCoinsString2(); // 0.000000005
// Extension mutation function let val = 10; val.hippityHoppity(); val; // 11
// Native function, called just like global functions getData(); // Cell with the contract's persistent storage data.
// Assembly function, called just like global functions rawReserveExtra(ton("0.1"), emptyCell(), 0);}
// The functions discussed above are helpful but not mandatory for// the contracts to operate, unlike the receiver functions,// which can only be defined at the contract and trait level.//// See the "Contracts and traits" section below for more info.Read more: Functions.
Message exchange and communication
// On TON, contracts cannot read each other's states and cannot synchronously// call each other's functions. Instead, the actor model of communication is// applied — contracts send or receive asynchronous messages that may or may not// influence each other's state.//// Each message is a Cell with a well-defined, complex structure of serialization.// However, Tact provides you with simple abstractions to send, receive, and// (de)serialize messages to and from various structures.//// Each message has a so-called message body,// which can be represented by message structs with certain opcodes.message(123) MyMsg { someVal: Int as uint8 }
// Messages can also omit their bodies, or have them be empty,// in which case they won't have any opcode and could only be handled// by the empty message body receiver or "empty receiver" for short.//// See the "Contracts and traits" section below for more info.
// Finally, sending messages is not free and requires// some forward fees to be paid upfront.fun examples() { // To keep some amount of nanoToncoins on the balance, // use nativeReserve() prior to calling message-sending functions: nativeReserve(ton("0.01"), ReserveAtMost);
// There are many message-sending functions for various cases. // See the links given right after this code block. // // This is most general and simple function to send an internal message: message(MessageParameters { // Recipient address. to: address("UQDtFpEwcFAEcRe5mLVh2N6C0x-_hJEM7W61_JLnSF74p9dz"),
// Optional message body. body: null, // empty body, no opcodes, nothing
// Some nanoToncoins to send, possibly none. value: 0, // do not attach any nanoToncoins to the message
// Configure various modes of sending the message in regards to how // the funds will be charged, how message will be processed, etc. mode: SendPayGasSeparately | SendIgnoreErrors,
// Whether to allow this message to bounce back to this contract // in case the recipient contract doesn't exist or wasn't able to // process the message. bounce: true, // to handle messages that bounced back, a special bounced // receiver function is used. See the "Contracts and traits" // section below for more info. });
// To do refunds and forward excess values from the incoming message // back to the original sender, use the cashback message-sending function: cashback(sender());
// Note that all message-sending functions only queue the messages when called. // The actual processing and sending will be done in the next, action phase // of the transaction, where many messages can fail for various reasons. // // For example, if the remaining value from the incoming message was used by // the first function, the subsequent functions that would try to do the same // will fail. The optional SendIgnoreErrors flag seen above hides those failures // and ignores the unprocessed messages. It's not a silver bullet, and you're // advised to always double-check the message flow in your contracts.}
// Already bounced messages cannot be sent by any contract and are guaranteed// to be received only by the current contract that did sent them as internal// messages first.//// Additionally, external messages cannot be sent by the contract,// only processed by it.//// To receive internal messages (bounced or sent directly) and external messages,// provide corresponding receiver functions. See the "Contracts and traits"// section below for more info.//// To organize child-parent contract message exchange, see the "Jetton contracts"// section below.Read more:
Contracts and traits
// Tact allows you to import Tact and FunC code.// Additionally, there's a versatile set of standard libraries// which come bundled in with a compiler, but are not included// in projects right away.//// To import a standard library, instead of specifying a path to a file// start the import string with @stdlib/.import "@stdlib/ownable"; // for the Ownable trait
// Traits have the same structure as contracts and are used// to provide some means of inheritance and common code reuse.//// Like contracts, traits can also inherit other traits.trait MyTrait with Ownable { owner: Address; // required field from the Ownable trait
// Within traits and contracts, you can define scoped functions // and only accessible from them or their successors. Those functions // are often called internal functions. // // If you won't be using any contract fields, it's better to define // such functions as global, i.e., on the top-level. fun addIfOwner(a: Int, b: Int): Int { self.requireOwner(); return a + b; }
// Adding an abstract attribute to the internal function requires us // to omit their body definitions and demand that from contracts that // will inherit the trait. abstract fun trust(msg: MyMsg);
// Adding a virtual attribute to the internal function allows their // body definitions to be be overridden in the contracts that will // inherit the trait. virtual fun verify(msg: MyMsg) { self.requireOwner(); require(msg.someVal > 42, "Too low!"); }}
// Contract definitions in Tact conveniently represent smart contracts// on TON Blockchain. They hold all variables, functions, getters and receivers,// while providing accessible abstractions for working with them.contract MyContract( // Persistent state variables of the contract: owner: Address, // required field from the Ownable trait accumulator: Int as uint8, // Their default or initial values are supplied during deployment.) with MyTrait, Ownable {
// The internal message receiver is a function that handles messages received // by this contract on-chain: from other contracts and never from outside. receive(msg: MyMsg) { self.requireOwner(); self.accumulator += msg.someVal;
// Send a message back to the sender() with MyMsg message(MessageParameters { to: sender(), value: ton("0.04"), body: MyMsg{ someVal: self.accumulator }.toCell(), }); }
// For deployments, it is common to use the following receiver // often called an "empty receiver", which handles `null` (empty) // message bodies of internal messages. receive() { // Forward the remaining value in the // incoming message (surplus) back to the sender. cashback(sender()); }
// The bounced message receiver is a function that handles messages sent // from this contract and bounced back to it because of a malformed payload or // some issues on the recipient side. bounced(msg: bounced<MyMsg>) { // Bounced message bodies are limited by their first 256 bits, which // means that excluding their 32-bit opcode there are only 224 bits left // for other contents of MyMsg. // // Thus, in message structs prefer to put small important fields first. require(msg.someVal > 42, "Unexpected bounce!"); self.accumulator = msg.someVal; }
// The external message receiver is a function that handles messages sent // to this contract from outside the blockchain. That is often the case // for user wallets, where apps that present some UI for them have to // communicate with contracts on chain to perform transfers on their behalf. external(msg: MyMsg) { // There is no sender, i.e., calling sender() here won't work. // Additionally, there are no guarantees that the received message // is authentic and is not malicious. Therefore, when receiving // such messages one has to first check the signature to validate the sender, // and explicitly agree to accept the message and fund its processing // in the current transaction with acceptMessage() function. require(msg.someVal > 42, "Nothing short of 42 is allowed!"); self.accumulator = msg.someVal; acceptMessage(); }
// Getter functions or get methods are special functions that can only // be called from within this contract or off-chain, and never by other contracts. // They cannot modify the contract's state and they do not affect its balance. // The IO analogy would be that they can only "read", not "write". get fun data(): MyContract { // This getter returns the current state of the contract's variables, // which is convenient for tests but not advised for production. return self; }
// Finally, for each inherited trait contract may override its virtual internal // functions and it MUST override its abstract internal functions as to provide // their defined bodies. override fun trust(msg: MyMsg) { require(msg.someVal == 42, "Always bring your towel with you"); }}
// Message struct with 123 as its 32-bit opcode.message(123) MyMsg { someVal: Int as uint8;}Read more: Contracts.
Constants
// Global, top-level constants.// Type ascription is mandatory.const MY_CONSTANT: Int = ascii("⚡"); // expressions are computed at compile-time
// Trait-level constants.trait MyTrait { const I_AM_ON_THE_TRAIT_LEVEL: Int = 420;
// On the trait-level, you can make constants abstract, // which requires the contracts that inherit this trait // to override those constants with some values. abstract const OVERRIDE_ME: Int;
// Virtual constants allow overrides, but do not require them. virtual const YOU_CAN_OVERRIDE_ME: Int = crc32("babecafe");}
// Contract-level constants.contract MyContract() with MyTrait { const iAmOnTheContractLevel: Int = 4200;
// Because this contract inherits from MyTrait, // the I_AM_ON_THE_TRAIT_LEVEL constant is also in scope of this contract, // but we cannot override it.
// However, we can override the virtual constant. override const YOU_CAN_OVERRIDE_ME: Int = crc32("deadbeef");
// And we MUST override and define the value of the abstract constant. override const OVERRIDE_ME: Int = ton("0.5");}
// All constants are inlined, i.e., their values are embedded in the resulting// code in all places where their values are referenced in Tact code.//// The main difference is the scope — global can be referenced// from anywhere, while contract and trait-level constants are// only accessible within them via `self` references.Read more: Constants.
Persistent state
// Contracts can define state variables that persist between contract calls,// and thus commonly referred to as storage variables.// Contracts in TON pay rent in proportion to the amount of persistent space// they consume, so compact representations via serialization are encouraged.// Unlike variables, constants do not consume space in the persistent state.
contract StateActor( // Persistent state variables oneByte: Int as uint8, // ranges from -128 to 127 twoBytes: Int as int16, // ranges from -32,768 to 32,767 currency: Int as coins, // variable bit-width format, which occupies // between 4 and 124 bits // and ranges from 0 to 2^120 - 1) {
receive() { // Serialization to smaller values only applies for the state between // transactions and incoming or outgoing message bodies. // This is because at runtime everything is computed // at their maximum capacity and all integers are assumed to // be 257-bit signed ones. // // That is, the following would not cause any runtime overflows, // but will throw an out of range error only after the execution of // this receiver is completed. self.oneByte = -1; // this won't fail immediately self.oneByte = 500; // and this won't fail right away either
} // only here, at the end of the compute phase, // would an error be thrown with exit code 5: // Integer out of expected range}Read more: Persistent state variables.
Testing and debugging
fun showcase() { // An exit code is an integer that indicates whether the transaction // was successful and, if not — holds the code of the exception that occurred. // // They are the simplest litmus tests for your contracts, // indicating what happened and, if you are lucky, where, and why. try { dump( beginCell() .storeInt(0, 250) .storeInt(0, 250) .storeInt(0, 250) .storeInt(0, 250) .storeUint(0, 24) // oh no, we're trying to store 1024 bits, // while cells can only store up to 1023 bits ); } catch (exitCode) { // exitCode is 8, which means the cell overflow has happened somewhere. // However, there is no clear indication there just from the code alone — // you need to either view transaction logs or know many pitfalls in advance. // // Additionally, runtime computations aren't free and require gas to be spent. // The catch block can revert the state before the try block, // but it cannot revert the gas spent for computations in the try block. }
// Contrary to this reactive approach, we can be proactive and state // our expectations upfront with require() function. // It will also throw an exit code, but this time we can map that exit code // onto the error message and trace our way back to the this call to require(). require(now() >= 1000, "We're too early, now() is less than 1000");
// Sometimes, its also helpful to log the events as they unfold to view // the data off-chain. Use the emit() message-sending function for this. emit("Doing X, then Y, currently at R".asComment());}
// Those tools are handy, but they are not enough.// To fully understand what went wrong, you might need to read the TON Virtual// Machine (TVM) execution logs when running contracts in the Sandbox.// And to ensure that encountered issues won't happen again, do write tests// in Tact and TypeScript using the Sandbox + Jest toolkit provided by// the Blueprint framework or tact-template.Read more:
- Debugging and testing.
- Exit codes.
- Gas best practices.
- Security best practices.
- Transaction retracer on retracer.ton.org.
- Blueprint framework on GitHub.
- tact-template on GitHub.
Contract deployment
fun showcase() { // Contract deployment is done by sending messages. For deployments, // one must send the new contract's initial state, i.e., // its initial code and initial data, to its Address, // which is deterministically obtained from the initial state. deploy(DeployParameters { // Use initOf expression to provide initial code and data init: initOf MyContract(), // Attaching 1 Toncoin, which will be used to pay various fees value: ton("1"), // Notice that we do not need to explicitly specify the Address. // That's because it will be computed on the fly from the initial package. });}
// However, before your contracts can start deploying other contracts on-chain,// it is common to first send an external message to your TON wallet from off-chain.// Then, your wallet will be the contract that deploys the target one.
// An empty contract needed for the showcase above to work.contract MyContract() {}Read more: Deployment.
Example contracts
Let’s put our newly acquired information of syntax and some semantics to the test.
Counter contract
// Defining a new Message type, which has one field// and an automatically assigned 32-bit opcode prefixmessage Add { // unsigned integer value stored in 4 bytes amount: Int as uint32;}
// Defining a contractcontract SimpleCounter( // Persistent state variables of the contract: counter: Int as uint32, // actual value of the counter id: Int as uint32, // a unique id to deploy multiple instances // of this contract in a same workchain // Their default or initial values are supplied during deployment.) { // Registers a receiver of empty messages from other contracts. // It handles internal messages with `null` body // and is very handy and cheap for the deployments. receive() { // Forward the remaining value in the // incoming message back to the sender. cashback(sender()); }
// Registers a binary receiver of the Add message bodies. receive(msg: Add) { self.counter += msg.amount; // <- increase the counter // Forward the remaining value in the // incoming message back to the sender. cashback(sender()); }
// A getter function, which can only be called from off-chain // and never by other contracts. This one is useful to see the counter state. get fun counter(): Int { return self.counter; // <- return the counter value }
// Another getter function, but for the id: get fun id(): Int { return self.id; // <- return the id value }}Jetton contracts
The tokens on TON Blockchain are commonly called Jettons. The distinction is made because they work differently from ERC-20 tokens or others.
This is due to the scalable and distributed approach that works best on TON: instead of having a single giant contract with a big hashmap of addresses of token holders, Jettons instead have a single minter contract that creates individual contracts called Jetton wallets per each holder.
For more, refer to the following resources:
- TON Jettons Processing in TON Docs.
- Jetton standards: TEP-64, TEP-74, and TEP-89.
JettonWallet
/// Child contract per each holder of N amount of given Jetton (token)contract JettonWallet( /// Balance in Jettons. balance: Int as coins,
/// Address of the user's wallet which owns this JettonWallet, and messages /// from whom should be recognized and fully processed. owner: Address,
/// Address of the main minting contract, /// which deployed this Jetton wallet for the specific user's wallet. master: Address,) { /// Registers a binary receiver of the JettonTransfer message body. /// Transfers Jettons from the current owner to the target user's JettonWallet. /// If that wallet does not exist, it is deployed on-chain in the same transfer. receive(msg: JettonTransfer) { // Ensure the basechain (workchain ID = 0) require(parseStdAddress(msg.destination.asSlice()).workchain == 0, "Invalid destination workchain"); // Ensure the owner. require(sender() == self.owner, "Incorrect sender");
// Ensure the balance does not go negative. self.balance -= msg.amount; require(self.balance >= 0, "Incorrect balance after send"); // Ensure the payload has enough bits. require(msg.forwardPayload.bits() >= 1, "Invalid forward payload");
let ctx = context(); // msg.forwardTonAmount cannot be negative // because its serialized as "coins" let fwdCount = 1 + sign(msg.forwardTonAmount);
// Ensure there are enough Toncoin for transferring Jettons. require( ctx.value > msg.forwardTonAmount + fwdCount * ctx.readForwardFee() + (2 * getComputeFee(GAS_FOR_TRANSFER, false) + MIN_TONCOIN_FOR_STORAGE), "Insufficient amount of TON attached", );
// Transfer Jetton from the current owner to the target user's JettonWallet. // If that wallet does not exist, it is deployed on-chain in the same transfer. deploy(DeployParameters { value: 0, mode: SendRemainingValue, bounce: true, body: JettonTransferInternal{ queryId: msg.queryId, amount: msg.amount, sender: self.owner, responseDestination: msg.responseDestination, forwardTonAmount: msg.forwardTonAmount, forwardPayload: msg.forwardPayload, }.toCell(), // Notice that we do not need to explicitly specify the Address, // because it will be computed on the fly from the initial package. init: initOf JettonWallet(0, msg.destination, self.master), }); }
/// Registers a binary receiver of messages with JettonTransferInternal opcode. /// Those are expected to be sent from the JettonMinter /// or from other JettonWallets, and indicate incoming Jetton transfers. receive(msg: JettonTransferInternal) { self.balance += msg.amount;
// This message should come only from JettonMinter, // or from other JettonWallet. let wallet: StateInit = initOf JettonWallet(0, msg.sender, self.master); if (sender() != contractAddress(wallet)) { require(self.master == sender(), "Incorrect sender"); }
let ctx: Context = context(); let msgValue: Int = ctx.value; let tonBalanceBeforeMsg = myBalance() - msgValue;
// If there are some funds to forward a message // let's notify the user's wallet about the Jetton transfer we just got. if (msg.forwardTonAmount > 0) { let fwdFee: Int = ctx.readForwardFee(); msgValue -= msg.forwardTonAmount + fwdFee; message(MessageParameters { to: self.owner, value: msg.forwardTonAmount, mode: SendPayGasSeparately, bounce: false, body: JettonNotification{ queryId: msg.queryId, amount: msg.amount, sender: msg.sender, forwardPayload: msg.forwardPayload, }.toCell(), }); }
// In general, let's try to reserve minimal amount of Toncoin // to keep this contract running and paying storage fees. nativeReserve(max(tonBalanceBeforeMsg, MIN_TONCOIN_FOR_STORAGE), ReserveAtMost);
// And forward excesses (cashback) to the original sender. if (msg.responseDestination != null && msgValue > 0) { message(MessageParameters { to: msg.responseDestination!!, value: msgValue, mode: SendRemainingBalance + SendIgnoreErrors, bounce: false, body: JettonExcesses{ queryId: msg.queryId }.toCell(), }); } }
/// Registers a binary receiver of messages with JettonBurn opcode. receive(msg: JettonBurn) { // Ensure the owner. require(sender() == self.owner, "Incorrect sender");
// Ensure the balance does not go negative. self.balance -= msg.amount; require(self.balance >= 0, "Incorrect balance after send");
// Ensure there are enough Toncoin for transferring Jettons. let ctx = context(); let fwdFee: Int = ctx.readForwardFee(); require( ctx.value > (fwdFee + 2 * getComputeFee(GAS_FOR_BURN, false)), "Insufficient amount of TON attached", );
// Send a message to the JettonMinter to reduce the total supply // of the Jettons. That is, to burn some. message(MessageParameters { to: self.master, value: 0, mode: SendRemainingValue, bounce: true, body: JettonBurnNotification{ queryId: msg.queryId, amount: msg.amount, sender: self.owner, responseDestination: msg.responseDestination, }.toCell(), }); }
/// Registers a bounced binary receiver of messages /// with the JettonTransferInternal opcode. /// It handles such outgoing messages that bounced back to this contract. bounced(msg: bounced<JettonTransferInternal>) { self.balance += msg.amount; }
/// Registers a bounced binary receiver of messages /// with the JettonBurnNotification opcode. /// It handles such outgoing messages that bounced back to this contract. bounced(msg: bounced<JettonBurnNotification>) { self.balance += msg.amount; }
/// An off-chain getter function which returns useful data about this wallet. get fun get_wallet_data(): JettonWalletData { return JettonWalletData{ balance: self.balance, owner: self.owner, master: self.master, code: myCode(), }; }}
//// Helper structs, message structs and constants,// which would otherwise be imported from another file//
struct JettonWalletData { balance: Int; owner: Address; master: Address; code: Cell;}
message(0xf8a7ea5) JettonTransfer { queryId: Int as uint64; amount: Int as coins; destination: Address; responseDestination: Address?; customPayload: Cell?; forwardTonAmount: Int as coins; forwardPayload: Slice as remaining;}
message(0x178d4519) JettonTransferInternal { queryId: Int as uint64; amount: Int as coins; sender: Address; responseDestination: Address?; forwardTonAmount: Int as coins; forwardPayload: Slice as remaining;}
message(0x7362d09c) JettonNotification { queryId: Int as uint64; amount: Int as coins; sender: Address; forwardPayload: Slice as remaining;}
message(0x595f07bc) JettonBurn { queryId: Int as uint64; amount: Int as coins; responseDestination: Address?; customPayload: Cell?;}
message(0x7bdd97de) JettonBurnNotification { queryId: Int as uint64; amount: Int as coins; sender: Address; responseDestination: Address?;}
message(0xd53276db) JettonExcesses { queryId: Int as uint64;}
const GAS_FOR_BURN: Int = 6000;const GAS_FOR_TRANSFER: Int = 8000;const MIN_TONCOIN_FOR_STORAGE: Int = ton("0.01");JettonMinter
It is a parent contract that deploys individual Jetton wallets per each holder.