Tact Hello World

Introduction to TON Smart Contracts using Tact

Tact's facts #4

Here we will learn more about existing tools in Tact for defining contract's behavior in general. init() - is one of them, and below listed all of them:

Entity	Description
`fun`	Special Tact keyword for declaring function. Functions serves for convenient abstraction of logic and optimization that helps code be more readable.
`init()`	Function that declares initial values will be stored in contract's data. It will be use in Deployment process, when contract creates in Blockchain.
`context()`	Tact stdlib helper that gets common properties about current incoming messages such as sender's Address, message's body and message value.
`recieve()`	Special function of contract serves to declare behaviours of contract with messages.
`get fun`	Special keywords of contract serves to declare get functions. According to name, this uses to get information from its data.

Tactical Practice #4 - add contract's behaviour

Ok, one more time, we need extend our contract's behaviour

//previous code
 
fun add(v: Int) {
 
    let ctx: Context = context();
    require(ctx.sender == self.owner, "Invalid sender");
    
    self.counter = (self.counter + v);
}

Declare function add that will do following things:

Get incoming integer argument v.
Check current incoming message sender is equal to owner we set for contract in deployment process.
If this check passed add to current counter value v and update counter value in contract. If required is not pass, throw exception message "Invalid sender".

//previous code
 
receive(msg: Add) {
    self.add(msg.amount);
  }
}
 
receive("increment") {
    self.add(1);
  }
}

Here we are specified two receive functions. This means, that if we get message in contract that correspond of one the receiver contract execute one of them. In other case, contract will do nothing. receive() function expects an incoming argument of String type, Message struct or custom Tact Struct. Take a look our receivers:

receive(msg: Add) - specifies actions with messages defined in the beginning by Message add. It will content only one integer number of 32-bits length, that we called amount and nothing else. This receiver invokes function add(amount).
receive("increment") - specifies actions with messages, that contents comment string "increment" in its body. Here we invoke function add(v) with v = 1, so we increment contract's counter by one.

 
contract SampleTactContract with Deployable {
    //previous code
 
    get fun counter(): Int {
      return self.counter;
    }

Last function we need to specifier is get function counter, that will return value of current counter value from contract. Now our contract is ready, we need compile this, so run:

yarn build

Command yarn build will compile contract.tact and place result in tact-template/src/output.

Tact compile scheme

If you faced some compile issue and can't figure out what is wrong, just compare with target contract placed in sources/example/increment.tact or here (opens in a new tab).

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