Decimal Point

All numbers in Tact are integers. Floating point types are not used in smart contracts because they're unpredictable.

Arithmetics with dollars, for example, requires 2 decimal places. How can we represent the number 1.25 if we can only work with integers? The answer is to work with cents. So 1.25 becomes 125. We just remember that the two lowest digits are coming after the decimal point.

In the same way, working with TON coins has 9 decimal places instead of 2. So the amount 1.25 TON is actually the number 1250000000 - we call these nano-tons instead of cents.

Calculating interest

This example calculates the earned interest over a deposit of 500 TON coins. The yearly interest rate in the example is 3.25%.

Since we can't hold the number 3.25 we will use thousandth of a percent as unit (percent-mille). So 3.25% becomes 3250 (3.25 * 1000).

On withdraw, to calculate earned interest, we multiply the amount by the fraction of a year that passed (duration in seconds divided by total seconds in a year) and then by the interest rate divided by 100,000 (100% in percent-mille, meaning 100 * 1000).

Info: Notice that total is returned in nano-tons.

All Examples

import "@stdlib/deploy";

struct TokenInfo {
    ticker: String;
    decimals: Int as uint8;
}

// this is a global static function that can be called from anywhere
fun average(a: Int, b: Int): Int {
    return (a + b) / 2;
}

contract Functions with Deployable {

    deployer: Address;

    init() {
        self.deployer = sender();
    }

    // this contract method can be called from within this contract and access its variables
    fun onlyDeployer() {
        require(sender() == self.deployer, "Only the deployer is permitted here");
    }

    receive("priviliged") {
        self.onlyDeployer();
    }

    // this contract method returns multiple return values using a struct
    fun getInfo(index: Int): TokenInfo {
        if (index == 1) {
            return TokenInfo{ticker: "TON", decimals: 9};
        }
        if (index == 2) {
            return TokenInfo{ticker: "ETH", decimals: 18};
        }
        return TokenInfo{ticker: "unknown", decimals: 0};
    }

    receive("best L1") {
        let best: TokenInfo = self.getInfo(1);
        self.reply(best.ticker.asComment());
    }

    get fun result(): Int {
        return average(1, 10);
    }
}