4_functions.md

Functions

In this section, we will introduce functions in Sui Move and write our first Sui Move function as a part of the Hello World example.

Function Visibility

Sui Move functions have three types of visibility:

• private: the default visibility of a function; it can only be accessed by functions inside the same module
• public: the function is accessible by functions inside the same module and by functions defined in another module
• public(package): the function is accessible by functions of modules inside the same package

Return Value

The return type of a function is specified in the function signature after the function parameters, separated by a colon.

A function's last line (of execution) without a semicolon is the return value.

Example:

    public fun addition (a: u8, b: u8): u8 {
        a + b    
    }

Transaction Context

Functions called directly through a transaction typically have an instance of TxContext as the last parameter. This is a special parameter set by the Sui Move VM and does not need to be specified by the user calling the function.

The TxContext object contains essential information about the transaction used to call the entry function, such as the sender's address, the tx's digest ID, the tx's epoch, etc.

Create the mint Function

We can define our minting function in the Hello World example as the following:

    public fun mint(ctx: &mut TxContext) {
        let object = HelloWorldObject {
            id: object::new(ctx),
            text: string::utf8(b"Hello World!")
        };
        transfer::public_transfer(object, tx_context::sender(ctx));
    }

This function simply creates a new instance of the HelloWorldObject custom type, then uses the Sui system public_transfer function to send it to the transaction caller.