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(friend): the function is accessible by functions inside the same module and by functions defined in modules that are included on the module's friends list.

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    
    }

Entry Functions

In Sui Move, entry functions are simply functions that can be called by a transactions. They must satisfy the following three requirements:

• Denoted by the keyword entry
• have no return value
• (optional) have a mutable reference to an instance of the TxContext type in the last parameter

Transaction Context

Entry functions 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 signer's address, the tx's epoch, etc.

Create the mint Function

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

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

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