One Time Witness (OTW)

In this tutorial, we will discuss the use of One Time Witness (OTW) in Sui Move. OTW is a special instance of a type that is created only in the module initializer and is guaranteed to be unique and have only one instance. We will explore the properties of OTW, how to check whether an instance is an OTW, and how to use it in a smart contract.

1. Import the required dependencies:

use sui::tx_context::TxContext;
use sui::object::{Self, UID};
use std::string::String;
use sui::transfer;
use sui::types;

2. Define error constants:

/// For when someone tries to send a non OTW struct
const ENotOneTimeWitness: u64 = 0;

3. Define the struct for the Unique Type Record:

struct UniqueTypeRecord<phantom T> has key {
    id: UID,
    name: String
}

4. Expose a public function to allow registering new types with custom names:

public fun add_record<T: drop>(
    witness: T,
    name: String,
    ctx: &mut TxContext
) {
    assert!(types::is_one_time_witness(&witness), ENotOneTimeWitness);

    transfer::share_object(UniqueTypeRecord<T> {
        id: object::new(ctx),
        name
    });
}

5. Define the struct for the One Time Witness:

struct MY_OTW has drop {}

6. Define the initialization function:

fun init(witness: MY_OTW, ctx: &mut TxContext) {
    registry::add_record(
        witness,
        string::utf8(b"My awesome record"),
        ctx
    )
}

In this example, we use OTW to ensure that there is only one record per type. We define a struct called UniqueTypeRecord that has a key and a name. We also define a function called add_record that checks whether the type is an OTW and shares the record for the world to see.

To use OTW, we define a struct called MY_OTW and use it as the witness in the initialization function of our smart contract. We then call the add_record function and pass in the witness, a custom name, and the context.

One Time Witness is a powerful tool in Sui Move that guarantees uniqueness and can be used to ensure that certain actions are performed only once. It is essential to understand its properties and how to use it in smart contracts.

To check whether an instance is an OTW, sui::types::is_one_time_witness(witness) should be used.