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Foundations · medium

12. One-Time Witness (OTW)

Guaranteeing singleton module initialization.

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An OTW is a special struct with only `drop` named after the module in uppercase. It is passed into `init` exactly once upon publication to create unique coins or registries.

move
module my_token::my_token {
    public struct MY_TOKEN has drop {}

    fun init(witness: MY_TOKEN, ctx: &mut TxContext) {
        // Initializes coin currency
    }
}

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Check your understanding

1. What rules govern a One-Time Witness struct in Sui Move?
Has only the 'drop' ability and has the exact same name as the module in ALL_CAPSHas key and store abilitiesCan be copied anytimeContains u64 fields
2. Why is a One-Time Witness required when creating a new Coin via coin::create_currency?
Guarantees that TreasuryCap for that specific coin type can never be duplicatedTo pay gas feesTo verify user signaturesTo freeze the coin

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