Transfers

The simplest Tx3 transactions move value from one party to another. They consume a UTxO, produce one output for the recipient, and return any leftover value as change. Every block uses two pre-bound identifiers: Ada (the chain’s primary asset) and fees (the resolved transaction fee).

Send Ada

A parameterised template that sends quantity lovelace from Sender to Receiver.

party Sender;

party Receiver;

tx transfer(
    quantity: Int
) {
    input source {
        from: Sender,
        min_amount: Ada(quantity),
    }

    output {
        to: Receiver,
        amount: Ada(quantity),
    }

    output {
        to: Sender,
        amount: source - Ada(quantity) - fees,
    }
}

min_amount: Ada(quantity) is a lower bound — the resolver picks a real UTxO with at least that much value. The change output subtracts both the transferred amount and fees from source, which forces the resolver to pick a UTxO large enough to cover fees as well.

Transfer respecting minimum UTxO size

Cardano outputs must carry a minimum amount of Ada to be valid. The built-in min_utxo(<output_name>) returns that minimum for a named output, computed from its contents.

party Sender;
party Receiver;

tx transfer_min(
    quantity: Int
) {
    input source {
        from: Sender,
        min_amount: fees + min_utxo(minimal_utxo) + min_utxo(change),
    }

    output minimal_utxo {
        to: Receiver,
        amount: min_utxo(minimal_utxo),
    }

    output change {
      to: Sender,
      amount: source - fees - min_utxo(minimal_utxo),
    }
}

Both outputs reference themselves by name. The resolver evaluates min_utxo after the rest of the output is known, so cyclic-looking definitions like amount: min_utxo(self_name) resolve cleanly.