Tickets

Tickets are a special class of assets on Tezos that contracts can construct and store, and that can then be transferred between contracts, user accounts, or rollups. They are useful to model ownership of some digital artefact, existing in a limited number of copies (or instances); this can further serve as a basis for restricting access to the artefact and permissions to use it. In this document, we will explain the semantics and lifetime of tickets, including creation, transfer, and destruction.

Semantics

A ticket represents a digital artefact existing in a limited number of copies (or instances). As such, a ticket has the following four attributes:

  1. ticketer is the creator of the ticket, which is always a Tezos smart contract.

  2. contents is the Michelson expression attached to the ticket.

  3. content_ty is the Michelson type of the contents data. It can be any comparable type.

  4. amount is the amount of the ticket. It is a strictly positive whole number.

Furthermore, a ticket cannot be duplicated with the DUP or the DUP n instructions.

Operations on tickets

Creating

Tickets can be constructed by smart contracts. To construct tickets, smart contracts use the TICKET instruction with some ticket amount, ticket content type, and value as inputs. Smart contracts then may store them in contract storage or transfer them along to either other contracts, rollups, or user accounts.

Tickets cannot be constructed with instructions for duplication, such as DUP. Therefore, the issuance of tickets is completely within the control of the ticketer contract by invoking TICKET instructions.

Splitting

A ticket can be split into two tickets with the same contents and ticketer values, as long as the sum of the amount of the two tickets is equal to that of the original. This operation allows tickets to be spent across several transactions by breaking tickets into smaller tickets. Smart contracts may split tickets by invoking the SPLIT_TICKET instruction. The ticket splitting is done automatically when a user account transfers part of a ticket.

Joining

Tickets issued by the same ticketer contract with the same contents data of the same contents_ty type are considered of the same kind. Therefore, two such tickets can be joined into one ticket and the output amount will be the sum of those of the two input tickets. Smart contracts can join tickets via the JOIN_TICKETS instruction. Tickets of the same kind are automatically joined when they belong to the same user account.

Transferring

Once a ticket has been constructed by a smart contract, it may be transferred to other contracts as follows:

  • Smart contract to user account: Smart contracts can transfer a ticket to user accounts via TRANSFER_TOKENS. To do so, the contract needs to cast the address of the target user account to type contract (ticket cty) where cty is the type of the content of the ticket to be sent. This can be done using CONTRACT (ticket cty). The rest is the same as making a contract call. The following Michelson snippet is an example sending a ticket of amount 10 with a string content "some ticket" to a user account address made available at the top of the stack.

# Stack: address :: S
CONTRACT (ticket string) ;
# Stack: option (contract (ticket string)) :: S
ASSERT_SOME ;
# Stack: contract (ticket string) :: S
PUSH mutez 0 ;
# Stack: mutez :: contract (ticket string) :: S
PUSH nat 10 ;
# Stack: nat :: mutez :: contract (ticket string) :: S
PUSH string "some ticket" ;
# Stack: string :: nat :: mutez :: contract (ticket string) :: S
TICKET ;
# Stack: option (ticket string) :: mutez :: contract (ticket string) :: S
ASSERT_SOME ;
# Stack: ticket string :: mutez :: contract (ticket string) :: S
TRANSFER_TOKENS ;
# Stack: operation :: S
  • Between smart contracts: Contracts can send tickets to other contracts via regular contract calls using the instruction TRANFSER_TOKENS, as long as target contracts accept tickets of matching content type in their parameters.

  • Between user accounts: User accounts can transfer existing tickets they own to other user accounts with Transfer_ticket operation from their wallets. For instance, octez-client can be invoked in the following way by a user account holder alice to transfer a ticket of amount 10, type string, content "some ticket" and ticketer ticketer to another user account held by bob.

octez-client transfer 10 tickets from alice to bob with entrypoint default and type string and content '"some ticket"' and ticketer 'ticketer'

If the amount of the ticket "some ticket" owned by alice was greater than 10, this transfer would cause an automatic ticket split, after which alice would continue to own the remaining amount.

Note that the above command uses the expression “transfer 10 tickets” of the given kind, instead of “transfer a ticket of amount 10”. Indeed, by virtue of automatic joining of tickets of the same kind belonging to the same user account, this formulation does not introduce any ambiguity: the user account cannot hold several distinct tickets of this kind. Of course, these expressions would not be interchangeable for tickets belonging to a smart contract.

  • User account to smart contract: Using the same Transfer_ticket operation, user accounts can also send their tickets to smart contracts. In this case, the entrypoint, as defined by the specification of the target contract, must accept this kind of tickets. Here is an example using octez-client to transfer a ticket of amount 10, type string, content "some ticket" and ticketer ticketer owned by alice to a contract receiver accepting tickets at the entrypoint save.

octez-client transfer 10 tickets from alice to receiver with entrypoint save and type string and content '"some ticket"' and ticketer 'ticketer'

In this case, too, automatic ticket splitting may happen, under the same circumstances.

Note

This page does not cover transfers of tickets to/from rollups. For that, refer to the documentation pages of particular rollups (e.g. Smart Optimistic Rollups).

Destroying

Only smart contracts can destroy tickets, by simply dropping them. That is, by not storing them anymore in the contract storage and not sending them to other contracts. User accounts, on the other hand, cannot destroy any ticket in their possession.

For instance, a user account A may receive a string ticket of amount 2 with content Lorem ipsum created by a smart contract B. A may send amount 1 of it to a smart contract C. This automatically splits the ticket into two tickets of amount 1. Now C may destroy this ticket by dropping it and A will still hold a ticket of amount 1 with a string content Lorem ipsum created by B. It will remain in the possession of A until A sends it to another user account or smart contract.