Reference: updates

This page gives reference information on Updates:

For the structure around them, see Overview: template structure.

Background

  • An Update is ledger update. There are many different kinds of these, and they’re listed below.
  • They are what can go in a choice body.

Binding variables

boundVariable <- UpdateExpression1
  • One of the things you can do in a choice body is bind (assign) an Update expression to a variable. This works for any of the Updates below.

do

do
   updateExpression1
   updateExpression2
  • do can be used to group Update expressions. You can only have one update expression in a choice, so any choice beyond the very simple will use a do block.

  • Anything you can put into a choice body, you can put into a do block.

  • By default, do returns whatever is returned by the last expression in the block.

    So if you want to return something else, you’ll need to use return explicitly - see return for an example.

create

  create NameOfTemplate with exampleParty; exampleParty2; exampleParty3
  • create keyword.

  • Creates an instance of that contract on the ledger. When a contract is committed to the ledger, it is given a unique contract identifier of type ContractId <name of template>.

    Creating the contract returns that ContractId.

  • Use with to specify the template parameters.

  • Requires authorization from the signatories of the contract being created. This is given by being signatories of the contract from which the other contract is created, being the controller, or explicitly creating the contract itself.

    If the required authorization is not given, the transaction fails. For more detail on authorization, see Signatory parties.

exercise

exercise IdOfContract NameOfChoiceOnContract with choiceArgument1 = value1
  • exercise keyword.
  • Exercises the specified choice on the specified contract.
  • Use with to specify the choice parameters.
  • Requires authorization from the controller(s) of the choice. If the authorization is not given, the transaction fails.

fetch

fetchedContract <- fetch IdOfContract
  • fetch keyword.
  • Fetches the contract instance with that ID. Usually used with a bound variable, as in the example above.
  • Often used to check the details of a contract before exercising a choice on that contract. Also used when referring to some reference data.
  • fetch cid fails if cid is not the contract id of an active contract, and thus causes the entire transaction to abort.

abort

abort errorMessage
  • abort function.
  • Fails the transaction - nothing in it will be committed to the ledger.
  • errorMessage is of type Text. Use the error message to provide more context to an external system (e.g., it gets displayed in DAML Studio scenario results).
  • You could use assert False as an alternative.

assert

assert (condition == True)
  • assert keyword.
  • Fails the transaction if the condition is false. So the choice can only be exercised if the boolean expression evaluates to True.
  • Often used to restrict the arguments that can be supplied to a contract choice.

Here’s an example of using assert to prevent a choice being exercised if the Party passed as a parameter is on a blacklist:

      Transfer : ContractId RestrictedPayout
        with newReceiver : Party
        do
          assert (newReceiver /= blacklisted)
          create RestrictedPayout with receiver = newReceiver; giver; blacklisted; qty

getTime

currentTime <- getTime
  • getTime keyword.
  • Gets the ledger effective time. (You will usually want to immediately bind it to a variable in order to be able to access the value.)
  • Used to restrict when a choice can be made. For example, with an assert that the time is later than a certain time.

Here’s an example of a choice that uses a check on the current time:

      Complete : ()

        do
          -- bind the ledger effective time to the tchoose variable using getTime
          tchoose <- getTime

return

return ()
  • return keyword.
  • Used to return a value from do block that is not of type Update.

Here’s an example where two contracts are created in a choice and both their ids are returned as a tuple:

do
  firstContract <- create SomeContractTemplate with arg1; arg2
  secondContract <- create SomeContractTemplate with arg1; arg2
  return (firstContract, secondContract)

let

See the documentation on Let.

Let looks similar to binding variables, but it’s very different! This code example shows how:

do
  -- defines a function, createdContract, taking a single argument that when
  -- called _will_ create the new contract using argument for issuer and owner
  let createContract x = create NameOfContract with issuer = x; owner = x

  createContract party1
  createContract party2

this

this lets you refer to the current contract from within the choice body. This refers to the contract, not the contract ID.

It’s useful, for example, if you want to pass the current contract to a helper function outside the template.