Reference: templates

This page gives reference information on templates:

For the structure of a template, see Overview: template structure.

Template name

template NameOfTemplate
  • This is the name of the template. It’s preceded by template keyword. Must begin with a capital letter.
  • This is the highest level of nesting.
  • The name is used when creating a contract instance of this template (usually, from within a choice).

Template parameters

  with
    exampleParty : Party
    exampleParty2 : Party
    exampleParty3 : Party
    exampleParam : Text
    -- more parameters here
  • with keyword. The parameters are in the form of a record type.
  • Passed in when creating a contract instance from this template. These are then in scope inside the template body.
  • A template parameter can’t have the same name as any choice arguments inside the template.
  • For all parties involved in the contract (whether they’re a signatory, observer, or controller) you must pass them in as parameters to the contract, whether individually or as a list ([Party]).

Signatory parties

  where
    signatory exampleParty
  • signatory keyword. After where. Followed by at least one Party.

  • Signatories are the parties (see the Party type) who must consent to the creation of an instance of this contract. They are the parties who would be put into an obligable position when this contract is created.

    DAML won’t let you put someone into an obligable position without their consent. So if the contract will cause obligations for a party, they must be a signatory. If they haven’t authorized it, you won’t be able to create the contract. In this situation, you may see errors like:

    NameOfTemplate requires authorizers Party1,Party2,Party, but only Party1 were given.

  • When a signatory consents to the contract creation, this means they also authorize the consequences of choices that can be exercised on this contract.

  • The contract instance is visible to all signatories (as well as the other stakeholders of the contract). That is, the compiler automatically adds signatories as observers.

  • You must have least one signatory per template. You can have many, either as a comma-separated list or reusing the keyword. You could pass in a list (of type [Party]).

Observers

    observer exampleParty2
  • observer keyword. After where. Followed by at least one Party.
  • Observers are additional stakeholders, so the contract instance is visible to these parties (see the Party type).
  • Optional. You can have many, either as a comma-separated list or reusing the keyword. You could pass in a list (of type [Party]).
  • Use when a party needs visibility on a contract, or be informed or contract events, but is not a signatory or controller.
  • If you start your choice with choice rather than controller (see Choices below), you must make sure to add any potential controller as an observer. Otherwise, they will not be able to exercise the choice, because they won’t be able to see the contract.

Choices

    -- option 1 for specifying choices: choice name first
    choice NameOfChoice1
          : ()  -- replace () with the actual return type
        with
          exampleParameter : Text -- parameters here
      controller exampleParty
        do
          return () -- replace this line with the choice body

    -- option 2 for specifying choices: controller first
    controller exampleParty can
      NameOfChoice2
          : () -- replace () with the actual return type
        with
          exampleParameter : Text -- parameters here
        do
          return () -- replace this line with the choice body
      nonconsuming NameOfChoice3
          : ()  -- replace () with the actual return type
        with
          exampleParameter : Text -- parameters here
        do
          return () -- replace this line with the choice body
  • A right that the contract gives the controlling party. Can be exercised.

  • This is essentially where all the logic of the template goes.

  • By default, choices are consuming: that is, exercising the choice archives the contract, so no further choices can be exercised on it. You can make a choice non-consuming using the nonconsuming keyword.

  • There are two ways of specifying a choice: start with the choice keyword or start with the controller keyword.

    Starting with choice lets you pass in a Party to use as a controller. But you must make sure to add that party as an observer.

  • See Reference: choices for full reference information.

Agreements

    agreement
      -- text representing the contract
      ""
  • agreement keyword, followed by text.

  • Represents what the contract means in text. They’re usually the boundary between on-ledger and off-ledger rights and obligations.

  • Usually, they look like agreement tx, where tx is of type Text.

    You can use the built-in operator show to convert party names to a string, and concatenate with <> .

Preconditions

    ensure
      True -- a boolean condition goes here
  • ensure keyword, followed by a boolean condition.
  • Used on contract creation. ensure limits the values on parameters that can be passed to the contract: the contract can only be created if the boolean condition is true.

Contract keys and maintainers

    key (exampleParty, exampleParam) : (Party, Text)
    maintainer (exampleFunction key)
  • key and maintainer keywords.

  • This feature lets you specify a “key” that you can use to uniquely identify an instance of this contract template.

  • If you specify a key, you must also specify a maintainer. This is a Party that will ensure the uniqueness of all the keys it is aware of.

    Because of this, the key must include the maintainer Party or parties (for example, as part of a tuple or record), and the maintainer must be a signatory or observer of the contract instance.

  • For a full explanation, see Contract keys.