Reference: Interfaces¶

In Daml, an interface defines an abstract type together with a behavior specified by its view type, method signatures, and choices. For a template to conform to this interface, there must be a corresponding interface instance definition where all the methods of the interface (including the special view method) are implemented. This allows decoupling such behavior from its implementation, so other developers can write applications in terms of the interface instead of the concrete template.

Configuration¶

In order to use this new feature, your Daml project needs to explicitly target Daml-LF version 1.15 or higher which is specified by adding the following line to the project’s daml.yaml file:

build-options: [--target=1.15]

If using Canton, the protocol version of the domain should be 4 or higher, see Canton protocol version for more details.

Interface Declaration¶

An interface declaration is somewhat similar to a template declaration.

Interface Name¶

interface MyInterface where

This is the name of the interface.
It’s preceded by the keyword interface and followed by the keyword where.
It must begin with a capital letter, like any other type name.

Implicit abstract type¶

Whenever an interface is defined, an abstract type is defined with the same name. “Abstract” here means:
- Values of this type cannot be created using a data constructor. Instead, they are constructed by applying the function toInterface to a template value.
- Values of this type cannot be inspected directly via case analysis. Instead, use functions such as fromInterface.
- See Interface Functions for more information on these and other functions for interacting with interface values.
An interface value carries inside it the type and parameters of the template value from which it was constructed.
As for templates, the existence of a local binding b of type I, where I is an interface does not necessarily imply the existence on the ledger of a contract with the template type and parameters used to construct b. This can only be assumed if b the result of a fetch from the ledger within the same transaction.

Interface Methods¶

  method1 : Party
  method2 : Int
  method3 : Bool -> Int -> Int -> Int

An interface may define any number of methods.
A method definition consists of the method name and the method type, separated by a single colon :. The name of the method must be a valid identifier beginning with a lowercase letter or an underscore.
A method definition introduces a top level function of the same name:
- If the interface is called I, the method is called m, and the method type is M (which might be a function type), this introduces the function m : I -> M:
```
func1 : MyInterface -> Party
func1 = method1

func2 : MyInterface -> Int
func2 = method2

func3 : MyInterface -> Bool -> Int -> Int -> Int
func3 = method3
```
- The first argument’s type I means that the function can only be applied to values of the interface type I itself. Methods cannot be applied to template values, even if there exists an interface instance of I for that template. To use an interface method on a template value, first convert it using the toInterface function.
- Applying the function to such argument results in a value of type M, corresponding to the implementation of m in the interface instance of I for the underlying template type t (the type of the template value from which the interface value was constructed).
One special method, view, must be defined for the viewtype. (see Interface View Type below).

Interface View Type¶

data MyInterfaceViewType =
  MyInterfaceViewType { name : Text, value : Int }

  viewtype MyInterfaceViewType

All interface instances must implement a special view method which returns a value of the type declared by viewtype.
The type must be a record.
This type is returned by subscriptions on interfaces.

Interface Choices¶

  choice MyChoice : (ContractId MyInterface, Int)
    with
      argument1 : Bool
      argument2 : Int
    controller method1 this
    do
      let n0 = method2 this
      let n1 = method3 this argument1 argument2 n0
      pure (self, n1)

  nonconsuming choice MyNonConsumingChoice : Int
    controller method1 this
    do
      pure $ method2 this

Interface choices work in a very similar way to template choices. Any contract of a template type for which an interface instance exists will grant the choice to the controlling party.
Interface choices can only be exercised on values of the corresponding interface type. To exercise an interface choice on a template value, first convert it using the toInterface function.
Interface methods can be used to define the controller of a choice (e.g. method1) as well as the actions that run when the choice is exercised (e.g. method2 and method3).
As for template choices, the choice keyword can be optionally prefixed with the nonconsuming keyword to specify that the contract will not be consumed when the choice is exercised. If not specified, the choice will be consuming. Note that the preconsuming and postconsuming qualifiers are not supported on interface choices.
See Reference: Choices for full reference information, but note that controller-first syntax is not supported for interface choices.

Empty Interfaces¶

data EmptyInterfaceView = EmptyInterfaceView {}

interface YourInterface where
  viewtype EmptyInterfaceView

It is possible (though not necessarily useful) to define an interface without methods, precondition or choices. However, a view type must always be defined, though it can be set to unit.

Interface Instances¶

For context, a simple template definition:

template NameOfTemplate
  with
    field1 : Party
    field2 : Int
  where
    signatory field1

`interface instance` clause¶

    interface instance MyInterface for NameOfTemplate where
      view = MyInterfaceViewType "NameOfTemplate" 100
      method1 = field1
      method2 = field2
      method3 False _ _ = 0
      method3 True x y
        | x > 0 = x + y
        | otherwise = y

To make a template an instance of an existing interface, an interface instance clause must be defined in the template declaration.
The template of the clause must match the enclosing declaration. In other words, a template T declaration can only contain interface instance clauses where the template is T.
The clause must start with the keywords interface instance, followed by the name of the interface, then the keyword for and the name of the template, and finally the keyword where, which introduces a block where all the methods of the interface must be implemented.
Within the clause, there’s an implicit local binding this referring to the contract on which the method is applied, which has the type of the template’s data record. The template parameters of this contract are also in scope.
Method implementations can be defined using the same syntax as for top level functions, including pattern matches and guards (e.g. method3).
Each method implementation must return the same type as specified for that method in the interface declaration.
The implementation of the special view method must return the type specified as the viewtype in the interface declaration.

`interface instance` clause in the interface¶

interface MyNewInterface where
  viewtype EmptyInterfaceView
  myNewMethod1 : Party
  myNewMethod2 : Int -> Int

  choice MyNewChoice : Int
    controller myNewMethod1 this
    do pure $ myNewMethod2 this 0

  interface instance MyNewInterface for NameOfTemplate where
    view = EmptyInterfaceView
    myNewMethod1 = field1
    myNewMethod2 x = field2 `min` x

To make an existing template an instance of a new interface, the interface instance clause must be defined in the interface declaration.
In this case, the interface of the clause must match the enclosing declaration. In other words, an interface I declaration can only contain interface instance clauses where the interface is I.
All other rules for interface instance clauses are the same whether the enclosing declaration is a template or an interface. In particular, the implicit local binding this always has the type of the template’s record.

Empty `interface instance` clause¶

If the interface has no methods, the interface instance only needs to implement the view method:

    interface instance YourInterface for NameOfTemplate where
      view = EmptyInterfaceView

Interface Functions¶

`interfaceTypeRep`¶

Type	`HasInterfaceTypeRep i =>` `i -> TemplateTypeRep`
Instantiated Type	`MyInterface -> TemplateTypeRep`
Notes	The value of the resulting `TemplateTypeRep` indicates what template was used to construct the interface value.

`toInterface`¶

Type	`forall i t.` `HasToInterface t i =>` `t -> i`
Instantiated Type	`MyTemplate -> MyInterface`
Notes	Converts a template value into an interface value.

`fromInterface`¶

Type	`HasFromInterface t i =>` `i -> Optional t`
Instantiated Type	`MyInterface -> Optional MyTemplate`
Notes	Attempts to convert an interface value back into a template value. The result is `None` if the expected template type doesn’t match the underlying template type used to construct the contract.

`toInterfaceContractId`¶

Type	`forall i t.` `HasToInterface t i =>` `ContractId t -> ContractId i`
Instantiated Type	`ContractId MyTemplate -> ContractId MyInterface`
Notes	Converts a template Contract ID into an Interface Contract ID.

`fromInterfaceContractId`¶

Type	`forall t i.` `HasFromInterface t i =>` `ContractId i -> ContractId t`
Instantiated Type	`ContractId MyInterface -> ContractId MyTemplate`
Notes	Converts an interface contract id into a template contract id. This function does not verify that the given contract id actually points to a contract of the resulting type; if that is not the case, a subsequent `fetch`, `exercise` or `archive` will fail. Therefore, this should only be used when the underlying contract is known to be of the resulting type, or when the result is immediately used by a `fetch`, `exercise` or `archive` action and a transaction failure is the desired behavior in case of mismatch. In all other cases, consider using `fetchFromInterface` instead.

`coerceInterfaceContractId`¶

Type	`forall j i.` `(HasInterfaceTypeRep i, HasInterfaceTypeRep j) =>` `ContractId i -> ContractId j`
Instantiated Type	`ContractId SourceInterface -> ContractId TargetInterface`
Notes	Converts an interface contract id into a contract id of a different interface. This function does not verify that the given contract id actually points to a contract of the resulting type; if that is not the case, a subsequent `fetch`, `exercise` or `archive` will fail. Therefore, this should only be used when the underlying contract is known to be of the resulting type, or when the result is immediately used by a `fetch`, `exercise` or `archive` action and a transaction failure is the desired behavior in case of mismatch.

`fetchFromInterface`¶

Type	`forall t i.` `(HasFromInterface t i, HasFetch i) =>` `ContractId i -> Update (Optional (ContractId t, t))`
Instantiated Type	`ContractId MyInterface ->` `Update (Optional (ContractId MyTemplate, MyTemplate))`
Notes	Attempts to fetch and convert an interface contract id into a template, returning both the converted contract and its contract id if the conversion is successful, or `None` otherwise.

Required Interfaces¶

interface OurInterface requires MyInterface, YourInterface where
  viewtype EmptyInterfaceView

An interface can depend on other interfaces. These are specified with the requires keyword after the interface name but before the where keyword, separated by commas.
For an interface declaration to be valid, its list of required interfaces must be transitively closed. In other words, an interface I cannot require an interface J without also explicitly requiring all the interfaces required by J. The order, however, is irrelevant.

For example, given
```
interface Shape where
  viewtype EmptyInterfaceView

interface Rectangle requires Shape where
  viewtype EmptyInterfaceView
```
This declaration for interface Square would cause a compiler error
```
-- Compiler error! "Interface Square is missing requirement [Shape]"
interface Square requires Rectangle where
  viewtype EmptyInterfaceView
```
Explicitly adding Shape to the required interfaces fixes the error
```
interface Square requires Rectangle, Shape where
  viewtype EmptyInterfaceView
```
For a template T to be a valid interface instance of an interface I, T must also be an interface instance of each of the interfaces required by I.

Interface Functions¶

Function	Notes
`toInterface`	Can also be used to convert an interface value to one of its required interfaces.
`fromInterface`	Can also be used to convert a value of an interface type to one of its requiring interfaces.
`toInterfaceContractId`	Can also be used to convert an interface contract id into a contract id of one of its required interfaces.
`fromInterfaceContractId`	Can also be used to convert an interface contract id into a contract id of one of its requiring interfaces.
`fetchFromInterface`	Can also be used to fetch and convert an interface contract id into a contract and contract id of one of its requiring interfaces.

Reference: Interfaces¶

Configuration¶

Interface Declaration¶

Interface Name¶

Implicit abstract type¶

Interface Methods¶

Interface View Type¶

Interface Choices¶

Empty Interfaces¶

Interface Instances¶

interface instance clause¶

interface instance clause in the interface¶

Empty interface instance clause¶

Interface Functions¶

interfaceTypeRep¶

toInterface¶

fromInterface¶

toInterfaceContractId¶

fromInterfaceContractId¶

coerceInterfaceContractId¶

fetchFromInterface¶

Required Interfaces¶

Interface Functions¶

`interface instance` clause¶

`interface instance` clause in the interface¶

Empty `interface instance` clause¶

`interfaceTypeRep`¶

`toInterface`¶

`fromInterface`¶

`toInterfaceContractId`¶

`fromInterfaceContractId`¶

`coerceInterfaceContractId`¶

`fetchFromInterface`¶