Interfaces

-- Interfaces.daml
module Interfaces where

import Daml.Script


-- ── 1. Define the interface ──────────────────────────────
-- The viewtype is a record that must be implemented by every instance.
-- It is what callers see when they fetch the interface.
data AssetView = AssetView
  with
    issuer : Party
    owner  : Party
    amount : Decimal
  deriving (Eq, Show)

interface IAsset where
  viewtype AssetView

  -- Method signatures (no implementations here).
  getOwner  : Party
  getAmount : Decimal

  -- Interface choice — controller uses method, not a field.
  choice ITransfer : ContractId IAsset
    with newOwner : Party
    controller getOwner this
    do
      setOwner this newOwner

  -- Method that subclasses must implement.
  setOwner : ContractId IAsset -> Party -> Update (ContractId IAsset)


-- ── 2. Implement the interface in a template ─────────────
template Cash
  with
    issuer : Party
    owner  : Party
    amount : Decimal
  where
    signatory issuer
    observer  owner
    ensure amount > 0.0

    -- `interface instance IAsset for Cash` links the template to the interface.
    interface instance IAsset for Cash where
      view = AssetView with issuer; owner; amount

      getOwner  = owner
      getAmount = amount

      setOwner _self newOwner =
        toInterfaceContractId <$> create this with owner = newOwner


-- A second template implementing the same interface.
template Bond
  with
    issuer    : Party
    owner     : Party
    faceValue : Decimal
  where
    signatory issuer
    observer  owner
    ensure faceValue > 0.0

    interface instance IAsset for Bond where
      view = AssetView with issuer; owner; amount = faceValue

      getOwner  = owner
      getAmount = faceValue

      setOwner _self newOwner =
        toInterfaceContractId <$> create this with owner = newOwner


-- ── 3. Write code against the interface ──────────────────
-- This function works on any IAsset, regardless of template type.
transferAsset : ContractId IAsset -> Party -> Update (ContractId IAsset)
transferAsset assetId newOwner = do
  exercise assetId ITransfer with newOwner


-- ── Test ─────────────────────────────────────────────────
interfaceTest : Script ()
interfaceTest = do
  bank  <- allocateParty "Bank"
  alice <- allocateParty "Alice"
  bob   <- allocateParty "Bob"

  -- Create a Cash contract and view it as IAsset
  cashId <- submit bank do
    createCmd Cash with issuer = bank; owner = alice; amount = 100.0

  let iAssetId = toInterfaceContractId @IAsset cashId

  -- Transfer using the interface choice
  newId <- submit alice do
    exerciseCmd iAssetId ITransfer with newOwner = bob

  -- Fetch via interface to get the view
  Some iAsset <- queryInterfaceContractId @IAsset alice newId
  assert (iAsset.owner == bob)

Key points

Interface basics

• Every interface must declare a viewtype: a record type returned when the interface is fetched.
• The interface instance T for Template block must implement every method and the view function.

Viewtype & methods

• toInterfaceContractId @I cid upcasts a ContractId Template to a ContractId I.
• fromInterfaceContractId @T icid attempts a downcast to a specific template; it returns Optional (ContractId T).

Polymorphism

• Interface choices are defined on the interface and dispatch to the template's interface instance implementation at runtime.
• Interfaces require DAML-LF 1.15 or higher (the default in SDK 2.x).