Composition and Design Patterns

Compose multi-step transactions, understand Daml’s execution model and privacy, and apply common multi-party workflow patterns

Compose choices

It’s time to put everything you’ve learned so far together into a complete and secure Daml model for asset issuance, management, transfer, and trading. This application will have capabilities similar to the one in the CN Quickstart. In the process you will learn about a few more concepts:

• Daml projects, packages, and modules
• Composition of transactions
• Observers and stakeholders
• Daml’s execution model
• Privacy

The model in this section is not a single Daml file, but a Daml project consisting of several files that depend on each other.

Remember that you can load all the code for this section into a folder called intro-compose by running dpm new intro-compose --template daml-intro-compose

Daml projects

Daml is organized in projects, packages, and modules. A Daml project is specified using a single daml.yaml file, and compiles into a package in Daml’s intermediate language, or bytecode equivalent, Daml-LF. Each Daml file within a project becomes a Daml module, which is a bit like a namespace. Each Daml project has a source root specified in the source parameter in the project’s daml.yaml file. The package will include all modules specified in *.daml files beneath that source directory.

You can start a new project with a skeleton structure using dpm new project-name in the terminal. A minimal project would contain just a daml.yaml file and an empty directory of source files.

Take a look at the daml.yaml for the this chapter’s project:

sdk-version: __VERSION__
name: __PROJECT_NAME__
source: daml
version: 1.0.0
dependencies:
  - daml-prim
  - daml-stdlib
  - daml-script

You can generally set name and version freely to describe your project. dependencies does what the name suggests: it includes dependencies. You should always include daml-prim and daml-stdlib. The former contains internals of the compiler and the Daml Runtime, the latter gives access to the Daml standard library. daml-script contains the types and functions for Daml Script.

You compile a Daml project by running dpm build from the project root directory. This creates a DAR file in .daml/dist/dist/${project_name}-${project_version}.dar. A DAR file is Daml’s equivalent of a JAR file in Java: it’s the artifact that gets deployed to a ledger to load the package and its dependencies. dar files are fully self-contained in that they contain all dependencies of the main package. More on all of this in Building and Packaging.

Project structure

This project contains an asset holding model for transferable, fungible assets and a separate trade workflow. The templates are structured in three modules: Intro.Asset, Intro.Asset.Role, and Intro.Asset.Trade.

In addition, there are tests in modules Test.Intro.Asset, Test.Intro.Asset.Role, and Test.Intro.Asset.Trade.

All but the last .-separated segment in module names correspond to paths relative to the project source directory, and the last one to a file name. The folder structure therefore looks like this:

.
├── daml
│   ├── Intro
│   │   ├── Asset
│   │   │   ├── Role.daml
│   │   │   └── Trade.daml
│   │   └── Asset.daml
│   └── Test
│       └── Intro
│           ├── Asset
│           │   ├── Role.daml
│           │   └── Trade.daml
│           └── Asset.daml
└── daml.yaml

Each file contains a module header. For example, daml/Intro/Asset/Role.daml:

module Intro.Asset.Role where

You can import one module into another using the import keyword. The LibraryModules module imports all six modules:

import Intro.Asset

Imports always have to appear just below the module declaration. You can optionally add a list of names after the import to import only the selected names:

import DA.List (sortOn, groupOn)

If your module contains any Daml Scripts, you need to import the corresponding functionality:

import Daml.Script

Project overview

The project both changes and adds to the Iou model presented in Authorization:

• Assets are fungible in the sense that they have Merge and Split choices that allow the owner to manage their holdings.

• Transfer proposals now need the authorities of both issuer and newOwner to accept. This makes Asset safer than Iou from the issuer’s point of view.

  With the Iou model, an issuer could end up owing cash to anyone as transfers were authorized by just owner and newOwner. In this project, only parties having an AssetHolder contract can end up owning assets. This allows the issuer to determine which parties may own their assets.

• The Trade template adds a swap of two assets to the model.

Composed choices and scripts

This project showcases how you can put the Update and Script actions you learned about in Authorization to good use. For example, the Merge and Split choices each perform several actions in their consequences.

• Two create actions in case of Split
• One create and one archive action in case of Merge

choice Split
      : SplitResult
      with
        splitQuantity : Decimal
      controller owner
      do
        splitAsset <- create this with
          quantity = splitQuantity
        remainder <- create this with
          quantity = quantity - splitQuantity
        return SplitResult with
          splitAsset
          remainder

    choice Merge
      : ContractId Asset
      with
        otherCid : ContractId Asset
      controller owner
      do
        other <- fetch otherCid
        assertMsg
          "Merge failed: issuer does not match"
          (issuer == other.issuer)
        assertMsg
          "Merge failed: owner does not match"
          (owner == other.owner)
        assertMsg
          "Merge failed: symbol does not match"
          (symbol == other.symbol)
        archive otherCid
        create this with
          quantity = quantity + other.quantity

The return function used in Split is available in any Action context. The result of return x is a no-op containing the value x. It has an alias pure, indicating that it’s a pure value, as opposed to a value with side-effects. The return name makes sense when it’s used as the last statement in a do block as its argument is indeed the “return”-value of the do block in that case.

Taking transaction composition a step further, the Trade_Settle choice on Trade composes two exercise actions:

choice Trade_Settle
      : (ContractId Asset, ContractId Asset)
      with
        quoteAssetCid : ContractId Asset
        baseApprovalCid : ContractId TransferApproval
      controller quoteAsset.owner
      do
        fetchedBaseAsset <- fetch baseAssetCid
        assertMsg
          "Base asset mismatch"
          (baseAsset == fetchedBaseAsset with
            observers = baseAsset.observers)

        fetchedQuoteAsset <- fetch quoteAssetCid
        assertMsg
          "Quote asset mismatch"
          (quoteAsset == fetchedQuoteAsset with
            observers = quoteAsset.observers)

        transferredBaseCid <- exercise
          baseApprovalCid TransferApproval_Transfer with
            assetCid = baseAssetCid

        transferredQuoteCid <- exercise
          quoteApprovalCid TransferApproval_Transfer with
            assetCid = quoteAssetCid

        return (transferredBaseCid, transferredQuoteCid)

The resulting transaction, with its two nested levels of consequences, can be seen in the test_trade script in Test.Intro.Asset.Trade:

TX 14 1970-01-01T00:00:00Z (Test.Intro.Asset.Trade:79:23)
#14:0
│   disclosed to (since): 'Alice' (14), 'Bob' (14)
└─> 'Bob' exercises Trade_Settle on #12:0 (Intro.Asset.Trade:Trade)
          with
            quoteAssetCid = #9:1; baseApprovalCid = #13:1
    children:
    #14:1
    │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'USD_Bank' (14)
    └─> 'Alice' and 'USD_Bank' fetch #10:1 (Intro.Asset:Asset)

    #14:2
    │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'EUR_Bank' (14)
    └─> 'Bob' and 'EUR_Bank' fetch #9:1 (Intro.Asset:Asset)

    #14:3
    │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'USD_Bank' (14)
    └─> 'Alice' and 'Bob' exercise TransferApproval_Transfer on #13:1 (Intro.Asset:TransferApproval)
                          with
                            assetCid = #10:1
        children:
        #14:4
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'USD_Bank' (14)
        └─> 'Alice' and 'USD_Bank' fetch #10:1 (Intro.Asset:Asset)

        #14:5
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'USD_Bank' (14)
        └─> 'Alice' and 'USD_Bank' exercise Archive on #10:1 (Intro.Asset:Asset)

        #14:6
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'USD_Bank' (14)
        └─> 'Bob' and 'USD_Bank' create Intro.Asset:Asset
                                 with
                                   issuer = 'USD_Bank';
                                   owner = 'Bob';
                                   symbol = "USD";
                                   quantity = 100.0000000000;
                                   observers = []

    #14:7
    │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'EUR_Bank' (14)
    └─> 'Alice',
        'Bob' exercises TransferApproval_Transfer on #11:1 (Intro.Asset:TransferApproval)
              with
                assetCid = #9:1
        children:
        #14:8
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'EUR_Bank' (14)
        └─> 'Bob' and 'EUR_Bank' fetch #9:1 (Intro.Asset:Asset)

        #14:9
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'EUR_Bank' (14)
        └─> 'Bob' and 'EUR_Bank' exercise Archive on #9:1 (Intro.Asset:Asset)

        #14:10
        │   disclosed to (since): 'Alice' (14), 'Bob' (14), 'EUR_Bank' (14)
        └─> 'Alice' and 'EUR_Bank' create Intro.Asset:Asset
                                   with
                                     issuer = 'EUR_Bank';
                                     owner = 'Alice';
                                     symbol = "EUR";
                                     quantity = 90.0000000000;
                                     observers = []

Similar to choices, you can see how the scripts in this project are built up from each other:

test_issuance = do
  setupResult@(alice, bob, bank, aha, ahb) <- setupRoles

  assetCid <- submit bank do
    exerciseCmd aha Issue_Asset
      with
        symbol = "USD"
        quantity = 100.0

  Some asset <- queryContractId bank assetCid
  assert (asset == Asset with
      issuer = bank
      owner = alice
      symbol = "USD"
      quantity = 100.0
      observers = []
        )

  return (setupResult, assetCid)

In the above, the test_issuance script in Test.Intro.Asset.Role uses the output of the setupRoles script in the same module.

The same line shows a new kind of pattern matching. Rather than writing setupResult <- setupRoles and then accessing the components of setupResult using _1, _2, etc., you can give them names. It’s equivalent to writing:

setupResult <- setupRoles
case setupResult of
  (alice, bob, bank, aha, ahb) -> ...

Just writing (alice, bob, bank, aha, ahb) <- setupRoles would also be legal, but setupResult is used in the return value of test_issuance so it makes sense to give it a name, too. The notation with @ allows you to give both the whole value as well as its constituents names in one go.

Daml’s execution model

Daml’s execution model is fairly easy to understand, but has some important consequences. You can imagine the life of a transaction as follows:

1. Command submission: A user submits a list of commands via the Ledger API of a participant node, acting as a Party hosted on that node. That party is called the requester.

2. Interpretation: Each command corresponds to one or more actions. During this step, the Update corresponding to each action is evaluated in the context of the ledger to calculate all consequences, including transitive ones (consequences of consequences, etc.). The result of this is a complete transaction. Together with its requestor, this is also known as a commit.

3. Blinding: On ledgers with strong privacy, projections (see Privacy Model) for all involved parties are created. This is also called projecting.

4. Transaction submission: The transaction/commit is submitted to the network.

5. Validation: The transaction/commit is validated by the network. Who exactly validates can differ from implementation to implementation. Validation also involves scheduling and collision detection, ensuring that the transaction has a well-defined place in the (partial) ordering of commits, and no double spends occur.

6. Commitment: The commit is actually committed according to the commit or consensus protocol of the ledger.

7. Confirmation: The network sends confirmations of the commitment back to all involved participant nodes.

8. Completion: The user gets back a confirmation through the Ledger API of the submitting participant node.

The first important consequence of the above is that all transactions are committed atomically. Either a transaction is committed as a whole and for all participants, or it fails.

That’s important in the context of the Trade_Settle choice shown above. The choice transfers a baseAsset one way and a quoteAsset the other way. Thanks to transaction atomicity, there is no chance that either party is left out of pocket.

The second consequence is that the requester of a transaction knows all consequences of their submitted transaction — there are no surprises in Daml. However, it also means that the requester must have all the information to interpret the transaction. We also refer to this as Principle 2 a bit later on this page.

That’s also important in the context of Trade. In order to allow Bob to interpret a transaction that transfers Alice’s cash to Bob, Bob needs to know both about Alice’s Asset contract, as well as about some way for Alice to accept a transfer — remember, accepting a transfer needs the authority of issuer in this example.

Observers

Observers are Daml’s mechanism to disclose contracts to other parties. They are declared just like signatories, but using the observer keyword, as shown in the Asset template:

template Asset
  with
    issuer : Party
    owner : Party
    symbol : Text
    quantity : Decimal
    observers : [Party]
  where
    signatory issuer, owner
    ensure quantity > 0.0

    observer observers

The Asset template also gives the owner a choice to set the observers, and you can see how Alice uses it to show her Asset to Bob just before proposing the trade. You can try out what happens if she didn’t do that by removing that transaction:

usdCid <- submit alice do
    exerciseCmd usdCid SetObservers with
      newObservers = [bob]

Observers have guarantees in Daml. In particular, they are guaranteed to see actions that create and archive the contract on which they are an observer.

Since observers are calculated from the arguments of the contract, they always know about each other. That’s why, rather than adding Bob as an observer on Alice’s AssetHolder contract, and using that to authorize the transfer in Trade_Settle, Alice creates a one-time authorization in the form of a TransferAuthorization. If Alice had lots of counterparties, she would otherwise end up leaking them to each other.

Choice controllers are not automatically made observers, as they can only be calculated at the point in time when the choice arguments are known.

Privacy

Daml’s privacy model is based on two principles:

Principle 1. Parties see those actions that they have a stake in. Principle 2. Every party that sees an action sees its (transitive) consequences.

Principle 2 is necessary to ensure that every party can independently verify the validity of every transaction they see.

A party has a stake in an action if

• they are a required authorizer of it
• they are a signatory of the contract on which the action is performed
• they are an observer on the contract, and the action creates or archives it

What does that mean for the exercise tradeCid Trade_Settle action from test_trade?

Alice is the signatory of tradeCid and Bob a required authorizer of the Trade_Settled action, so both of them see it. According to principle 2 above, that means they get to see everything in the transaction.

The consequences contain, next to some fetch actions, two exercise actions of the choice TransferApproval_Transfer.

Each of the two involved TransferApproval contracts is signed by a different issuer, which see the action on “their” contract. So the EUR_Bank sees the TransferApproval_Transfer action for the EUR Asset and the USD_Bank sees the TransferApproval_Transfer action for the USD Asset.

Some Daml ledgers, like the script runner and the Sandbox, work on the principle of “data minimization”, meaning nothing more than the above information is distributed. That is, the “projection” of the overall transaction that gets distributed to EUR_Bank in step 4 (transaction submission) of Daml’s execution model would consist only of the TransferApproval_Transfer and its consequences.

Other implementations, in particular those on public blockchains, may have weaker privacy constraints.

Divulgence

Note that principle 2 of the privacy model means that sometimes parties see contracts that they are not signatories or observers on. If you look at the final ledger state of the test_trade script, for example, you may notice that both Alice and Bob now see both assets, as indicated by the Xs in their respective columns:

Alice	Bob	EUR_Bank	USD_Bank	id	status	issuer	owner	symbol	quantity
X	X	-	X	#15:6	active	USD_Bank	Bob	USD	100.0
X	X	X	-	#15:10	active	EUR_Bank	Alice	EUR	90.0

This is because the create action of these contracts are in the transitive consequences of the Trade_Settle action both of them have a stake in. This kind of disclosure is often called “divulgence” and needs to be considered when designing Daml models for privacy sensitive applications.

Common Daml design patterns

Beyond the composition patterns above, this section covers common multi-party workflow patterns used in Daml. All examples below use a Coin asset model to illustrate each pattern.

You can check out the examples locally by running dpm new daml-patterns --template daml-patterns.

The diagrams below use a shared visual key for contracts, signatories, and choices:

Propose-Accept

The most common way to get multiple parties to agree on a shared contract. One party creates a proposal contract that the other party can accept, reject, or let expire. The IouProposal in the authorization module is another example of this pattern.

It takes two to tango, but one party has to propose. It is no different in the business world. The contractual relationship between two businesses often starts with an invite, a business proposal, a bid offering, etc.

Invite — When a market operator wants to set up a market, they need to go through an onboarding process in which they invite participants to sign master service agreements and fulfill different roles in the market. Receiving participants need to evaluate the rights and responsibilities of each role and respond accordingly.

Propose — When issuing an asset, an issuer is making a business proposal to potential buyers. The proposal lays out what is expected from buyers, and what they can expect from the issuer. Buyers need to evaluate all aspects of the offering, e.g. price, return, and tax implications, before making a decision.

The Propose and Accept pattern demonstrates how to write a Daml program to model the initiation of an inter-company contractual relationship. Daml modelers often have to follow this pattern to ensure that no participant is forced into an obligation.

The issuer creates a CoinMaster contract, then uses it to invite an owner. The invitation is a proposal contract with the issuer as signatory and the owner as observer:

template CoinMaster
  with
    issuer: Party
  where
    signatory issuer

    nonconsuming choice Invite : ContractId CoinIssueProposal
      with owner: Party
      controller issuer
      do create CoinIssueProposal
            with coinAgreement = CoinIssueAgreement with issuer; owner

The proposal gives the owner a choice to accept. In a complete model, it would also include Reject and Counter choices:

template CoinIssueProposal
  with
    coinAgreement: CoinIssueAgreement
  where
    signatory coinAgreement.issuer
    observer coinAgreement.owner

    choice AcceptCoinProposal
      : ContractId CoinIssueAgreement
      controller coinAgreement.owner
      do create coinAgreement

When the owner accepts, the result contract has both parties as signatories — neither can be forced into the agreement without consent:

template CoinIssueAgreement
  with
    issuer: Party
    owner: Party
  where
    signatory issuer, owner

    nonconsuming choice Issue : ContractId Coin
      with amount: Decimal
      controller issuer
      do create Coin with issuer; owner; amount; delegates = []

This pattern can be verbose when more than two signatures are needed — see Multiple Party Agreement below for that case.

Delegation

Gives one party the right to exercise a choice on behalf of another. The principal creates a delegation contract that authorizes an agent to act for them, without the principal committing each action. This models real-world custodian relationships where a bank holds securities and settles transactions on a client’s behalf.

Delegation is prevalent in the business world. In fact, the entire custodian business is based on delegation. When a company chooses a custodian bank, it is effectively giving the bank the rights to hold their securities and settle transactions on their behalf. The securities are not legally possessed by the custodian banks, but the banks should have full rights to perform actions in the client’s name, such as making payments or changing investments.

The Delegation pattern enables Daml modelers to model the real-world business contractual agreements between custodian banks and their customers. Ownership and administration rights can be segregated easily and clearly.

The delegation contract (CoinPoA — Power of Attorney) has the principal as signatory. The attorney controls a TransferCoin choice that exercises Transfer on the principal’s coin:

template CoinPoA
  with
    attorney: Party
    principal: Party
  where
    signatory principal
    observer attorney

    choice WithdrawPoA
      : ()
      controller principal
      do return ()

    -- Attorney has the delegated right to Transfer
    nonconsuming choice TransferCoin
      : ContractId TransferProposal
      with
        coinId: ContractId Coin
        newOwner: Party
      controller attorney
      do
        exercise coinId Transfer with newOwner

Whether or not the attorney should be a signatory of CoinPoA is subject to the business agreements between principal and attorney. For simplicity, in this example, the attorney is not a signatory.

The coin must be disclosed to the attorney before they can exercise the delegated choice. This is done by adding them as an observer via a Disclose choice on Coin:

choice Disclose : ContractId Coin
      with p : Party
      controller owner
      do create this with delegates = p :: delegates

Authorization

Verifies that a controlling party has the right permissions before they take certain actions. An authorization contract serves as proof — the choice body checks for its existence and validity before proceeding.

Authorization is a universal concept in the business world, as access to most business resources is a privilege and not given freely. For example, security trading may seem to be a plain bilateral agreement between the two trading counterparties, but this could not be further from the truth. To be able to trade, the trading parties need to go through a series of authorization processes and gain permission from a list of service providers such as exchanges, market data streaming services, clearing houses, and security registrars.

The Authorization pattern shows how to model these authorization checks prior to a business transaction.

For example, an issuer wants to ensure that only accredited parties can receive coin transfers. The issuer creates an authorization token for approved owners:

template CoinOwnerAuthorization
  with
    owner: Party
    issuer: Party
  where
    signatory issuer
    observer owner

    choice WithdrawAuthorization
      : ()
      controller issuer
      do return ()

The AcceptTransfer choice on TransferProposal requires the new owner to supply their authorization token. The asserts verify the token matches the issuer and the new owner:

choice AcceptTransfer
      : ContractId Coin
      with token: ContractId CoinOwnerAuthorization
      controller newOwner
      do
        t <- fetch token
        assert (coin.issuer == t.issuer)
        assert (newOwner == t.owner)
        create coin with owner = newOwner

If the issuer withdraws the authorization before the transfer is accepted, the transfer fails.

Locking

Prevents choices from being exercised on a contract while it is in a locked state. Useful for scenarios like securities settlement where assets must be frozen during clearing.

Locking is a common real-life requirement in business transactions. During the clearing and settlement process, once a trade is registered and novated to a central clearing house, the trade is considered locked-in. This means the securities under the ownership of the seller need to be locked so they cannot be used for other purposes, and so should the funds on the buyer’s account. The locked state should remain throughout the settlement payment-versus-delivery process. Once the ownership is exchanged, the lock is lifted for the new owner to have full access.

There are three ways to achieve locking:

Locking by archiving

Archiving is a straightforward choice for locking because once a contract is archived, all choices on the contract become unavailable. Archiving can be done either through a consuming choice or an archiving contract.

Consuming choice

The steps below show how to use a consuming choice in the original contract to achieve locking:

• Add a consuming choice, Lock, to the Coin template that creates a LockedCoin.
• The controller party on Lock may vary depending on business context. In this example, owner is a good choice.
• The parameters to this choice are also subject to business use case. Normally, it should at least have locking terms (e.g. lock expiry time) and a party authorized to unlock.

choice Lock : ContractId LockedCoin
  with maturity: Time; locker: Party
  controller owner
  do create LockedCoin with coin=this; maturity; locker

Create a LockedCoin to represent Coin in the locked state. LockedCoin has the following characteristics, all in order to be able to recreate the original Coin:

• The signatories are the same as the original contract.
• It has all data of Coin, either through having a Coin as a field, or by replicating all data of Coin.
• It has an Unlock choice to lift the lock.

template LockedCoin
  with
    coin: Coin
    maturity: Time
    locker: Party
  where
    signatory coin.issuer, coin.owner
    observer locker

    choice Unlock
      : ContractId Coin
      controller locker
      do create coin

Archiving contract

In the event that changing the original contract is not desirable, and assuming the original contract already has an Archive choice, you can introduce another contract, CoinCommitment, to archive Coin and create LockedCoin.

Examine the controller party and archiving logic in the Archives choice on the Coin contract. A coin can only be archived by the issuer under the condition that the issuer is the owner of the coin. This ensures the issuer cannot archive any coin at will:

-- a coin can only be archived by the issuer under the condition that the issuer is the owner of the coin. This ensures the issuer cannot archive coins at will.
choice Archives
  : ()
  controller issuer
  do assert (issuer == owner)

Since we need to call the Archives choice from CoinCommitment, its signatory has to be the issuer. The controller party and parameters on the Lock choice are the same as described above for locking by consuming choice — the additional logic required is to transfer the asset to the issuer, and then explicitly call the Archive choice on the Coin contract. Once a Coin is archived, the Lock choice creates a LockedCoin that represents Coin in the locked state:

template CoinCommitment
  with
    owner: Party
    issuer: Party
    amount: Decimal
  where
    signatory issuer
    observer owner

    nonconsuming choice LockCoin
      : ContractId LockedCoin
      with
        coinCid: ContractId Coin
        maturity: Time
        locker: Party
      controller owner
      do
        inputCoin <- fetch coinCid
        assert (inputCoin.owner == owner && inputCoin.issuer == issuer && inputCoin.amount == amount)
        -- the original coin is transferred to the issuer, then archived
        prop <- exercise coinCid Transfer with newOwner = issuer
        id <- exercise prop AcceptTransfer
        exercise id Archives
        create LockedCoin with
          coin = inputCoin with owner; issuer; amount
          maturity; locker

This pattern achieves locking in a fairly straightforward way. However, there are some trade-offs:

• Locking by archiving disables all choices on the original contract. Usually for consuming choices this is exactly what is required, but if a party needs to selectively lock only some choices, remaining active choices need to be replicated on the LockedCoin contract, which can lead to code duplication.
• The choices on the original contract need to be altered for the lock choice to be added. If this contract is shared across multiple participants, it will require agreement from all involved.

Locking by state change

In its original form, all choices on Coin are actionable as long as the contract is active. Locking by state requires introducing fields to track state. This allows for the creation of an active contract in two possible states: locked or unlocked. A Daml modeler can selectively make certain choices actionable only if the contract is in an unlocked state. This effectively makes the asset lockable.

The state can be stored in many ways. This example demonstrates how to create a LockableCoin through a party. Alternatively, you can add a lock contract to the asset contract, use a boolean flag, or include lock activation and expiry terms as part of the template parameters.

Here are the changes made to the original Coin contract to make it lockable:

• Add a locker party to the template parameters.
• Define the states: if owner == locker, the coin is unlocked; if owner != locker, the coin is in a locked state.
• The contract state is checked on choices: Transfer is only actionable if the coin is unlocked; Lock is only actionable if the coin is unlocked and a third-party locker is supplied; Unlock is available to the locker party only if the coin is locked.

template LockableCoin
  with
    owner: Party
    issuer: Party
    amount: Decimal
    locker: Party
  where
    signatory issuer, owner
    observer locker

    ensure amount > 0.0

    -- Transfer can only happen if not locked
    choice Transfer : ContractId TransferProposal
      with newOwner: Party
      controller owner
      do
        assert (locker == owner)
        create TransferProposal with coin=this; newOwner

    -- Lock by bringing a locker on board
    choice Lock : ContractId LockableCoin
      with newLocker: Party
      controller owner
      do
        assert (newLocker /= owner)
        create this with locker = newLocker

    -- Unlock restores owner control
    choice Unlock
      : ContractId LockableCoin
      controller locker
      do
        assert (locker /= owner)
        create this with locker = owner

Trade-offs:

• It requires changes made to the original contract template. Furthermore, every choice intended to be locked needs to change too.
• If locking and unlocking terms (e.g. lock triggering event, expiry time, etc.) need to be added to the template parameters to track the state change, the template can get overloaded.

Locking by safekeeping

Safekeeping is a realistic way to model locking, as it is a common practice in many industries. For example, during a real estate transaction, purchase funds are transferred to the seller’s lawyer’s escrow account after the contract is signed and before closing.

There is no need to make a change to the original contract. With two additional contracts, we can transfer the Coin ownership to a locker party:

• LockRequest has a locker party as the single signatory, allowing the locker party to unilaterally initiate the process and specify locking terms.
• Once the owner exercises Accept on the lock request, the ownership of the coin is transferred to the locker.
• The Accept choice also creates a LockedCoinV2 that represents Coin in the locked state.

template LockRequest
  with
    locker: Party
    maturity: Time
    coin: Coin
  where
    signatory locker
    observer coin.owner

    choice Accept : LockResult
      with coinCid : ContractId Coin
      controller coin.owner
      do
        inputCoin <- fetch coinCid
        assert (inputCoin == coin)
        tpCid <- exercise coinCid Transfer with newOwner = locker
        coinCid <- exercise tpCid AcceptTransfer
        lockCid <- create LockedCoinV2 with locker; maturity; coin
        return LockResult {coinCid; lockCid}

LockedCoinV2 represents Coin in the locked state. It is fairly similar to the LockedCoin described above for locking by consuming choice. The additional logic is to transfer ownership from the locker back to the owner when Unlock or Clawback is called:

template LockedCoinV2
  with
    coin: Coin
    maturity: Time
    locker: Party
  where
    signatory locker, coin.owner

    choice UnlockV2
      : ContractId Coin
      with coinCid : ContractId Coin
      controller locker
      do
        inputCoin <- fetch coinCid
        assert (inputCoin.owner == locker)
        tpCid <- exercise coinCid Transfer with newOwner = coin.owner
        exercise tpCid AcceptTransfer

    choice ClawbackV2
      : ContractId Coin
      with coinCid : ContractId Coin
      controller coin.owner
      do
        currTime <- getTime
        assert (currTime >= maturity)
        inputCoin <- fetch coinCid
        assert (inputCoin == coin with owner=locker)
        tpCid <- exercise coinCid Transfer with newOwner = coin.owner
        exercise tpCid AcceptTransfer

Ownership transfer may give the locking party too much access to the locked asset. A rogue lawyer could run away with the funds. In a similar fashion, a malicious locker party could introduce code to transfer assets away while they are under their ownership.

Multiple party agreement

Collects signatures from more than two parties. A Pending contract wraps the final Agreement and tracks who has signed. Each party signs by exercising a Sign choice, and once all parties have signed, any of them can Finalize to create the agreement.

Propose-Accept (above) shows how to create bilateral agreements in Daml. However, a project or a workflow often requires more than two parties to reach a consensus and put their signatures on a multi-party contract. For example, in a large construction project, there are at least three major stakeholders: owner, architect, and builder. All three parties need to establish agreement on key responsibilities and project success criteria before starting the construction.

If such an agreement were modeled as three separate bilateral agreements, no party could be sure if there are conflicts between their two contracts and the third contract between their partners. If Propose-Accept were used to collect three signatures on a multi-party agreement, unnecessary restrictions would be put on the order of consensus, and a number of additional contract templates would be needed as intermediate steps. Both solutions are suboptimal.

Following the Multiple Party Agreement pattern, it is easy to write an agreement contract with multiple signatories and have each party accept explicitly.

The final agreement contract has multiple signatories:

template Agreement
  with
    signatories: [Party]
  where
    signatory signatories
    ensure unique signatories

The Pending contract collects signatures one by one. It is observable by all required signatories, so each can see when it is their turn to sign:

toSign : Pending -> [Party]
toSign Pending { alreadySigned, finalContract } =
  filter (`notElem` alreadySigned) finalContract.signatories

template Pending
  with
    finalContract: Agreement
    alreadySigned: [Party]
  where
    signatory alreadySigned
    observer finalContract.signatories
    ensure unique alreadySigned

    choice Sign : ContractId Pending with
        signer : Party
      controller signer
        do
          assert (signer `elem` toSign this)
          create this with alreadySigned = signer :: alreadySigned

    choice Finalize : ContractId Agreement with
        signer : Party
      controller signer
        do
          assert (sort alreadySigned == sort finalContract.signatories)
          create finalContract

One party kicks off the workflow by creating a Pending contract listing only themselves as signed. The others sign in any order, and once complete, any signatory can finalize:

-- Any party can kick off by creating a Pending listing only themselves
pending <- person1 `submit` do
  createCmd Pending with finalContract; alreadySigned = [person1]

-- Each party signs
pending <- person2 `submit` do exerciseCmd pending Sign with signer = person2
pending <- person3 `submit` do exerciseCmd pending Sign with signer = person3
pending <- person4 `submit` do exerciseCmd pending Sign with signer = person4

-- Once all have signed, any signatory can finalize
person1 `submit` do exerciseCmd pending Finalize with signer = person1