Canton for Blockchain Developers

// Solidity: Mutable state
contract Token {
    mapping(address => uint256) public balances;
    
    function transfer(address to, uint256 amount) public {
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}

-- Daml: Immutable contracts with choices
template Token
  with
    owner : Party
    issuer : Party
    amount : Decimal
  where
    signatory issuer
    observer owner
    
    choice Transfer : ContractId Token
      with
        newOwner : Party
      controller owner
      do
        create this with owner = newOwner

template Asset
  with
    owner : Party
    issuer : Party
    auditor : Party
  where
    signatory issuer        -- Must authorize creation; always sees contract
    observer owner, auditor -- Can see but cannot act unless also controller
    
    choice Transfer : ContractId Asset
      with
        newOwner : Party
      controller owner      -- Only owner can exercise this choice
      do
        create this with owner = newOwner

    choice Transfer : ContractId Asset
      controller owner      -- Only owner can transfer
      do
        create this with owner = newOwner

    choice Audit : AuditReport
      controller auditor    -- Only auditor can audit
      do
        return AuditReport with ...

// Solidity: Manual signature collection
contract MultiSig {
    mapping(address => bool) public approved;
    uint256 public approvalCount;
    
    function approve() public {
        require(!approved[msg.sender], "Already approved");
        approved[msg.sender] = true;
        approvalCount++;
    }
    
    function execute() public {
        require(approvalCount >= 2, "Need 2 approvals");
        // ... execute action
    }
}

-- Daml: Native multi-party agreement
template Agreement
  with
    partyA : Party
    partyB : Party
    terms : Text
  where
    signatory partyA, partyB  -- Both must agree to create
    
    choice Execute : ()
      controller partyA, partyB  -- Both must agree to execute
      do
        -- ... execute action
        return ()