Canton Daml Design Patterns

Overview

Beyond single templates, real Canton apps compose multi-step transactions and use standard multi-party patterns. All Daml transactions are atomic (all-or-nothing across all participants), which is what makes safe swaps and settlements possible.

Atomic composition

A choice can compose several actions; thanks to atomicity neither party is left out of pocket. A swap settles two transfers in one transaction:

choice Trade_Settle : (ContractId Asset, ContractId Asset)
  with
    quoteAssetCid : ContractId Asset
    baseApprovalCid : ContractId TransferApproval
  controller buyer
  do
    -- exercise transfers both ways; either the whole thing commits or none of it
    ...

The requester sees all consequences of their submission (no surprises), but must therefore have the information to interpret them — which is why counterparties disclose the needed contracts first.