Choices

-- Choices.daml
module Choices where


template Token
  with
    issuer  : Party
    owner   : Party
    amount  : Decimal
  where
    signatory issuer
    observer  owner
    ensure amount > 0.0

    -- ── Consuming choice (default) ───────────────────────
    -- Archives this contract, returns a new ContractId.
    -- Only `owner` can exercise this.
    choice Transfer : ContractId Token
      with
        newOwner : Party
      controller owner
      do
        -- `this` is the current contract record.
        -- `create this with field = value` makes a modified copy.
        create this with owner = newOwner

    -- ── Nonconsuming choice ──────────────────────────────
    -- Contract survives. Can be exercised multiple times.
    nonconsuming choice GetAmount : Decimal
      controller owner
      do
        return amount

    -- ── Choice with multiple controllers ────────────────
    -- ALL listed parties must authorize.
    choice MutualBurn : ()
      controller [issuer, owner]
      do
        return ()

    -- ── Choice with arguments and assertions ─────────────
    choice Split : (ContractId Token, ContractId Token)
      with
        splitAmount : Decimal
      controller owner
      do
        assert (splitAmount > 0.0)
        assert (splitAmount < amount)
        part1 <- create this with amount = splitAmount
        part2 <- create this with amount = amount - splitAmount
        return (part1, part2)

    -- ── Issuer can archive independently ─────────────────
    choice Revoke : ()
      controller issuer
      do
        return ()

Key points

Choice structure

• A choice name must start with a capital letter and must be unique across the entire project (not just the template).
• The choice body is a do block of type Update a, where a is the declared return type.
• this refers to the current contract's field values. self is the ContractId of the current contract.
• The do block can contain: create, exercise, fetch, archive, assert, assertMsg, getTime, return, and let.

Consuming vs nonconsuming

• nonconsuming allows a choice to be exercised repeatedly without archiving the contract.

Controller rules

• Multiple controllers form a conjunction: all must authorize. To require any one of several parties, use separate choices.