Your First Script

-- FirstScript.daml
module FirstScript where

import Daml.Script

-- The template under test
template Asset
  with
    issuer : Party
    owner  : Party
    name   : Text
  where
    signatory issuer
    observer  owner

    choice Transfer : ContractId Asset
      with newOwner : Party
      controller owner
      do create this with owner = newOwner


-- A Script is just a value of type Script ().
-- `do` sequences Script actions top-to-bottom.
assetTest : Script ()
assetTest = do
  -- 1. Allocate parties.
  --    allocateParty creates a party on the sandbox ledger.
  alice <- allocateParty "Alice"
  bob   <- allocateParty "Bob"

  -- 2. Alice creates an asset.
  --    `submit party do` submits a transaction as `party`.
  --    `createCmd T with ...` is the create command.
  assetId <- submit alice do
    createCmd Asset with
      issuer = alice
      owner  = alice
      name   = "Gold"

  -- 3. Alice transfers the asset to Bob.
  --    `exerciseCmd contractId ChoiceName with ...` is the exercise command.
  assetId2 <- submit alice do
    exerciseCmd assetId Transfer with newOwner = bob

  -- 4. Query the ledger to verify state.
  --    queryContractId returns Optional T.
  Some asset <- queryContractId bob assetId2
  assert (asset.owner == bob)
  assert (asset.name  == "Gold")

  -- 5. Confirm the original contract is archived.
  original <- queryContractId alice assetId
  assert (original == None)

Key points

Parties

• allocateParty "Name" : Script Party creates a party on the sandbox. Party names are strings for human readability.

Transactions

• submit party do createCmd T with ... is a transaction: party creates contract T.
• createCmd and exerciseCmd are the Script variants of create and exercise. They are only valid inside a submit block.

Queries

• queryContractId party cid : Script (Optional T) reads the party's view of the ledger. A party can only query contracts visible to them.

Execution model

• Scripts run sequentially. Each submit is committed before the next line executes.
• The Script monad is separate from the Update monad. You cannot use Update functions like create directly inside do at the Script level: they must be inside a submit.