Sample DAML Contract Template

User.hs

module User where

template User with
    username: Party
    following: [Party]
  where
    signatory username
    observer following

    agreement
      "I hereby agree that by signing this contract I become a user and I can authorize other users to follow me."

    key username: Party
    maintainer key

    nonconsuming choice Follow: ContractId User with
        userToFollow: Party
      controller username
      do
        assertMsg "You cannot follow yourself" (userToFollow /= username)
        assertMsg "You cannot follow the same user twice" (notElem userToFollow following)
        archive self
        create this with following = userToFollow :: following