# Writing a contract using clauses

This tutorial will take you through restructuring the commercial paper contract to use clauses. You should have already completed “Writing a contract”.

Clauses are essentially micro-contracts which contain independent verification logic, and can be logically composed together to form a contract. Clauses are designed to enable re-use of common logic, for example issuing state objects is generally the same for all fungible contracts, so a common issuance clause can be inherited for each contract’s issue clause. This cuts down on scope for error, and improves consistency of behaviour. By splitting verification logic into smaller chunks, they can also be readily tested in isolation.

Clauses can be composed of subclauses, for example the `AllClause` or `AnyClause` clauses take list of clauses that they delegate to. Clauses can also change the scope of states and commands being verified, for example grouping together fungible state objects and running a clause against each distinct group.

The commercial paper contract has a `Group` outermost clause, which contains the `Issue`, `Move` and `Redeem` clauses. The result is a contract that looks something like this:

> 1. Group input and output states together, and then apply the following clauses on each group:
>      1. If an `Issue` command is present, run appropriate tests and end processing this group.
>      2. If a `Move` command is present, run appropriate tests and end processing this group.
>      3. If a `Redeem` command is present, run appropriate tests and end processing this group.

## Commercial paper class

To use the clause verification logic, the contract needs to call the `verifyClause` function, passing in the transaction, a clause to verify, and a collection of commands the clauses are expected to handle all of. This list of commands is important because `verifyClause` checks that none of the commands are left unprocessed at the end, and raises an error if they are. The top level clause would normally be a composite clause (such as `AnyComposition`, `AllComposition`, etc.) which contains further clauses. The following examples are trimmed to the modified class definition and added elements, for brevity:

    class CommercialPaper : Contract {
        override val legalContractReference: SecureHash = SecureHash.sha256("https://en.wikipedia.org/wiki/Commercial_paper")

        override fun verify(tx: TransactionForContract) = verifyClause(tx, Clauses.Group(), tx.commands.select<Commands>())

    public class CommercialPaper implements Contract {
        @Override
        public SecureHash getLegalContractReference() {
            return SecureHash.Companion.sha256("https://en.wikipedia.org/wiki/Commercial_paper");
        }

        @Override
        public void verify(@NotNull TransactionForContract tx) throws IllegalArgumentException {
            ClauseVerifier.verifyClause(tx, new Clauses.Group(), extractCommands(tx));
        }

## Clauses

We’ll tackle the inner clauses that contain the bulk of the verification logic, first, and the clause which handles grouping of input/output states later. The clauses must implement the `Clause` interface, which defines the `verify` function, and the `requiredCommands` property used to determine the conditions under which a clause is triggered. Normally clauses would extend `ConcreteClause` which provides defaults suitable for a clause which verifies transactions, rather than delegating to other clauses.

The `verify` function defined in the `Clause` interface is similar to the conventional `Contract` verification function, although it adds new parameters and returns the set of commands which it has processed. Normally this returned set is identical to the `requiredCommands` used to trigger the clause, however in some cases the clause may process further optional commands which it needs to report that it has handled.

The `Move` clause for the commercial paper contract is relatively simple, so we will start there:

    class Move: ConcreteClause<State, Commands, Issued<Terms>>() {
        override val requiredCommands: Set<Class<out CommandData>>
            get() = setOf(Commands.Move::class.java)

        override fun verify(tx: TransactionForContract,
                            inputs: List<State>,
                            outputs: List<State>,
                            commands: List<AuthenticatedObject<Commands>>,
                            groupingKey: Issued<Terms>?): Set<Commands> {
            val command = commands.requireSingleCommand<Commands.Move>()
            val input = inputs.single()
            requireThat {
                "the transaction is signed by the owner of the CP" by (input.owner in command.signers)
                "the state is propagated" by (outputs.size == 1)
                // Don't need to check anything else, as if outputs.size == 1 then the output is equal to
                // the input ignoring the owner field due to the grouping.
            }
            return setOf(command.value)
        }
    }

    class Move extends ConcreteClause<State, Commands, State> {
        @NotNull
        @Override
        public Set<Class<? extends CommandData>> getRequiredCommands() {
            return Collections.singleton(Commands.Move.class);
        }

        @NotNull
        @Override
        public Set<Commands> verify(@NotNull TransactionForContract tx,
                                       @NotNull List<? extends State> inputs,
                                       @NotNull List<? extends State> outputs,
                                       @NotNull List<? extends AuthenticatedObject<? extends Commands>> commands,
                                       @NotNull State groupingKey) {
            AuthenticatedObject<Commands.Move> cmd = requireSingleCommand(tx.getCommands(), Commands.Move.class);
            // There should be only a single input due to aggregation above
            State input = single(inputs);

            if (!cmd.getSigners().contains(input.getOwner()))
                throw new IllegalStateException("Failed requirement: the transaction is signed by the owner of the CP");

            // Check the output CP state is the same as the input state, ignoring the owner field.
            if (outputs.size() != 1) {
                throw new IllegalStateException("the state is propagated");
            }
            // Don't need to check anything else, as if outputs.size == 1 then the output is equal to
            // the input ignoring the owner field due to the grouping.
            return Collections.singleton(cmd.getValue());
        }
    }

## Group Clause

We need to wrap the move clause (as well as the issue and redeem clauses - see the relevant contract code for their full specifications) in an outer clause that understands how to group contract states and objects. For this we extend the standard `GroupClauseVerifier` and specify how to group input/output states, as well as the top-level to run on each group. As with the top level clause on a contract, this is normally a composite clause that delegates to subclauses.

    class Group : GroupClauseVerifier<State, Commands, Issued<Terms>>(
        AnyComposition(
            Redeem(),
            Move(),
            Issue())) {
        override fun groupStates(tx: TransactionForContract): List<TransactionForContract.InOutGroup<State, Issued<Terms>>>
                = tx.groupStates<State, Issued<Terms>> { it.token }
    }

    class Group extends GroupClauseVerifier<State, Commands, State> {
        public Group() {
            super(new AnyComposition<>(
                new Clauses.Redeem(),
                new Clauses.Move(),
                new Clauses.Issue()
            ));
        }

        @NotNull
        @Override
        public List<InOutGroup<State, State>> groupStates(@NotNull TransactionForContract tx) {
            return tx.groupStates(State.class, State::withoutOwner);
        }
    }

For the `CommercialPaper` contract, this is the top level clause for the contract, and is passed directly into `verifyClause` (see the example code at the top of this tutorial).

## Summary

In summary the top level contract `CommercialPaper` specifies a single grouping clause of type `CommercialPaper.Clauses.Group` which in turn specifies `GroupClause` implementations for each type of command (`Redeem`, `Move` and `Issue`). This reflects the flow of verification: In order to verify a `CommercialPaper` we first group states, check which commands are specified, and run command-specific verification logic accordingly.

## Debugging

Debugging clauses which have been composed together can be complicated due to the difficulty in knowing which clauses have been matched, whether specific clauses failed to match or passed verification, etc. There is “trace” level logging code in the clause verifier which evaluates which clauses will be matched and logs them, before actually performing the validation. To enable this, ensure trace level logging is enabled on the `Clause` interface.