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The semantics of Marlowe 2.0

This tutorial gives a high-level formal semantics of Marlowe 2.0 by describing the types of inputs supported, and the main semantic functions.

In general, participants in a Marlowe contract communicate with the contract by issuing transactions. At each block, a sequence of zero or more transactions can be processed – in sequence – by a contract.

From the point of view of the semantics, a transaction consists of a list of inputs, and the whole transaction may be signed by one or more participants. Concretely, we represent a transaction as a list of AnyInputs together with a set of integers representing the set of signatories.

As we mentioned in the previous tutorial, inputs can have one out of four kinds: Commit, Pay, Choice, and Oracle. Of these, Commit and Pay are considered to be actions and are typically associated with the transfer of money between a participant and the contract. Choice and Oracle are used to provide information from the external world to the contract.

The applyTransaction function

The effect of executing a transaction in a Marlowe contract is modelled by the top-level function applyTransaction. This takes the transaction to process and the current state of the contract (including the internal state, the remaining contract, and the amount of funds remaining in the contract), and it returns the new state of the contract together with the amount of money that each participant is supposed to transfer to or from the contract as part of the transaction.

The applyTransaction function has the following Haskell type:

applyTransaction :: [AnyInput] -> S.Set Person -> BlockNumber -> State -> Contract -> Integer
                    -> MApplicationResult (Integer, TransactionOutcomes, State, Contract)

In turn, the applyTransaction function calls three principal auxiliary functions: reduce, fetchPrimitive, and eval.

  • The reduce function (and its auxiliary function reduceRec) are applied before and after each input,
  • fetchPrimitive is applied only for inputs that are actions (i.e: Commits and Pays), and
  • eval is applied to the result of fetchPrimitive whenever appropriate.

In addition to these three functions, there are three additional functions that are applied in every transaction. Before processing the inputs, the function expireCommits is applied, and after processing the inputs the functions redeemMoney and simplify are applied.

Action processing

Let us start by looking at the parts of the processing that the semantics does exclusively for actions (that is, inputs of the kind Commit and Pay).

An action is represented by its IdAction. The function fetchPrimitive will traverse the parts of the contract that are currently in force (activated) and look for a primitive that matches the IdAction provided by the input. fetchPrimitive will only succeed if there is exactly one primitive that corresponds to the given IdAction.

fetchPrimitive idAction blockNum state (Both leftContract rightContract) =
  case (go leftContract, go rightContract) of
     (Picked (result, cont), NoMatch) -> Picked (result, (Both cont rightContract))
     (NoMatch, Picked (result, cont)) -> Picked (result, (Both leftContract cont))
     (NoMatch, NoMatch) -> NoMatch
     _                  -> MultipleMatches
  where go = fetchPrimitive idAction blockNum state

If it succeeds, fetchPrimitive will return a DetachedPrimitive:

data DetachedPrimitive = DCommit IdCommit Person Integer Timeout
                       | DPay IdCommit Person Integer

fetchPrimitive will also return the contract after removing the chosen primitive; this new contract will be taken as the remaining contract if the evaluation succeeds. The DetachedPrimitive is passed to eval, together with the current State of the contract.

fetchPrimitive :: IdAction -> BlockNumber -> State -> Contract
               -> FetchResult (DetachedPrimitive, Contract)

Commit

A Commit allows a participant person to transfer to the contract an amount of money value. This money will be recorded in the contract under the identifier idCommit and future payments can use this identifier as a source of money. Once the block specified by timeout (the second Timeout in Commit) is reached, any money from the Commit that has not been spent (through payments) will be frozen, and the participant that performed the commitment will be able to withdraw it with the next transaction that he or she signs.

eval (DCommit idCommit person value timeout) state =
  if (isCurrentCommit idCommit state) || (isExpiredCommit idCommit state)
  then InconsistentState
  else Result ( addOutcome person (- value) emptyOutcome
              , addCommit idCommit person value timeout state )
              NoProblem

For a commitment to be valid, no commitment with the identifier idCommit must have been issued before (only one Commit per identifier is allowed).

If the Commit does not timeout and is issued correctly, the remaining contract, as returned by the fetchPrimitive function, will use the first continuation (the first Contract in Commit).

fetchPrimitive idAction blockNum state (Commit idActionC idCommit person value _ timeout continuation _) =
  if (idAction == idActionC && notCurrentCommit && notExpiredCommit)
  then Picked ((DCommit idCommit person actualValue timeout),
               continuation)
  else NoMatch
  where notCurrentCommit = not (isCurrentCommit idCommit state)
        notExpiredCommit = not (isExpiredCommit idCommit state)
        actualValue = evalValue blockNum state value

On the other hand, if any of the timeouts of the Commit are reached, reduceRec (the auxiliary function of reduce) specifies that a Commit will be reduced to its second continuation.

reduceRec blockNum state env [email protected](Commit _ _ _ _ timeout_start timeout_end _ continuation) =
  if (isExpired blockNum timeout_start) || (isExpired blockNum timeout_end)
  then reduceRec blockNum state env continuation
  else c

Pay

A Pay allows a participant person to claim from the contract an amount of money value. This money will be discounted from the money committed under the identifier idCommit, and the contract will only pay up to the amount that remains available under idCommit, even if there is extra money available in the contract. In any case, Pay will pay no money if the commit idCommit has expired.

eval (DPay idCommit person value) state =
  if (not $ isCurrentCommit idCommit state)
  then (if (not $ isExpiredCommit idCommit state)
        then Result (emptyOutcome, state) CommitNotMade
        else Result (emptyOutcome, state) CommitIsExpired)
  else (if value > maxValue
        then Result ( addOutcome person maxValue emptyOutcome
                    , fromJust $ discountAvailableMoneyFromCommit idCommit maxValue state )
                    NotEnoughMoneyLeftInCommit
        else Result ( addOutcome person value emptyOutcome
                    , fromJust $ discountAvailableMoneyFromCommit idCommit value state )
                    NoProblem)
  where maxValue = getAvailableAmountInCommit idCommit state

If the Pay does not timeout and is claimed correctly, the remaining contract, as returned by the fetchPrimitive function, will use the first continuation (the first Contract in Pay).

fetchPrimitive idAction blockNum state (Pay idActionC idCommit person value _ continuation _) =
  if (idAction == idActionC)
  then Picked ((DPay idCommit person actualValue), continuation)
  else NoMatch
  where actualValue = evalValue blockNum state value

On the other hand, if the timeouts of the Pay is reached, reduceRec (the auxiliary function of reduce) specifies that a Pay will be reduced to its second continuation.

reduceRec blockNum state env [email protected](Pay _ _ _ _ timeout _ continuation) =
  if isExpired blockNum timeout
  then reduceRec blockNum state env continuation
  else c

Non-action input processing

Choices and Oracles inputs are processed very differently to actions. They are relatively independent of the state of the contract, and they may be issued at any time, as long as the values provided can potentially be used by the contract. In other words, there must be somewhere in the code of the contract that inspects the Choice or Oracle value in order for a participant to be able to provide that value. Otherwise, the contract does not need to know the value, and providing it anyway would just be adding clutter and load to the contract and blockchain, which could end up translating into problems like DoS. For these reasons, the Marlowe 2.0 semantics disallows providing information that is not required.

Other than that, the only thing that Marlowe does when provided with Choices and Oracles is to record them in the state so that the reduce function can access them.

Combinators and Null

In this section, we describe the semantics of the remaining Marlowe contracts, which can be used to combine other contracts together and to decide between them, depending on the information known to the contract at any given moment. The semantics of these combinators are mainly defined by reduceRec (the auxiliary function of reduce). However, their behaviour also affects other functions, in particular fetchPrimitive and simplify.

For example, the activation rules of each construct are reflected in fetchPrimitive, that is, if the construct immediately activates its subcontracts that is translated in that fetchPrimitive will be able to recursively examine some of its subcontracts. This is the case of the second subcontract of Let:

fetchPrimitive idAction blockNum state (Let label boundContract subContract) =
  case fetchPrimitive idAction blockNum state subContract of
     Picked (result, cont) -> Picked (result, Let label boundContract cont)
     NoMatch -> NoMatch
     MultipleMatches -> MultipleMatches

Whereas, in the case of other constructs like When, fetchPrimitive will just leave the whole construct unchanged:

fetchPrimitive _ _ _ _ = NoMatch

Null

The Null contract does nothing and stays quiescent forever.

reduceRec _ _ _ Null = Null

Nevertheless, it is used by the simplify function and it can be used to replace a contract by a smaller but equivalent one. For example, Both Null Null can be reduced to Null.

Both

The Both construct allows two contracts to be active simultaneously. It would be like having two separate contracts deployed simultaneously, except in that when using Both they will share State, and thus Commits made in one of the contracts can be used for Pays in the other contract. We have also taken a lot of care in ensuring that Both is symmetric, that is, writing Both A B should be equivalent to writing Both B A, no matter what A and B are.

reduceRec blockNum state env (Both cont1 cont2) = Both (go cont1) (go cont2)
  where go = reduceRec blockNum state env

Choice

The Choice construct immediately chooses between two contracts depending on the value of an Observation. The moment that Choice is activated, the value of obs will decide whether Choice gets reduced to cont1 (if it is true) or to cont2 (if it is false).

reduceRec blockNum state env (Choice obs cont1 cont2) =
  reduceRec blockNum state env (if (evalObservation blockNum state obs)
                                then cont1
                                else cont2)

When

The When construct delays a contract in time until an Observation becomes true. When will not activate any of its subcontracts until either the Observation becomes true, or until the timeout block is reached. If obs is true, then When is reduced to cont1, if timeout has been reached, then When is reduced to cont2.

reduceRec blockNum state env [email protected](When obs timeout cont1 cont2) =
  if isExpired blockNum timeout
  then go cont2
  else if evalObservation blockNum state obs
       then go cont1
       else c
  where go = reduceRec blockNum state env

It is worth noting that, because Marlowe follows a ‘pull’ model, it is not just enough for the Observation to become true for When to evolve; the contract needs to be triggered while the Observation is true. The contract can be triggered at any time by issuing a transaction that does not need to have any inputs and does not need to be signed; indeed anyone can trigger the contract.

While

The While construct works in the opposite way to When, in the sense that while When gets reduced when it Observation becomes true, While gets reduced when its Observation becomes false.

reduceRec blockNum state env (While obs timeout contractWhile contractAfter) =
  if isExpired timeout blockNum
  then go contractAfter
  else if evalObservation blockNum state obs
       then (While obs timeout (go contractWhile) contractAfter)
       else go contractAfter
  where go = reduceRec blockNum state env

However, there is one fundamental difference: When does not activate its subcontract until it gets reduced, and While activates its subcontract immediately, similarly to the behaviour of Both. And there is something unique about While: it may delete a contract that has already been activated once the Observation becomes true. For example, in the following contract:

While 
  (NotObs 
     (ChoseSomething (1, 1))) 20 
  (Commit 1 1 1 
     (ValueFromChoice (1, 1) 
        (Constant 20)) 10 20 Null Null) Null

once the choice (1, 1) is made, it will no longer be possible to use the Commit.

Scale

The Scale construct scales the amounts paid by Commit and Pay. It takes three Values, the first one is the numerator, the second one is the denominator, and the third one is the default.

As soon as the Scale construct is activated, it activates its subcontract contract, and it evaluates all the three Values and replaces them with a Constant (so that they may not change any more).

reduceRec blockNum state env (Scale divid divis def contract) =
  Scale (Constant vsDivid) (Constant vsDivis) (Constant vsDef) (go contract)
  where go = reduceRec blockNum state env
        vsDivid = evalValue blockNum state divid
        vsDivis = evalValue blockNum state divis
        vsDef = evalValue blockNum state def

Once evaluated, any inner Commit or Pay (in contract) will get their amount scaled as follows:

  • If the divisor is 0, then the amount is replace with the default.
  • If the divisor is not 0, then the amount is multiplied by the numerator, and divided (using integer division) by the denominator.
scaleValue :: Integer -> Integer -> Integer -> Integer -> Integer
scaleValue divid divis def val = if (divis == 0) then def else ((val * divid) `div` divis)

The process of scaling Commits and Pays is carried out by the fetchPrimitive function.

fetchPrimitive idAction blockNum state (Scale divid divis def subContract) =
  case fetchPrimitive idAction blockNum state subContract of
     Picked (result, cont) -> Picked (scaleResult sDivid sDivis sDef result,
                                      Scale divid divis def cont)
     NoMatch -> NoMatch
     MultipleMatches -> MultipleMatches
  where sDivid = evalValue blockNum state divid
        sDivis = evalValue blockNum state divis
        sDef = evalValue blockNum state def

Once there are no Commits or Pays inside a Scale, it gets removed by the simplify function.

Let and Use

The Let construct binds its first subcontract boundContract to an identifier label.

reduceRec blockNum state env (Let label boundContract contract) =
  case lookupEnvironment label env of
    Nothing -> let newEnv = addToEnvironment label checkedBoundContract env in
               Let label checkedBoundContract $ reduceRec blockNum state newEnv contract
    Just _ -> let freshLabel = getFreshLabel env contract in
              let newEnv = addToEnvironment freshLabel checkedBoundContract env in
              let fixedContract = relabel label freshLabel contract in
              Let freshLabel checkedBoundContract $ reduceRec blockNum state newEnv fixedContract
  where checkedBoundContract = nullifyInvalidUses env boundContract

We think that every Let should have a different identifier label, because reusing them leads to confusion, confusion leads to errors, and errors lead to the dark… hem… I mean… to potential loss of money.

However, we have written Marlowe semantics so that the most recent occurrence of a Let (the innermost) has priority in case of conflict (this is commonly known as shadowing). The way this is implemented is that whenever there is a conflict of identifiers, the newly defined identifier gets renamed to a fresh identifier that has not been used yet (in the current subtree), that is what the relabel function does.

Inside the second continuation (contract) of a Let may use the Use construct to represent a copy of boundContract that will only be expanded when the contract Use is activated:

reduceRec blockNum state env (Use label) =
  case lookupEnvironment label env of
    Just contract -> reduceRec blockNum state env contract
    Nothing -> Null

Note that if Use is not defined it will expand to Null.

Analogously to Scale, Let construct will only get reduced when there are no corresponding Use constructs that use it, this clean-up procedure is carried out by simplify function.

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