Marlowe step by step¶
Marlowe has six ways of building contracts. Five of these –
Assert – build a complex contract from
simpler contracts, and the sixth,
Close, is a simple contract. At
each step of execution, as well as returning a new state and
continuation contract, it is possible that effects – payments – and
warnings can be generated too.
In explaining these contracts we will also explain Marlowe values, observations and actions, which are used to supply external information and inputs to a running contract to control how it will evolve.
A payment contract
Pay a p t v cont will make a payment of value
v of token
t from the account
a to a payee
p, which will
be one of the contract participants or another account in the contract.
Warnings will be generated if the value
v is negative, or if there
is not enough in the account to make the payment in full (even if there
are positive balances of other tokens in the account). In the latter
case a partial payment (of all the money available) is made. The
continuation contract is the one given in the contract:
Close provides for the contract to be closed (or
terminated). The only action that is performs is to provide refunds to
the owners of accounts that contain a positive balance. This is
performed one account per step, but all accounts will be refunded in a
Before discussing other forms of contracts, we need to describe values, observations and actions.
Values, observations and actions¶
Values include some quantities that change with time, including “the current slot number”, 1 “the current balance of some token in an account”, and any choices that have already been made; we call these volatile values. Values can also be combined using addition, subtraction and negation, and can be conditional on an observation.
Observations are Boolean values derived by comparing values, and can be combined using the standard Boolean operators. It is also possible to observe whether any choice has been made (for a particular identified choice).
Observations will have a value at every step of execution. On the other hand, actions happen at particular points during execution. As noted earlier, actions can be
making a choice between various alternatives, including an oracle value (see next section), or
notifying an external value of some kind.
Oracles are being developed for the Cardano blockchain in general, and will be available for use within Marlowe on Cardano. In the meantime we have introduced an oracle prototype, which is implemented in the Marlowe Playground.
We model Oracles as choices that made by a participant with a specific Oracle role, “kraken”.
If a role in a contract is
"kraken", and that role makes a choice
"dir-adausd" then, in the Playground simulation, this choice
will be pre-filled, based on data from Cryptowat.ch, with the current
value of the direct ADA/USD conversion rate. You can find all supported
currency pairs here https://api.cryptowat.ch/markets/kraken
It is also possible to obtain the inverse rates of currency pairs listed
by adding the prefix
inv- instead. For example,
return the value of the USD/ADA conversion rate.
Note, that we support only whole numbers as choice inputs. How then do
we use current ADA/USD price, which might be $0.098924? We simply multiply the
price by 108, so the price would appear as 9892400. You can
DivValue the value after doing your calculations.
For example, you’d like to buy USDT for 12 ADA, using Oracle price.
Get the price:
When [Choice (ChoiceId "dir-adausdt" (Role "kraken") [Bound 1000000 10000000] ...
Calculate USDT amount
Let (ValueId "amount of USDT in microcents") (MulValue (Constant 12) (ChoiceValue (ChoiceId "dir-adausdt" (Role "kraken"))))
Scale the result down by 106 to get amount in USDT cents.
DivValue (UseValue (ValueId "amount of USDT in microcents")) (Constant 1000000)
If obs cont1 cont2 will continue as
cont2, depending on the Boolean value of the observation
when this construct is executed.
This is the most complex constructor for contracts, with the form
When cases timeout cont. It is a contract that is triggered on
actions, which may or may not happen at any particular slot: what
happens when various actions happen is described by the cases in the
In the contract
When cases timeout cont, the list
a collection of cases. Each case has the form
Case ac co where
ac is an action and
co a continuation (another contract). When a
particular action, e.g.
ac, happens, the state is updated
accordingly and the contract will continue as the corresponding
In order to make sure that the contract makes progress eventually, the
When cases timeout cont will continue as
cont once the
timeout, a slot number, is reached.
A let contract
Let id val cont allows a contract to name a value
using an identifier. In this case, the expression
val is evaluated,
and stored with the name
id. The contract then continues as
As well as allowing us to use abbreviations, this mechanism also means that we can capture and save volatile values that might be changing with time, e.g. the current price of oil, or the current slot number, at a particular point in the execution of the contract, to be used later on in contract execution.
An assert contract
Assert obs cont does not have any effect on the
state of the contract, it immediately continues as
cont, but it
issues a warning when the observation
obs is false. It can be used
to ensure that a property holds in any given point of the contract,
since static analysis will fail if any execution causes an assert to be
The presentation here is a simplification of the concrete implementation, in which transactions are associated with a slot interval during which it is valid to add them to the blockchain.The reason for this is that in general it is difficult to predict the precise slot in which a transaction will be accepted for inclusion on the blockchain; it is therefore more robust to specify an interval in which the transaction should be accepted. The view presented here is a simplification in that effectively we consider only intervals of length one. So, a Marlowe contract is able to access the upper and lower bounds on the current slot interval, rather than the specific current slot value. Executing a contract can, in some circumstances, lead to an “ambiguous slot interval error”, but we do not cover that any further here.