A first example¶
This tutorial introduces a simple financial contract in pseudocode, before explaining how it is modified to work in Marlowe, giving the first example of a Marlowe contract.
A simple escrow contract¶
alice wants to buy a cat from
bob, but neither of
them trusts the other. Fortunately, they have a mutual friend
whom they both trust to be neutral (but not enough to give her the money
and act as an intermediary). They therefore agree on the following
contract, written using simple functional pseudocode. This kind of
contract is a simple example of escrow.
When aliceChoice (When bobChoice (If (aliceChosen `ValueEQ` bobChosen) agreement arbitrate))
The contract is described using the constructors of a Haskell data
type. The outermost constructor
When has two arguments: the first is
an observation and the second is another contract. The intended
meaning of this is that when the action happens, the second contract
The second contract is itself another
When – asking for a decision
bob – but inside that, there is a choice:
bob agree on what to do, it is done; if not,
carol is asked
to arbitrate and make a decision.
When offers a list of cases, 1 each with an action
and a corresponding contract that is triggered when that action happens.
Using this we can allow for the option of
bob making the first
choice, rather than
alice, like this:
When [ Case aliceChoice (When [ Case bobChoice (If (aliceChosen `ValueEQ` bobChosen) agreement arbitrate) ], Case bobChoice (When [ Case aliceChoice (If (aliceChosen `ValueEQ` bobChosen) agreement arbitrate) ] ]
In this contract, either Alice or Bob can make the first choice; the
other then makes a choice. If they agree, then that is done; if not,
Carol arbitrates. In the remainder of the tutorial we’ll revert to the
simpler version where
alice chooses first.
Think about executing this contract in practice. Suppose that Alice has already committed some money to the contract. What will happen if Bob chooses not to participate any further?
We have assumed that Alice has already committed her payment, but suppose that we want to design a contract to ensure that: what would we need to do to?
Escrow in Marlowe¶
Marlowe contracts incorporate extra constructs to ensure that they
progress properly. Each time we see a
When, we need to provide two
a timeout after which the contract will progress, and
the continuation contract to which it progresses.
First, let us examine how to modify what we have written to take care of
the case that the condition of the
When never becomes true. So, we
add timeout and continuation values to each
When occurring in the
When [ Case aliceChoice (When [ Case bobChoice (If (aliceChosen `ValueEQ` bobChosen) agreement arbitrate) ] 60 -- ADDED arbitrate) -- ADDED ] 40 -- ADDED Close -- ADDED
When calls for the first choice to be made by Alice:
if Alice has not made a choice by slot
40, the contract is closed
and all the funds in the contract are refunded.
Close is typically the last step in every “path” through a Marlowe
contract, and its effect is to refund the money in the contract to the
participants; we will describe this in more detail when we look at
Marlowe step by step
in a later tutorial. In this particular case, refund will happen at slot
Looking at the inner constructs, if Alice’s choice has been made, then
we wait for one from Bob. If that is not forthcoming by slot
then Carol is called upon to arbitrate. 2
Next, we should look at how cash is committed as the first step of the contract.
When [Case (Deposit "alice" "alice" ada price) -- ADDED (When [ Case aliceChoice (When [ Case bobChoice (If (aliceChosen `ValueEQ` bobChosen) agreement arbitrate) ] 60 arbitrate) ] 40 Close) ] 10 -- ADDED Close -- ADDED
A deposit of
price is requested from
"alice": if it is given,
then it is held in an account, also called
"alice". Accounts like
this exist for the life of the contract only; each account belongs to a
There is a timeout at slot number
10 on making the deposit; if that
is reached without a deposit being made, the contract is closed and all
the money already in the contract is refunded. In this case, that is
simply the end of the contract.
We will see later that parts
of this contract description, such as
price, use the Haskell embedding of Marlowe DSL to give some
shorthand definitions. We also use overloaded strings to make some
descriptions – e.g. of accounts – more concise.
These are discussed in more detail when we look at Marlowe embedded in Haskell.
Comment on the choice of timeout values, and look at alternatives.
For example, what would happen if the timeout of
Whenwere to be replaced by
60, and vice versa? Would it be sensible to have the same timeout, of
100say, on each
When? If not, why not?
This example has shown many of the ingredients of the Marlowe contract language; in the next tutorial we will present the language in full.
While the names of Alice, Bob and so on are “hard wired” into the contract here, we will see later on that these can be represented by roles in an account, such as buyer and seller. These roles can then be associated with specific participants when a contract is run; we discuss this further in the next section.
These papers cover the original work on using functional programming to describe financial contracts.