5 Adding constraints to a contract¶
You will often want to constrain the data stored or the allowed data transformations in your contract models. In this section, you will learn about the two main mechanisms provided in DAML:
To make sense of the latter, you’ll also learn more about the
Scenario types and
do blocks, which will be good preparation for 7 Composing choices, where you will use
do blocks to compose choices into complex transactions.
Lastly, you will learn about time on the ledger and in scenarios.
The first kind of restriction you may want to put on the contract model are called template pre-conditions. These are simply restrictions on the data that can be stored on a contract from that template.
Suppose, for example, that the
SimpleIou contract from A simple cash model should only be able to store positive amounts. You can enforce this using the
template SimpleIou with issuer : Party owner : Party cash : Cash where signatory issuer ensure cash.amount > 0.0
ensure keyword takes a single expression of type
Bool. If you want to add more restrictions, use logical operators
not to build up expressions. The below shows the additional restriction that currencies are three capital letters:
&& T.length cash.currency == 3 && T.isUpper cash.currency
test_restrictions = scenario do alice <- getParty "Alice" bob <- getParty "Bob" dora <- getParty "Dora" -- Dora can't issue negative Ious submitMustFail dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = -100.0 currency = "USD" -- Or even zero Ious submitMustFail dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = 0.0 currency = "USD" -- Nor positive Ious with invalid currencies submitMustFail dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = 100.0 currency = "Swiss Francs" -- But positive Ious still work, of course iou <- submit dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = 100.0 currency = "USD"
A second common kind of restriction is one on data transformations.
For example, the simple Iou in A simple cash model allowed the no-op where the
owner transfers to themselves. You can prevent that using an
assert statement, which you have already encountered in the context of scenarios.
assert does not return an informative error so often it’s better to use the function
assertMsg, which takes a custom error message:
controller owner can Transfer : ContractId SimpleIou with newOwner : Party do assertMsg "newOwner cannot be equal to owner." (owner /= newOwner) create this with owner = newOwner
-- Alice can't transfer to herself submitMustFail alice do exercise iou Transfer with newOwner = alice -- but can transfer to bob iou2 <- submit alice do exercise iou Transfer with newOwner = bob
Similarly, you can write a
Redeem choice, which allows the
owner to redeem an
Iou during business hours on weekdays. The choice doesn’t do anything other than archiving the
SimpleIou. (This assumes that actual cash changes hands off-ledger.)
controller owner can Redeem : () do now <- getTime let today = toDateUTC now dow = dayOfWeek today timeofday = now `subTime` time today 0 0 0 hrs = convertRelTimeToMicroseconds timeofday / 3600000000 assertMsg ("Cannot redeem outside business hours. Current time: " <> show timeofday) (hrs >= 8 && hrs <= 18) case dow of Saturday -> abort "Cannot redeem on a Saturday." Sunday -> abort "Cannot redeem on a Sunday." _ -> return ()
-- June 1st 2019 is a Saturday passToDate (date 2019 Jun 1) -- Bob cannot redeem on a Saturday submitMustFail bob do exercise iou2 Redeem -- Not even at mid-day pass (hours 12) -- Bob cannot redeem on a Saturday submitMustFail bob do exercise iou2 Redeem -- Bob also cannot redeem at 6am on a Monday pass (hours 42) submitMustFail bob do exercise iou2 Redeem -- Bob can redeem 8am on Monday. pass (hours 2) submit bob do exercise iou2 Redeem
There are quite a few new time-related functions from the
DA.Date libraries here. Their names should be reasonably descriptive so how they work won’t be covered here, but given that DAML assumes it is run in a distributed setting, we will still discuss time in DAML.
There’s also quite a lot going on inside the
do block of the
Redeem choice, with several uses of the
do blocks and
<- deserve a proper explanation at this point.
Time on DAML ledgers¶
Each transaction on a DAML ledger has two timestamps called the ledger effective time (LET) and the record time (RT). The ledger effective time is set by the submitter of a transaction, the record time is set by the consensus protocol.
Each DAML ledger has a policy on the allowed difference between LET and RT called the skew. The submitter has to take a good guess at what the record time will be. If it’s too far off, the transaction will be rejected.
getTime is an action that gets the LET from the ledger. In the above example, that time is taken apart into day of week and hour of day using standard library functions from
DA.Time. The hour of the day is checked to be in the range from 8 to 18.
Suppose now that the ledger had a skew of 10 seconds, but a submission took less than 4 seconds to commit. At 18:00:05, Alice could submit a transaction with a LET of 17:59:59 to redeem an Iou. It would be a valid transaction and be committed successfully as
getTime will return 17:59:59 so
hrs == 17. Since RT will be before 18:00:09,
LET - RT < 10 seconds and the transaction won’t be rejected.
Time therefore has to be considered slightly fuzzy in DAML, with the fuzziness depending on the skew parameter.
Time in scenarios¶
In scenarios, record and ledger effective time are always equal. You can set them using the following functions:
passToDate, which takes a date sets the time to midnight (UTC) of that date and
pass, which takes a
Reltime(a relative time) and moves the ledger by that much
Time on ledgers¶
On a distributed DAML ledger, there are no guarantees that ledger effective time or relative time are strictly increasing. The only guarantee is that ledger effective time is increasing with causality. That is, if a transaction
TX2 depends on a transaction
TX1, then the ledger enforces that the LET of
TX2 is greater than or equal to that of
iou3 <- submit dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = 100.0 currency = "USD" pass (days (-3)) submitMustFail alice do exercise iou3 Redeem
You have come across
do blocks and
<- notations in two contexts by now:
Update. Both of these are examples of an
Action, also called a Monad in functional programming. You can construct
Actions conveniently using
do blocks is therefore crucial to being able to construct correct contract models and test them, so this section will explain them in some detail.
Pure expressions compared to Actions¶
Expressions in DAML are pure in the sense that they have no side-effects: they neither read nor modify any external state. If you know the value of all variables in scope and write an expression, you can work out the value of that expression on pen and paper.
However, the expressions you’ve seen that used the
<- notation are not like that. For example, take
getTime, which is an
Action. Here’s the example we used earlier:
getTime is a good example of an
Action. Here’s the example we used earlier
now <- getTime
You cannot work out the value of
now based on any variable in scope. To put it another way, there is no expression
expr that you could put on the right hand side of
now = expr. To get the ledger effective time, you must be in the context of a submitted transaction, and then look at that context.
Similarly, you’ve come across
fetch. If you have
cid : ContractId Account in scope and you come across the expression
fetch cid, you can’t evaluate that to an
Account so you can’t write
account = fetch cid. To do so, you’d have to have a ledger you can look that contract id up on.
Actions and impurity¶
Actions are a way to handle such “impure” expressions.
Action a is a type class with a single parameter
Scenario are instances of
Action. A value of such a type
m a where
m is an instance of
Action can be interpreted as “a recipe for an action of type
m, which, when executed, returns a value
You can always write a recipe using just pen and paper, but you can’t cook it up unless you are in the context of a kitchen with the right ingredients and utensils. When cooking the recipe you have an effect – you change the state of the kitchen – and a return value – the thing you leave the kitchen with.
Update ais “a recipe to update a DAML ledger, which, when committed, has the effect of changing the ledger, and returns a value of type
a”. An update to a DAML ledger is a transaction so equivalently, an
Update ais “a recipe to construct a transaction, which, when executed in the context of a ledger, returns a value of type
Scenario ais “a recipe for a test, which, when performed against a ledger, has the effect of changing the ledger in ways analogous to those available via the API, and returns a value of type
submit party update,
create contract and
exercise choice should make more sense in that light. For example:
getTime : Update Timeis the recipe for an empty transaction that also happens to return a value of type
pass (days 10) : Scenario ()is a recipe for a transaction that doesn’t submit any transactions, but has the side-effect of changing the LET of the test ledger. It returns
(), also called
Unitand can be thought of as a zero-tuple.
create iou : Update (ContractId Iou), where
iou : Iouis a recipe for a transaction consisting of a single
createaction, and returns the contract id of the created contract if successful.
submit alice (create iou) : Scenario (ContractId Iou)is a recipe for a scenario in which Alice evaluates the result of
create iouto get a transaction and a return value of type
ContractId Iou, and then submits that transaction to the ledger.
Any DAML ledger knows how to perform actions of type
Update a. Only some know how to run scenarios, meaning they can perform actions of type
Chaining up actions with do blocks¶
An action followed by another action, possibly depending on the result of the first action, is just another action. Specifically:
- A transaction is a list of actions. So a transaction followed by another transaction is again a transaction.
- A scenario is a list of interactions with the ledger (
pass, etc). So a scenario followed by another scenario is again a scenario.
This is where
do blocks come in.
do blocks allow you to combine small actions in to bigger actions, using the results of earlier actions in later ones.
sub_scenario1 : Scenario (ContractId SimpleIou) = scenario do alice <- getParty "Alice" dora <- getParty "Dora" submit dora do create SimpleIou with issuer = dora owner = alice cash = Cash with amount = 100.0 currency = "USD" sub_scenario2 : Scenario Int = scenario do getParty "Nobody" pass (days 1) pass (days (-1)) return 42 sub_scenario3 : Scenario (ContractId SimpleIou) = scenario do bob <- getParty "Bob" dora <- getParty "Dora" submit dora do create SimpleIou with issuer = dora owner = bob cash = Cash with amount = 100.0 currency = "USD" main_scenario : Scenario () = scenario do dora <- getParty "Dora" iou1 <- sub_scenario1 sub_scenario2 iou2 <- sub_scenario3 submit dora do archive iou1 archive iou2
Above, we see
do blocks in action for both
Wrapping values in actions¶
You may already have noticed the use of
return in the redeem choice.
return x is a no-op action which returns value
return 42 : Update Int. Since
do blocks always return the value of their last action,
sub_scenario2 : Scenario Int.
Not only are
Scenario examples of
Action, they are both examples of actions that can fail, e.g. because a transaction is illegal or the party retrieved via
getParty doesn’t exist on the ledger.
Each has a special action
abort txt that represents failure, and that takes on type
Update () or
Scenario () depending on context .
Transactions and scenarios succeed or fail atomically as a whole. So an occurrence of an
abort action will always fail the entire evaluation of the current
The last expression in the
do block of the
Redeem choice is a pattern matching expression on
dow. It has type
Update () and is either an
return depending on the day of week. So during the week, it’s a no-op and on weekends, it’s the special failure action. Thanks to the atomicity of transactions, no transaction can ever make use of the
Redeem choice on weekends, because it fails the entire transaction.
A sample Action¶
If the above didn’t make complete sense, here’s another example to explain what actions are more generally, by creating a new type that is also an action.
CoinGame a is an
Action a in which a
Coin is flipped. The
Coin is a pseudo-random number generator and each flip has the effect of changing the RNGs state. Based on the
Tails results, a return value of type
a is calulated.
data Face = Heads | Tails deriving (Eq, Show, Enum) data CoinGame a = CoinGame with play : Coin -> (Coin, a) flipCoin : CoinGame Face getCoin : Scenario Coin
CoinGame a exposes a function
play which takes a
Coin and returns a new
Coin and a result
a. More on the
-> syntax for functions later.
play are deliberately left obscure in the above. All you have is an action
getCoin to get your hands on a
Coin in a
Scenario context and an action
flipCoin which represents the simplest possible game: a single coin flip resulting in a
You can’t play any
CoinGame game on pen and paper as you don’t have a coin, but you can write down a script or recipe for a game:
coin_test = scenario do -- the coin is pseudo-random on LET so change the parameter to change the game passToDate (date 2019 Jun 1) pass (seconds 2) coin <- getCoin let game = do f1r <- flipCoin f2r <- flipCoin f3r <- flipCoin if all (== Heads) [f1r, f2r, f3r] then return "Win" else return "Loss" (newCoin, result) = game.play coin assert (result == "Win")
game expression is a
CoinGame in which a coin is flipped three times. If all three tosses return
Heads, the result is
"Win", or else
Scenario context you can get a
Coin using the
getCoin action, which uses the LET to calculate a seed, and play the game.
Coin is threaded through the various actions. If you want to look through the looking glass and understand in-depth what’s going on, you can look at the source file to see how the
CoinGame action is implemented, though be warned that the implementation uses a lot of DAML features we haven’t introduced yet in this introduction.
More generally, if you want to learn more about Actions (aka Monads), we recommend a general course on functional programming, and Haskell in particular. For example:
- Finding Success and Failure in Haskell (Julie Maronuki, Chris Martin)
- Haskell Programming from first principles (Christopher Allen, Julie Moronuki)
- Learn You a Haskell for Great Good! (Miran Lipovača)
- Programming in Haskell (Graham Hutton)
- Real World Haskell (Bryan O’Sullivan, Don Stewart, John Goerzen)
Above, you’ve learnt about
abort, which represent (potentially) failing actions. Actions only have an effect when they are performed, so the following scenario succeeds or fails depending on the value of
nonPerformedAbort = scenario do let abortScenario = False let failingAction : Scenario () = abort "Foo" let successfulAction : Scenario () = return () if abortScenario then failingAction else successfulAction
However, what about errors in contexts other than actions? Suppose we wanted to implement a function
pow that takes an integer to the power of another positive integer. How do we handle that the second parameter has to be positive?
One option is to make the function explicitly partial by returning an
optPow : Int -> Int -> Optional Int optPow x y | y == 0 = Some 1 | y > 0 = let Some z = optPow x (y - 1) in Some (y * z) | otherwise = None
This is a useful pattern if we need to be able to handle the error case, but is also forces us to always handle it as we need to extract the result from an
Optional. We can see the impact on conveneience in the definition of the above function. In cases, like division by zero or the above function, it can therefore be preferrable to fail catastrophically instead:
errPow : Int -> Int -> Int errPow x y | y == 0 = 1 | y > 0 = y * errPow x (y - 1) | otherwise = error "Negative exponent not supported"
The big downside to this is that even unused errors cause failures. The following scenario will fail, because
failingComputation is evaluated:
nonPerformedError = scenario do let causeError = False let failingComputation = errPow 1 (-1) let successfulComputation = errPow 1 1 return if causeError then failingComputation else successfulComputation
error should therefore only be used in cases where the error case is unlikely to be encountered, and where explicit partiality would unduly impact usability of the function.
You can now specify a precise data and data-transformation model for DAML ledgers. In 6 Parties and authority, you will learn how to properly involve multiple parties in contracts, how authority works in DAML, and how to build contract models with strong guarantees in contexts with mutually distrusting entities.