2 Testing templates using scenarios¶
In this section you will test the
Token model from 1 Basic contracts using DAML’s inbuilt
scenario language. You’ll learn about the basic features of scenarios:
- Getting parties
- Submitting transactions
- Creating contracts
- Testing for failure
- Archiving contracts
- Viewing ledger and final ledger state
Scenario is like a recipe for a test, where you can script different parties submitting a series of transactions, to check that your templates behave as you’d expect. You can also script some some external information like party identities, and ledger time.
Below is a basic scenario that creates a
Token for a party called “Alice”.
token_test_1 = do alice <- getParty "Alice" submit alice do create Token with owner = alice
You declare a
Scenario a top-level variable and introduce it using
do always starts a block, so the rest of the scenario is indented.
Before you can create any
Token contracts, you need some parties on the test ledger. The above scenario uses the function
getParty to put a party called “Alice” in a variable
alice. There are two things of note there:
The reason for that is
Actionthat can only be performed once the
Scenariois run in the context of a ledger.
<-means “run the action and bind the result”. It can only be run in that context because, depending on the ledger the scenario is running on,
getPartymay have to look up a party identity or create a new party.
doblocks in 5 Adding constraints to a contract.
If that doesn’t quite make sense yet, for the time being you can think of this arrow as extracting the right-hand-side value from the ledger and storing it into the variable on the left.
getPartydoes not have to be enclosed in brackets. Functions in DAML are called using the syntax
fn arg1 arg2 arg3.
With a variable
alice of type
Party in hand, you can submit your first transaction. Unsurprisingly, you do this using the
submit takes two arguments: a
Party and an
Scenario is a recipe for a test,
Update is a recipe for a transaction.
create Token with owner = alice is an
Update, which translates to the transaction creating a
Token with owner Alice.
You’ll learn all about the syntax
Token with owner = alice in 3 Data types.
You could write this as
submit alice (create Token with owner = alice), but just like scenarios, you can assemble updates using
do blocks. A
do block always takes the value of the last statement within it so the syntax shown in the scenario above gives the same result, whilst being easier to read.
There are two ways to run scenarios:
- In DAML Studio, providing visualizations of the resulting ledger
- Using the command line, useful for continuous integration
In DAML Studio, you should see the text “Scenario results” just above the line
token_test_1 = do. Click on it to display the outcome of the scenario.
This opens the scenario view in a separate column in VS Code. The default view is a tabular representation of the final state of the ledger:
What this display means:
- The big title reading
Token_Test:Tokenis the identifier of the type of contract that’s listed below.
Token_Testis the module name,
Tokenthe template name.
- The first columns, labelled vertically, show which parties know about which contracts. In this simple scenario, the sole party “Alice” knows about the contract she created.
- The second column shows the ID of the contract. This will be explained later.
- The third column shows the status of the contract, either
- The remaining columns show the contract arguments, with one column per field. As expected, field
'Alice'. The single quotation marks indicate that
Aliceis a party.
To run the same test from the command line, save your module in a file
Token_Test.daml and run
daml damlc -- test Token_Test.daml. If your file contains more than one scenario, all of them will be run.
Testing for failure¶
In 1 Basic contracts you learned that creating a
Token requires the authority of its owner. In other words, it should not be possible for Alice to create a Token for another party and vice versa. A reasonable attempt to test that would be:
failing_test_1 = do alice <- getParty "Alice" bob <- getParty "Bob" submit alice do create Token with owner = bob submit bob do create Token with owner = alice
However, if you open the scenario view for that scenario, you see the following message:
The scenario failed, as expected, but scenarios abort at the first failure. This means that it only tested that Alice can’t create a token for Bob, and the second
submit statement was never reached.
To test for failing submits and keep the scenario running thereafter, or fail if the submission succeeds, you can use the
token_test_2 = do alice <- getParty "Alice" bob <- getParty "Bob" submitMustFail alice do create Token with owner = bob submitMustFail bob do create Token with owner = alice submit alice do create Token with owner = alice submit bob do create Token with owner = bob
submitMustFail never has an impact on the ledger so the resulting tabular scenario view just shows the two Tokens resulting from the successful
submit statements. Note the new column for Bob as well as the visibilities. Alice and Bob cannot see each others’ Tokens.
Archiving contracts works just like creating them, but using
archive instead of
create takes an instance of a template,
archive takes a reference to a contract.
References to contracts have the type
ContractId a, where
a is a type parameter representing the type of contract that the ID refers to. For example, a reference to a
Token would be a
archive the Token Alice has created, you need to get a handle on its contract ID. In scenarios, you do this using
<- notation. That’s because the contract ID needs to be retrieved from the ledger. How this works is discussed in 5 Adding constraints to a contract.
This scenario first checks that Bob cannot archive Alice’s Token and then Alice successfully archives it:
token_test_3 = do alice <- getParty "Alice" bob <- getParty "Bob" alice_token <- submit alice do create Token with owner = alice submitMustFail bob do archive alice_token submit alice do archive alice_token
Exploring the ledger¶
The resulting scenario view is empty, because there are no contracts left on the ledger. However, if you want to see the history of the ledger, e.g. to see how you got to that state, tick the “Show archived” box at the top of the ledger view:
You can see that there was a
Token contract, which is now archived, indicated both by the “archived” value in the
status column as well as by a strikethrough.
Click on the adjacent “Show transaction view” button to see the entire transaction graph:
In the DAML Studio scenario runner, committed transactions are numbered sequentially. The lines starting with
TX indicate that there are three committed transactions, with ids
#2. These correspond to the three
submitMustFail statements in the scenario.
#0 has one sub-transaction
#0:0, which the arrow indicates is a
create of a
commit X, sub-transaction Y. All transactions have this format in the scenario runner. However, this format is a testing feature. In general, you should consider Transaction and Contract IDs to be opaque.
The lines above and below
create Token_Test:Token give additional information:
consumed by: #2:1tells you that the contract is archived in sub-transaction
referenced by #2:0, #2:1tells you that the contract was used in other transactions, and lists their IDs.
known to (since): 'Alice' (#0)tells you who knows about the contract. The fact that
'Alice'appears in the list is equivalent to a
xin the tabular view. The
(#0)gives you the additional information that
Alicelearned about the contract in commit
- Everything following
withshows the create arguments.
To get a better understanding of scenarios, try the following exercises:
- Write a template for a second type of Token.
- Write a scenario with two parties and two types of tokens, creating one token of each type for each party and archiving one token for each party, leaving one token of each type in the final ledger view.
- In Archiving contracts you tested that Bob cannot archive Alice’s token. Can you guess why the submit fails? How can you find out why the submit fails?
Remember that in Testing for failure we saw a proper error message for a failing submit.