# Daml Studio¶

Daml Studio is an integrated development environment (IDE) for Daml. It is an extension on top of Visual Studio Code (VS Code), a cross-platform, open-source editor providing a rich code editing experience.

## Installing¶

Daml Studio is included in the Daml SDK.

## Creating your first Daml file¶

1. Start Daml Studio by running daml studio in the current project.

This command starts Visual Studio Code and (if needs be) installs the Daml Studio extension, or upgrades it to the latest version.

2. Make sure the Daml Studio extension is installed:

1. Click on the Extensions icon at the bottom of the VS Code sidebar.
2. Click on the Daml Studio extension that should be listed on the pane.
3. Open a new file (⌘N) and save it (⌘S) as Test.daml.

4. Copy the following code into your file:

module Test where

double : Int -> Int
double x = 2 * x


Your screen should now look like the image below.

1. Introduce a parse error by deleting the = sign and then clicking the Ⓧ symbol on the lower-left corner. Your screen should now look like the image below.

2. Remove the parse error by restoring the = sign.

## Supported features¶

Visual Studio Code provides many helpful features for editing Daml files and we recommend reviewing Visual Studio Code Basics and Visual Studio Code Keyboard Shortcuts for OS X. The Daml Studio extension for Visual Studio Code provides the following Daml-specific features:

### Symbols and problem reporting¶

Use the commands listed below to navigate between symbols, rename them, and inspect any problems detected in your Daml files. Symbols are identifiers such as template names, lambda arguments, variables, and so on.

Command Shortcut (OS X)
Go to Definition F12
Peek Definition ⌥F12
Rename Symbol F2
Go to Symbol in File ⇧⌘O
Go to Symbol in Workspace ⌘T
Find all References ⇧F12
Problems Panel ⇧⌘M

Note

You can also start a command by typing its name into the command palette (press ⇧⌘P or F1). The command palette is also handy for looking up keyboard shortcuts.

Note

### Hover tooltips¶

You can hover over most symbols in the code to display additional information such as its type.

### Daml Script results¶

Top-level declarations of type Script are decorated with a Script results code lens. You can click on the code lens to inspect the execution transaction graph and the active contracts.

For the script from the Iou module, you get the following table displaying all contracts that are active at the end of the script. The first column displays the contract id. The columns afterwards represent the fields of the contract and finally you get one column per party with an X if the party can see the contract or a - if not.

If you want more details, you can click on the Show archived checkbox, which extends the table to include archived contracts, and on the Show detailed disclosure checkbox, which displays why the contract is visible to each party, based on four categories:

1. S, the party sees the contract because they are a signatory on the contract.
2. O, the party sees the contract because they are an observer on the contract.
3. W, the party sees the contract because they witnessed the creation of this contract, e.g., because they are an actor on the exercise that created it.
4. D, the party sees the contract because they have been divulged the contract, e.g., because they witnessed an exercise that resulted in a fetch of this contract.

For details on the meaning of those four categories, refer to the Daml Ledger Model. For the example above, the resulting table looks as follows. You can see the archived Bank contract and the active Bank contract whose creation Alice has witnessed by virtue of being an actor on the exercise that created it.

If you want to see the detailed transaction graph you can click on the Show transaction view button. The transaction graph consists of transactions, each of which contain one or more updates to the ledger, that is creates and exercises. The transaction graph also records fetches of contracts.

For example a script for the Iou module looks as follows:

Script results

Each transaction is the result of executing a step in the script. In the image below, the transaction #0 is the result of executing the first line of the script (line 20), where the Iou is created by the bank. The following information can be gathered from the transaction:

• The result of the first script transaction #0 was the creation of the Iou contract with the arguments bank, 10, and "USD".
• The created contract is referenced in transaction #1, step 0.
• The created contract was consumed in transaction #1, step 0.
• A new contract was created in transaction #1, step 1, and has been divulged to parties ‘Alice’, ‘Bob’, and ‘Bank’.
• At the end of the script only the contract created in #1:1 remains.
• The return value from running the script is the contract identifier #1:1.
• And finally, the contract identifiers assigned in script execution correspond to the script step that created them (e.g. #1).

You can navigate to the corresponding source code by clicking on the location shown in parenthesis (e.g. Iou:25:12, which means the Iou module, line 25 and column 1). You can also navigate between transactions by clicking on the transaction and contract ids (e.g. #1:0).

### Daml snippets¶

You can automatically complete a number of “snippets” when editing a Daml source file. By default, hitting ^-Space after typing a Daml keyword displays available snippets that you can insert.

To define your own workflow around Daml snippets, adjust your user settings in Visual Studio Code to include the following options:

{
"editor.tabCompletion": true,
"editor.quickSuggestions": false
}


With those changes in place, you can simply hit Tab after a keyword to insert the code pattern.

You can develop your own snippets by following the instructions in Creating your own Snippets to create an appropriate daml.json snippet file.

## Common script errors¶

During Daml execution, errors can occur due to exceptions (e.g. use of “abort”, or division by zero), or due to authorization failures. You can expect to run into the following errors when writing Daml.

When a runtime error occurs in a script execution, the script result view shows the error together with the following additional information, if available:

Location of the failed commit
If the failing part of the script was a submitCmd, the source location of the call to submitCmd will be displayed.
Stack trace
A list of source locations that were encountered before the error occurred. The last encountered location is the first entry in the list.
Ledger time
The ledger time at which the error occurred.
Partial transaction
The transaction that is being constructed, but not yet committed to the ledger.
Committed transaction
Transactions that were successfully committed to the ledger prior to the error.
Trace
Any messages produced by calls to trace and debug.

### Abort, assert, and debug¶

The abort, assert and debug inbuilt functions can be used in updates and scripts. All three can be used to output messages, but abort and assert can additionally halt the execution:

abortTest = script do
debug "hello, world!"
abort "stop"

Scenario execution failed:
Unhandled exception:  DA.Exception.GeneralError:GeneralError with
message = "stop"

Ledger time: 1970-01-01T00:00:00Z

Trace:
"hello, world!"


### Missing authorization on create¶

If a contract is being created without approval from all authorizing parties the commit will fail. For example:

template Example
with
party1 : Party; party2 : Party
where
signatory party1
signatory party2

example = script do
alice <- allocateParty "Alice"
bob <- allocateParty "Bob"
alice submit createCmd Example with
party1 = alice
party2 = bob


Execution of the example script fails due to ‘Bob’ being a signatory in the contract, but not authorizing the create:

Scenario execution failed:
#0: create of CreateAuthFailure:Example at unknown source
failed due to a missing authorization from 'Bob'

Ledger time: 1970-01-01T00:00:00Z

Partial transaction:
Sub-transactions:
#0
└─> create CreateAuthFailure:Example
with
party1 = 'Alice'; party2 = 'Bob'


To create the “Example” contract one would need to bring both parties to authorize the creation via a choice, for example ‘Alice’ could create a contract giving ‘Bob’ the choice to create the ‘Example’ contract.

### Missing authorization on exercise¶

Similarly to creates, exercises can also fail due to missing authorizations when a party that is not a controller of a choice exercises it.

template Example
with
owner : Party
friend : Party
where
signatory owner
observer friend

choice Consume : ()
controller owner
do return ()

choice Hello : ()
controller friend
do return ()

example = script do
alice <- allocateParty "Alice"
bob <- allocateParty "Bob"
cid <- alice submit createCmd Example with
owner = alice
friend = bob
bob submit exerciseCmd cid Consume


The execution of the example script fails when ‘Bob’ tries to exercise the choice ‘Consume’ of which he is not a controller

Scenario execution failed:
#1: exercise of Consume in ExerciseAuthFailure:Example at unknown source
failed due to a missing authorization from 'Alice'

Ledger time: 1970-01-01T00:00:00Z

Partial transaction:
Failed exercise:
exercises Consume on #0:0 (ExerciseAuthFailure:Example)
with
Sub-transactions:
0
└─> 'Alice' exercises Consume on #0:0 (ExerciseAuthFailure:Example)
with

Committed transactions:
TX #0 1970-01-01T00:00:00Z (unknown source)
#0:0
│   known to (since): 'Alice' (#0), 'Bob' (#0)
└─> create ExerciseAuthFailure:Example
with
owner = 'Alice'; friend = 'Bob'


From the error we can see that the parties authorizing the exercise (‘Bob’) is not a subset of the required controlling parties.

### Contract not visible¶

Contract not being visible is another common error that can occur when a contract that is being fetched or exercised has not been disclosed to the committing party. For example:

template Example
with owner: Party
where
signatory owner

choice Consume : ()
controller owner
do return ()

example = script do
alice <- allocateParty "Alice"
bob <- allocateParty "Bob"
cid <- alice submit createCmd Example with owner = alice
bob submit exerciseCmd cid Consume


In the above script the ‘Example’ contract is created by ‘Alice’ and makes no mention of the party ‘Bob’ and hence does not cause the contract to be disclosed to ‘Bob’. When ‘Bob’ tries to exercise the contract the following error would occur:

Scenario execution failed:
Attempt to fetch or exercise a contract not visible to the reading parties.
Contract:  #0:0 (NotVisibleFailure:Example)
actAs: 'Bob'
Disclosed to: 'Alice'

Ledger time: 1970-01-01T00:00:00Z

Partial transaction:

Committed transactions:
TX #0 1970-01-01T00:00:00Z (unknown source)
#0:0
│   known to (since): 'Alice' (#0)
└─> create NotVisibleFailure:Example
with
owner = 'Alice'


To fix this issue the party ‘Bob’ should be made a controlling party in one of the choices.

## Working with multiple packages¶

Often a Daml project consists of multiple packages, e.g., one containing your templates and one containing a Daml trigger so that you can keep the templates stable while modifying the trigger. It is possible to work on multiple packages in a single session of Daml studio but you have to keep some things in mind. You can see the directory structure of a simple multi-package project consisting of two packages pkga and pkgb below:

.
├── daml.yaml
├── pkga
│   ├── daml
│   │   └── A.daml
│   └── daml.yaml
└── pkgb
├── daml
│   └── B.daml
└── daml.yaml


pkga and pkgb are regular Daml projects with a daml.yaml and a Daml module. In addition to the daml.yaml files for the respective packages, you also need to add a daml.yaml to the root of your project. This file only needs to specify the SDK version. Replace X.Y.Z by the SDK version you specified in the daml.yaml files of the individual packages.

sdk-version: X.Y.Z


You can then open Daml Studio once in the root of your project and work on files in both packages. Note that if pkgb refers to pkga.dar in its dependencies field, changes will not be picked up automatically. This is always the case even if you open Daml Studio in pkgb. However, for multi-package projects there is an additional caveat: You have to both rebuild pkga.dar using daml build and then build pkgb using daml build before restarting Daml Studio.