# Daml REPL¶

The Daml REPL allows you to use the Daml Script API interactively. This is useful for debugging and for interactively inspecting and manipulating a ledger.

## Usage¶

First create a new project based on the script-example template. Take a look at the documentation for Daml Script for details on this template.

daml new script-example --template script-example # create a project called script-example based on the template
cd script-example # switch to the new project


Now, build the project and start Daml Sandbox, the in-memory ledger included in the SDK. Note that we are starting Sandbox in wallclock mode. Static time is not supported in daml repl.

daml build
daml sandbox --wall-clock-time --port=6865 --dar .daml/dist/script-example-0.0.1.dar


Now that the ledger has been started, you can launch the REPL in a separate terminal using the following command.

daml repl --ledger-host=localhost --ledger-port=6865 .daml/dist/script-example-0.0.1.dar --import script-example


The --ledger-host and --ledger-port parameters point to the host and port your ledger is running on. In addition to that, you also need to pass in the name of a DAR containing the templates and other definitions that will be accessible in the REPL. We also specify that we want to import all modules from the script-example package. If your modules provide colliding definitions you can also import modules individually from within the REPL. Note that you can also specify multiple DARs and they will all be available.

You should now see a prompt looking like

daml>


You can think of this prompt like a line in a do-block of the Script action. Each line of input has to have one of the following two forms:

1. An expression expr of type Script a for some type a. This will execute the script and print the result if a is an instance of Show and not ().
2. A pure expression expr of type a for some type a where a is an instance of Show. This will evaluate expr and print the result. If you are only interest in pure expressions you can also use Daml REPL without connecting to a ledger.
3. A binding of the form pat <- expr where pat is pattern, e.g., a variable name x to bind the result to and expr is an expression of type Script a. This will execute the script and match the result against the pattern pat bindings the matches to the variables in the pattern. You can then use those variables on subsequent lines.
4. A let binding of the form let pat = y, where pat is a pattern and y is a pure expression or let f x = y to define a function. The bound variables can be used on subsequent lines.
5. Next to Daml code the REPL also understands REPL commands which are prefixed by :. Enter :help to see a list of supported REPL commands.

First create two parties: A party with the display name "Alice" and the party id "alice" and a party with the display name "Bob" and the party id "bob".

daml> alice <- allocatePartyWithHint "Alice" (PartyIdHint "alice")
daml> bob <- allocatePartyWithHint "Bob" (PartyIdHint "bob")


Next, create a CoinProposal from Alice to Bob

daml> submit alice (createCmd (CoinProposal (Coin alice bob)))


As Bob, you can now get the list of active CoinProposal contracts using the query function. The debug : Show a => a -> Script () function can be used to print values.

daml> proposals <- query @CoinProposal bob
daml> debug proposals
[Daml.Script:39]: [(<contract-id>,CoinProposal {coin = Coin {issuer = 'alice', owner = 'bob'}})]


Finally, accept all proposals using the forA function to iterate over them.

daml> forA proposals \$ \(contractId, _) -> submit bob (exerciseCmd contractId Accept)


Using the query function we can now verify that there is one Coin and no CoinProposal:

daml> coins <- query @Coin bob
daml> debug coins
[Daml.Script:39]: [(<contract-id>,Coin {issuer = 'alice', owner = 'bob'})]
daml> proposals <- query @CoinProposal bob
[Daml.Script:39]: []


To exit daml repl press Control-D.

## What is in scope at the prompt?¶

In the prompt, all modules from DALFs specified in --import are imported automatically. In addition to that, the Daml.Script module is also imported and gives you access to the Daml Script API.

You can use the commands :module + ModA ModB … to import additional modules and :module - ModA ModB … to remove previously added imports. Modules can also be imported using regular import declarations instead of module +. The command :show imports lists the currently active imports.

daml> import DA.Time
daml> debug (days 1)


## Using Daml REPL without a Ledger¶

If you are only interested in pure expressions, e.g., because you want to test how some function behaves you can omit the --ledger-host and -ledger-port parameters. Daml REPL will work as usual but any attempts to call Daml Script APIs that interact with the ledger, e.g., submit will result in the following error:

daml> java.lang.RuntimeException: No default participant


## Connecting via TLS¶

You can connect to a ledger that requires TLS by passing --tls. A custom root certificate used for validating the server certificate can be set via --cacrt. Finally, you can also enable client authentication by passing --pem client.key --crt client.crt. If --cacrt or --pem and --crt are passed TLS is automatically enabled so --tls is redundant.

## Connection to a Ledger with Authorization¶

If your ledger requires an authorization token you can pass it via --access-token-file.

## Using Daml REPL to convert to JSON¶

Using the :json command you can encode serializable Daml expressions as JSON. For example using the definitions and imports from above:

daml> :json days 1
{"microseconds":86400000000}
daml> :json map snd coins
[{"issuer":"alice","owner":"bob"}]