Import library modules

In this tutorial, you are going to write a simple dapp that enables you to store and look up telephone numbers. This tutorial illustrates how to import and use a few basic Motoko library functions.

For this tutorial, the Motoko base library functions are defined in the List and AssocList modules and enable you to work with lists as linked key-value pairs. In this example, the key is a name and the value is the phone text string associated with that name.

This dapp supports the following function calls:

  • The insert function accepts the name and phone key-value pair as input stored in the book variable.

  • The lookup function is a query that uses the specified name key as input to find the associated phone number.

Before you begin

Before starting the tutorial, verify the following:

  • You have downloaded and installed the DFINITY Canister SDK package as described in Download and install.

  • You have stopped the local canister execution environment provided by dfx.

This tutorial takes approximately 10 minutes to complete.

Create a new project

To create a new project for this tutorial:

  1. Open a terminal shell on your local computer, if you don’t already have one open.

  2. Change to the folder you are using for your Internet Computer projects, if you are using one.

  3. Create a new project by running the following command:

    dfx new phonebook
  4. Change to your project directory by running the following command:

    cd phonebook

Modify the default dapp

For this tutorial, let’s create a new main.mo file for the simple phone number lookup dapp.

To modify the default template:

  1. Open the src/phonebook/main.mo file in a text editor and delete the existing content.

  2. Copy and paste the following sample code into the main.mo file:

    // Import standard library functions for lists
    
    import L "mo:base/List";
    import A "mo:base/AssocList";
    
    // The PhoneBook actor.
    actor {
    
        // Type aliases make the rest of the code easier to read.
        public type Name = Text;
        public type Phone = Text;
    
        // The actor maps names to phone numbers.
        flexible var book: A.AssocList<Name, Phone> = L.nil<(Name, Phone)>();
    
        // An auxiliary function checks whether two names are equal.
        func nameEq(l: Name, r: Name): Bool {
            return l == r;
        };
    
        // A shared invokable function that inserts a new entry
        // into the phone book or replaces the previous one.
        public func insert(name: Name, phone: Phone): async () {
            let (newBook, _) = A.replace<Name, Phone>(book, name, nameEq, ?phone);
            book := newBook;
        };
    
        // A shared read-only query function that returns the (optional)
        // phone number corresponding to the person with the given name.
        public query func lookup(name: Name): async ?Phone {
            return A.find<Name, Phone>(book, name, nameEq);
        };
    };

    In looking at this sample dapp, you might notice the following key elements:

    • The code defines Name and Phone as custom Text types. Creating user-defined types improves the readability of the code.

    • The insert function is an update call and the lookup function is a query call.

    • The Phone type is identified as an optional value by using the ?Phone syntax.

Start the local canister execution environment

For development purposes dfx provides a local canister execution environment. This requires a dfx.json file, so you should be sure you are in your project’s root directory. For this tutorial, you should have two separate terminal shells, so that you can start and see the output of the local canister execution environment in one terminal and manage your project in another.

To start the local canister execution environment:

  1. Open a new terminal window or tab on your local computer.

  2. Navigate to the root directory for your project, if necessary.

    • You should now have two terminals open.

    • You should have the project directory as your current working directory.

  3. Start the local canister execution environment on your local computer by running the following command:

    dfx start --clean

    For this tutorial, we’re using the --clean option to start the local canister execution environment in a clean state.

    This option removes any orphan background processes or canister identifiers that might disrupt normal operations. For example, if you forgot to issue a dfx stop when moving between projects, you might have a process running in the background or in another terminal. The --clean option ensures that you can start the local canister execution environment and continue to the next step without manually finding and terminating any running processes.

  4. Leave the terminal that displays the output of the local canister execution environment open and switch your focus to your original terminal where you created your new project.

Register, build, and deploy the dapp

Once the local canister execution environment is up and running in your development environment, you can register, build, and deploy your dapp onto it.

To deploy the dapp locally:

  1. Check that you are still in the root directory for your project, if needed.

  2. Register, build, and deploy your dapp locally by running the following command:

    dfx deploy phonebook

    The dfx.json file provides default settings for creating a dapp front-end entry point and assets canister smart contract.

    In previous tutorials, we deleted the entries for the asset canister because we were not adding a front-end for the sample dapp. That change kept our project workspace tidy by eliminating files that would go unused. There is no requirement to do this, however, and there is no harm in leaving the asset canister smart contract description in the dfx.json file. For example, you might want to use it as a placeholder if you intend to add front-end assets later.

    For this tutorial, you can deploy just the phonebook back-end canister smart contract using the dfx deploy phonebook command because the project doesn’t include any front-end assets and you will interact with it via the terminal.

    Although this tutorial illustrates how to skip compiling a front-end canister, you can add a simple user interface to this dapp later by exploring the phone-book project in the examples repository.

Add names and numbers using the insert function

You now have a dapp deployed as a canister smart contract on your local canister execution environment and can test your dapp by using dfx canister call commands.

To test the dapp you have deployed:

  1. Use the dfx canister call command to call the canister smart contract phonebook using the insert function and pass it a name and phone number by running the following command:

    dfx canister call phonebook insert '("Chris Lynn", "01 415 792 1333")'
  2. Add a second name and number pair by running the following command:

    dfx canister call phonebook insert '("Maya Garcia", "01 408 395 7276")'
  3. Verify that the command returns the number associated with "Chris Lynn" using the lookup function by running the following command:

    dfx canister call phonebook lookup '("Chris Lynn")'

    The command returns output similar to the following:

    (opt "01 415 792 1333")
  4. Try to call the lookup function with the number associated with "Maya Garcia" by running the following command:

    dfx canister call phonebook lookup '("01 408 395 7276")'

    Note that, in this case, the command returns (null) because the phone number is not a key associated with the "Maya Garcia" name entry.

  5. Try to call the lookup function again to return the phone numbers for both "Maya Garcia" and "Chris Lynn" by running the following command:

    dfx canister call phonebook lookup '("Maya Garcia","Chris Lynn")'

    Because the dapp is written to return one value for one key, the command only returns information associated with the first key, in this example the phone number for Maya Garcia.

Test functions in a browser

The canister interface description language—often referred to as Candid or more generally as the IDL—provides a common language for specifying the signature of a canister smart contract. Candid provides a unified way for you to interact with canister smart contracts that are written in different languages or accessed using different tools. For example, Candid provides a consistent view of a service whether the underlying program is native Rust, JavaScript, or Motoko. Candid also enables different tools—such as the dfx command-line interface and the Network Nervous System dapp—to share a common description for a service.

Based on the type signature of the actor, Candid also provides a web interface that allows you to call canister functions for testing and debugging.

After you have deployed your project in the local canister execution environment using the dfx deploy or dfx canister install command, you can access the Candid web interface endpoint in a browser. This web interface—the Candid UI—exposes the service description in a form, enabling you to quickly view and test functions and experiment with entering different data types without writing any front-end code.

To use the Candid web interface to test canister functions:

  1. Find the Candid UI canister identifier associated with the current project using the dfx canister id __Candid_UI command.

    dfx canister id __Candid_UI

    The command displays the canister identifier for the Candid UI with output similar to the following:

    r7inp-6aaaa-aaaaa-aaabq-cai
  2. Copy the Candid UI canister identifier so that it is available in the clipboard.

  3. If you’ve stopped the local canister execution environment, restart it locally by running the following command:

    dfx start --background
  4. Open a browser and navigate to the address and port number specified in the dfx.json configuration file.

    By default, the local canister execution environment binds to the 127.0.0.1:8000 address and port number.

  5. Add the required canisterId parameter and the Candid UI canister identifier returned by the dfx canister id __Candid_UI command to the URL.

    For example, the full URL should look similar to the following but with the CANDID-UI-CANISTER-IDENTIFIER that was returned by the dfx canister id __Candid_UI command:

    http://127.0.0.1:8000/?canisterId=<CANDID-UI-CANISTER-IDENTIFIER>

    For instance, with the example canister identifier for the Candid UI as shown above, this could look as follows:

    http://127.0.0.1:8000/?canisterId=r7inp-6aaaa-aaaaa-aaabq-cai

    The browser then displays a form for you to specify a canister identifier or choose a Candid description (.did) file. Note that this field refers to the canister identifier of the canister you would like to interact with (as opposed to the canister identifier for the Candid UI that we used in the last step).

  6. Specify the canister identifier of the canister you would like to test in the Provide a canister ID field, then click Go to display the service description.

    If you aren’t sure which canister identifier to use, you can run the dfx canister id command to look up the identifier for a specific canister name. For instance, to get the canister identifier for a canister named my_counter, you would use:

    dfx canister id my_counter
  7. Review the list of function calls and types defined in the dapp.

  8. Type a value of the appropriate type for a function or click Random to generate a value, then click Call or Query to see the result.

    Note that depending on the data type, the Candid interface might display additional configuration settings for testing functions. For example, if a function takes an array, you might need to specify the number of items in the array before entering values.

    Phonebook functions

Revise the source code in your dapp

To extend what you have learned in this tutorial, you might want to try modifying the source code to return different results.

For example, you might want to change the source code so that instead of a dapp that inserts and looks up a current key-value (name-phone) pair to create a dapp that stores contact information similar to a database "record" in which a primary key is associated with multiple fields. In this example, your dapp might enable users or another dapp to add information—such as a home phone number, a cell phone number, an email address, and a street address—and selectively return all or specific field values.

Stop the local canister execution environment

After you finish experimenting with your dapp, you can stop the local canister execution environment so that it does not continue running in the background.

To stop the local canister execution environment:

  1. In the terminal that displays network operations, press Control-C to interrupt the local network process.

  2. Stop the local canister execution environment by running the following command:

    dfx stop