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# Basic dependency

One common approach to dapp design is to calculate or store data in one canister smart contract – or canister for short – that you can then use in another canister. This ability to share and use functions defined in different canisters, even if the underlying smart contracts are written in different languages, is an important strategy for building dapps to run on the Internet Computer blockchain. This tutorial provides a basic introduction to how you can write functions in one language—in the example, Motoko—then use the data in another—in this case, Rust.

For this tutorial, both canisters are in the same project.

• The Motoko canister creates an actor with a cell variable to contain the current value that results from an operation.

• The mul function takes a natural number as input, multiplies the input value by three and stores the result in the cell variable.

• The Rust canister provides a simple read function that returns the current value of the cell variable. Notice that the function making inter-canister call should be a update method even though the method to be called is query.

## Before you begin​

Before you start your project, verify the following:

• You have an internet connection and access to a shell terminal on your local macOS or Linux computer.

• You have downloaded and installed the Rust programming language and Cargo as described in the Rust installation instructions for your operating system.

curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSf | sh

The Rust tool chain must be at version 1.46.0, or later.

• You have downloaded and installed the DFINITY Canister Software Development Kit (SDK) package as described in Download and install.

• You have cmake installed. For example, use Homebrew with the following command:

brew install cmake

For instructions on how to install Homebrew, see the Homebrew Documentation.

• You have stopped any local canister execution environment processes running on your computer.

This tutorial takes approximately 20 minutes to complete.

## Create a new project​

To create a new project directory for this tutorial:

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

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

dfx new --type=rust rust_deps
3. Change to your project directory by running the following command:

cd rust_deps

## Modify the default project​

To complete this tutorial, you’ll need to complete the following steps:

### Edit the default canister settings​

Because this sample project is going to consist of two canisters-the Motoko canister and the Rust canister—you need to modify the default dfx.json configuration file to include information for building both the Motoko canister and a Rust canister.

To modify the dfx.json configuration file:

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

2. Open the dfx.json configuration file in a text editor.

3. Insert a new section before the canisters.rust_deps_backend settings with settings for building a Motoko program.

For example, in the canisters section, add a new multiply_deps key with settings like these:

"multiply_deps": {  "main": "src/multiply_deps/main.mo",  "type": "motoko"}
4. Add a dependencies setting to the rust_deps_backend .

The dependencies setting enables you to import functions from one canisters for use in another canister. For this tutorial, we want to import a function from the multiply_deps canister—written in Motoko—and use it from the rust_deps canister written in Rust. Then the rust_deps_backend field will look like:

"rust_deps_backend": {  "candid": "src/rust_deps_backend/rust_deps_backend.did",  "package": "rust_deps_backend",  "type": "rust",  "dependencies": [    "multiply_deps"  ]},
5. Remove all of the rust_deps_frontend configuration settings from the file.

The sample dapp for this tutorial doesn’t use any frontend assets, so you can remove those settings from the configuration file.

You can also remove the defaults settings.

For example, your configuration file might look like this after you modify the settings.

{  "version": 1,  "canisters": {    "multiply_deps": {      "main": "src/multiply_deps/main.mo",      "type": "motoko"    },    "rust_deps_backend": {      "type": "rust",      "package": "rust_deps_backend",      "candid": "src/rust_deps_backend/rust_deps_backend.did",      "dependencies": [        "multiply_deps"      ]    }  }}
1. Save your change and close the dfx.json file to continue.

### Implement Motoko canister​

The next step is to create a file at src/multiply_deps/main.mo with code that implements the mul and read functions.

To write the Motoko source code:

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

2. Create the directory for the Motoko canister.

mkdir src/multiply_deps
3. Create and open the src/multiply_deps/main.mo file in a text editor.

4. Copy and paste this code into the main.mo file.

actor Multiply {    var cell : Nat = 1;    public func mul(n : Nat) : async Nat { cell *= n * 3; cell };    public query func read() : async Nat {        cell;    };};
1. Save your changes and close the file to continue.

### Replace the default Rust canister​

Now that we have the Motoko canister that the Rust canister depends upon, let’s add the Rust canister to the project.

To replace the default Rust canister:

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

2. Open the template src/rust_deps_backend/src/lib.rs file in a text editor and delete the existing content.

The next step is to write a Rust program that imports the Motoko canister and implements the read function.

3. Copy and paste this code into the lib.rs file.

use ic_cdk_macros::*;use ic_cdk::export::candid;#[import(canister = "multiply_deps")]struct CounterCanister;#[update]async fn read() -> candid::Nat {    CounterCanister::read().await.0}
4. Save your changes and close the src/rust_deps_backend/src/lib.rs file to continue.

### Update interface description file​

Candid is an interface description language (IDL) for interacting with canisters running on the Internet Computer blockchain. Candid files provide a language-independent description of a canister’s interfaces including the names, parameters, and result formats and data types for each function a canister defines.

By adding Candid files to your project, you can ensure that data is properly converted from its definition in Rust to run safely on the Internet Computer blockchain.

To see details about the Candid interface description language syntax, see the Candid Guide or the Candid crate documentation.

To update the Candid file for this tutorial:

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

2. Open the src/rust_deps_backend/rust_deps_backend.did file in a text editor.

3. Copy and paste the following service definition for the read function:

service : {  "read": () -> (nat);}
4. Save your changes and close the rust_deps_backend.did file to continue.

## Start the local canister execution environment​

Before you can build the project, you need to connect to the local canister execution environment in your development environment or the Internet Computer blockchain mainnet.

To start the network locally:

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

2. Start the local canister execution environment on your computer in the background by running the following command:

dfx start --clean --background

Depending on your platform and local security settings, you might see a warning displayed. If you are prompted to allow or deny incoming network connections, click Allow.

## Register, build, and deploy your project​

After you connect to the local canister execution environment running in your development environment, you can register, build, and deploy your project locally.

To register, build, and deploy:

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

2. Register, build, and deploy the canisters specified in the dfx.json file by running the following command:

dfx deploy

The dfx deploy command output displays information about each of the operations it performs similar to the following excerpt:

Creating a wallet canister on the local network.The wallet canister on the "local" network for user "default" is "rwlgt-iiaaa-aaaaa-aaaaa-cai"Deploying all canisters.Creating canisters...Creating canister multiply_deps...multiply_deps canister created with canister id: rrkah-fqaaa-aaaaa-aaaaq-caiCreating canister rust_deps_backend...rust_deps_backend canister created with canister id: ryjl3-tyaaa-aaaaa-aaaba-caiBuilding canisters...Checking for vulnerabilities in rust canisters.    Fetching advisory database from https://github.com/RustSec/advisory-db.git      Loaded 469 security advisories (from /Users/moritz/.cargo/advisory-db)    Updating crates.io index    Scanning Cargo.lock for vulnerabilities (110 crate dependencies)Audit found no vulnerabilities.Shrink WASM module size.Executing: cargo build --target wasm32-unknown-unknown --release -p rust_deps_backend --locked    Finished release [optimized] target(s) in 0.07sShrink WASM module size.Installing canisters...Creating UI canister on the local network.The UI canister on the "local" network is "r7inp-6aaaa-aaaaa-aaabq-cai"Installing code for canister multiply_deps, with canister ID rrkah-fqaaa-aaaaa-aaaaq-caiInstalling code for canister rust_deps_backend, with canister ID ryjl3-tyaaa-aaaaa-aaaba-caiDeployed canisters.URLs:  Backend canister via Candid interface:    multiply_deps: http://127.0.0.1:4943/?canisterId=r7inp-6aaaa-aaaaa-aaabq-cai&id=rrkah-fqaaa-aaaaa-aaaaq-cai    rust_deps_backend: http://127.0.0.1:4943/?canisterId=r7inp-6aaaa-aaaaa-aaabq-cai&id=ryjl3-tyaaa-aaaaa-aaaba-cai

## Call functions on the deployed canister​

After successfully deploying the canister, you can test the canister by invoking the functions it provides.

For this tutorial:

• Call the mul function to multiply the value of the cell variable by three each time it is called.

• Call the read function to return the current value of the cell variable.

To test the deployed canister:

1. Call the read function from the Motoko canister, which reads the current value of the cell variable on the deployed canister:

dfx canister call multiply_deps read

The command returns the current value of the cell variable as one:

(1 : nat)
2. Call the mul function to multiply the input argument by three by running the following command:

dfx canister call multiply_deps mul '(3)'

The command returns the new value of the cell variable:

(9 : nat)
3. Call the read function using the rust_deps canister that imports functions from the multiply_deps canister:

dfx canister call rust_deps_backend read

The command returns the current value of the cell variable:

(9 : nat)

## Stop the local canister execution environment​

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

To stop the local execution environment running on your computer, run the following command:

dfx stop