📝 Creating a draft for upcoming cross-contract tutorial

docs/zkapps/tutorials/12-cross-contract-calls.mdx

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+---
+title: 'Tutorial 12: Cross Contract Calls'
+hide_title: true
+sidebar_label: 'Tutorial 12: Cross Contract Calls'
+description: Guided steps to learn about the ways to interact with a smart contract from another smart contract.
+keywords:
+  - smart contracts
+  - zkapps
+  - zero knowledge proof programming
+  - zk proof
+  - zk
+  - cross contract call
+  - mina
+---
+
+:::info
+
+Please note that zkApp programmability is not yet available on Mina Mainnet, but zkApps can now be deployed to Berkeley Testnet.
+
+:::
+
+
+# Tutorial 12: Cross Contract Calls
+
+In this tutorial, you learn how smart contracts on a blockchain can interact by calling functions in each other's code, enabling building modular and complex decentralized applications.
+
+Cross contract calls allow smart contracts on a blockchain to interact with each other. This enables the building of complex decentralized applications (Dapps) from multiple modular components. In a cross-contract call, a function in one smart contract can call a function in another smart contract to leverage existing code and functionality.
+
+This tutorial demonstrates passing data between contracts, handling events, and returning values when contracts call each other.
+
+The full example code is provided in the [12-cross-contract-calls/src/](https://github.com/o1-labs/docs2/tree/main/examples/zkapps/12-cross-contract-calls/src) example files.
+
+## Prerequisites
+
+- Make sure you have the latest version of the zkApp CLI installed:
+
+    ```sh
+    $ npm install -g zkapp-cli
+    ```
+
+- Ensure your environment meets the [Prerequisites](/zkapps/tutorials#prerequisites) for zkApp Developer Tutorials.
+
+This tutorial has been tested with:
+
+- [zkApp CLI](https://github.com/o1-labs/zkapp-cli) version `0.16.0`
+- [o1js](https://www.npmjs.com/package/o1js) version `0.16.2`
+
+## Create a new project
+
+Now that you have the tooling installed, you can start building your application.
+
+1. Create or change to a directory where you have write privileges.
+1. Now, create a project using the `zk project` command:
+
+    ```sh
+    $ zk project 12-cross-contract-calls
+    ```
+
+    As you learned in earlier tutorials, the `zk project` command creates the `12-cross-contract-calls` directory that contains the scaffolding for your project.
+
+1. Change into the `12-cross-contract-calls` directory. 
+
+Like all projects, you run `zk` commands from the root of the `12-cross-contract-calls` directory as you work in the `src` directory on files that contain the TypeScript code for the smart contract. 
+
+Each time you make updates, then build or deploy, the TypeScript code is compiled into JavaScript in the `build` directory.
+
+### Prepare the project
+
+Like earlier tutorials, you can prepare your project by deleting the default files that come with the new project and creating a smart contract called `Composability`.
+
+## Write the ZkProgram
+
+Now, the fun part! Write your smart contract in the `src/Composability.ts` file.
+
+A final version of the smart contract is provided in the [Composability.ts](https://github.com/o1-labs/docs2/blob/main/examples/zkapps/12-cross-contract-calls/src/Composability.ts) example file.
+
+
+### Copy the example
+
+Use the existing code in the [Composability.ts](https://github.com/o1-labs/docs2/blob/main/examples/zkapps/12-cross-contract-calls/src/Composability.ts) example file.
+
+1. First, open the [Composability.ts](https://github.com/o1-labs/docs2/blob/main/examples/zkapps/12-cross-contract-calls/src/Composability.ts) example file.
+
+1. Copy the file's entire contents into your project `src/Composability.ts` file.
+
+### Imports and Incrementer smart contract 
+
+First, bring in imports and set up the first smart contract `Incrementer`.
+
+```typescript
+import {
+  Field,
+  method,
+  Mina,
+  AccountUpdate,
+  PrivateKey,
+  SmartContract,
+  state,
+  State,
+} from 'o1js';
+
+// Contract which adds 1 to a number
+class Incrementer extends SmartContract {
+  @method increment(x: Field): Field {
+    return x.add(1);
+  }
+}
+
+```
+This Incrementer contract adds `1` to the `Field` argument, which is passed.
+
+### Adder smart contract 
+
+Now bring in the second contract `Adder` that returns the addition of two numbers and adds `1` to the result.
+The addition of `1` to the result is outsourced to the `Incrementer` smart contract by creating a new object by passing its address.
+
+```typescript
+// Contract which add two numbers and plus 1 to their sum and return the result
+// Incrementing by one is outsourced to Incrementer contract
+class Adder extends SmartContract {
+  @method addPlus1(x: Field, y: Field): Field {
+    let sum = x.add(y);
+    let incrementer = new Incrementer(incrementerAddress);
+    return incrementer.increment(sum);
+  }
+}
+
+```
+
+### Caller smart contract 
+
+The final smart contract `Caller` calls the `addPlus1()` method of the `Adder` smart contract and emits the stored result that is returned.
+
+```typescript
+// Contract which calls the Adder contract, stores the result on chain & emits an event
+class Caller extends SmartContract {
+  @state(Field) sum = State<Field>();
+  events = { sum: Field };
+
+  @method callAddAndEmit(x: Field, y: Field) {
+    let adder = new Adder(adderAddress);
+    let sum = adder.addPlus1(x, y);
+    this.emitEvent('sum', sum);
+    this.sum.set(sum);
+  }
+}
+
+```
+
+The code to interact with the smart contract:
+
+```typescript
+const doProofs = true;
+
+// Deploy and interact with smart contract locally
+let Local = Mina.LocalBlockchain({ proofsEnabled: doProofs });
+Mina.setActiveInstance(Local);
+
+// Test account that pays all the fees, and puts additional funds into the zkapp
+let feePayerKey = Local.testAccounts[0].privateKey;
+let feePayer = Local.testAccounts[0].publicKey;
+
+// The Incrementer contract's address
+let incrementerKey = PrivateKey.random();
+let incrementerAddress = incrementerKey.toPublicKey();
+
+// The Adder contract's address
+let adderKey = PrivateKey.random();
+let adderAddress = adderKey.toPublicKey();
+
+// The Caller contract's address
+let callerKey = PrivateKey.random();
+let callerAddress = callerKey.toPublicKey();
+
+let callerZkapp = new Caller(callerAddress);
+let adderZkapp = new Adder(adderAddress);
+let incrementerZkapp = new Incrementer(incrementerAddress);
+
+// When doProofs is true, compile contracts to generate prover and verifier keys
+if (doProofs) {
+  console.log('compile (incrementer)');
+  await Incrementer.compile();
+  console.log('compile (adder)');
+  await Adder.compile();
+  console.log('compile (caller)');
+  await Caller.compile();
+}
+
+// Create transaction to deploy contracts
+console.log('deploy');
+let tx = await Mina.transaction(feePayer, () => {
+  AccountUpdate.fundNewAccount(feePayer, 3);
+  callerZkapp.deploy();
+  adderZkapp.deploy();
+  incrementerZkapp.deploy();
+});
+// Sign all four account updates by 
+// passing the corresponding private key for the mentioned public addresses
+await tx.sign([feePayerKey, callerKey, adderKey, incrementerKey]).send();
+
+// Create transaction to interact with the callAddAndEmit method of Caller contract
+console.log('call interaction');
+tx = await Mina.transaction(feePayer, () => {
+  zkapp.callAddAndEmit(Field(5), Field(6));
+});
+console.log('proving (3 proofs.. can take a bit!)');
+await tx.prove();
+console.log(tx.toPretty());
+await tx.sign([feePayerKey]).send();
+
+console.log('state: ' + zkapp.sum.get());
+
+``` 
+
+In this example, you spin up the Mina chain and deploy all three smart contracts locally.
+
+Then you call the `callAddAndEmit` method from the `Caller` smart contract, which takes two numbers as arguments, then call the `Adder` smart contract, which adds these two numbers and passes the result in the `Incrementer` smart contract, which increments the result by 1.
+
+When run successfully, the state is equal to 12.
+
+## Conclusion
+
+Congratulations! You have learned how to implement cross contract calls, allowing smart contracts to interact and unlocking new possibilities for modular blockchain applications.

examples/zkapps/12-cross-contract-calls/.eslintrc.cjs

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+module.exports = {
+  root: true,
+  env: {
+    browser: true,
+    node: true,
+    jest: true,
+  },
+  extends: [
+    'eslint:recommended',
+    'plugin:@typescript-eslint/eslint-recommended',
+    'plugin:@typescript-eslint/recommended',
+    'plugin:o1js/recommended',
+  ],
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    ecmaVersion: 'latest',
+  },
+  plugins: ['@typescript-eslint', 'o1js'],
+  rules: {
+    'no-constant-condition': 'off',
+    'prefer-const': 'off',
+  },
+};

examples/zkapps/12-cross-contract-calls/.gitattributes

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+# Use line endings appropriate for the system. This prevents Git from
+# complaining about project template line endings when committing on Windows.
+* text=auto eol=lf

examples/zkapps/12-cross-contract-calls/.github/workflows/ci.yml

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+#
+# ci.yml
+#
+# Run tests for all pushed commits and opened pull requests on Github.
+#
+
+name: CI
+on: [push, pull_request]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Set up NodeJS
+        uses: actions/setup-node@v4
+        with:
+          node-version: '16'
+      - name: Git checkout
+        uses: actions/checkout@v4
+      - name: NPM ci, build, & test
+        run: |
+          npm install
+          npm run build --if-present
+          npm test
+        env:
+          CI: true

examples/zkapps/12-cross-contract-calls/.gitignore

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+# NodeJS
+node_modules
+build
+coverage
+
+# Editor
+.vscode
+
+# System
+.DS_Store
+
+# Never commit keys to Git!
+keys

examples/zkapps/12-cross-contract-calls/.husky/pre-commit

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+#!/bin/sh
+. "$(dirname "$0")/_/husky.sh"
+
+npx lint-staged

examples/zkapps/12-cross-contract-calls/.npmignore

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+# TypeScript files
+src
+
+# Editor
+.vscode
+
+# System
+.DS_Store
+
+# Never reveal your keys!
+keys

examples/zkapps/12-cross-contract-calls/.prettierignore

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+# NodeJS
+node_modules
+build
+coverage
+.husky
+
+# Editor
+.vscode
+
+# System
+.DS_Store
+
+# Misc
+LICENSE

examples/zkapps/12-cross-contract-calls/.prettierrc

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+{
+  "semi": true,
+  "singleQuote": true,
+  "tabWidth": 2,
+  "trailingComma": "es5"
+}