---
layout: core
description: "Blockstack smart contracting language"
permalink: /:collection/:path.html
---
# Hello Clarity

| Audience | | **Beginners**  |
| Duration | | **15 minutes** |

In this tutorial, you learn how to use Clarity, Blockstack's smart contracting language inside of a virtual environment. The environment is run using a [Docker](https://www.docker.com/) image. Use this tutorial to get a quick introduction to Clarity and the default Blockstack test environment.

By the end of this tutorial you will ...

* Have working test environment to run Clarity smart contracts
* Have access to a set of sample contracts to play with
* Understand basic Clarity language design principles
* Deploy and run your own smart contract

## Overview

* TOC
{:toc}

<div class="uk-card uk-card-default uk-card-body">
<h5>Early Release</h5>
<p>Clarity and its accompanying toolset are in early release. If you encounter issues with or have feature requests regarding Clarity, please create an issue on the <a href='https://github.com/blockstack/stacks-blockchain/issues' target='_blank'>blockstack/stacks-blockchain</a> repository. To read previous or join ongoing discussions about smart contracts in general and Clarity in particular, visit the <strong><a href='https://forum.blockstack.org/c/clarity' target='_blank'>Smart Contracts</a></strong> topic in the Blockstack Forum.
</p>
</div>

## Before you begin (pre-requisites)

The Clarity language goes live with the release of Stacks2.0. Until then, you can run Clarity in a test environment. You run this test environment in a Docker container. Before you begin this tutorial, make sure you have <a href="https://docs.docker.com/get-docker/" target="_blank">Docker installed on your workstation</a>.

You can verify your Docker installation by running the following in your terminal:

```bash
docker verion
```

You should see the version of your Docker engine running on your workstation.

## Step 1: Set up the test environment

In this step, you use the [Docker CLI](https://docs.docker.com/engine/reference/commandline/cli/) to pull and run the image on your local workstation.

1. Using your terminal, download the latest [Stacks 2.0 blockchain Docker image](https://hub.docker.com/r/blockstack/stacks-blockchain) from the Docker Hub:

    ```bash
    docker pull blockstack/stacks-blockchain:latest
    ```

2. Now, you can start the Stack2.0 test environment in your terminal:

    ```bash
    docker run -it -v $HOME/blockstack-dev-data:/data/ blockstack/stacks-blockchain:latest bash
    ```

    The command opens a bash shell inside the Docker image, so that you can operate in the test environment.

## Step 2: Review sample contracts
With a test environment running on your workstation, let's have a look at some sample contracts implemented with Clarity.

1. Still inside the bash shell of the Docker image, list the contents of the `sample-contracts` directory.

    ```bash
    ls sample-contracts/
    ```

    This directory contains a set of simple Clarity contracts. Note that all Clarity files have a `.clar` suffix.

2. Let's review the contents of `tokens.clar` with the `cat` command.

    ```bash
    cat sample-contracts/tokens.clar
    ```

    You should see the contract source code. Take a few minutes to review the content.

    Clarity is a programming language based on [LISP](https://en.wikipedia.org/wiki/Lisp_(programming_language)). Most notably, Clarity is designed for static analysis, not compiled, and **not** [Turing complete](https://en.wikipedia.org/wiki/Turing_completeness).

    Let's go through the source code:

    ```cl
    (define-map tokens ((account principal)) ((balance uint)))
    (define-private (get-balance (account principal))
        (default-to u0 (get balance (map-get? tokens (tuple (account account))))))
    ```

    Notice the program and each statement is enclosed in `()` (parentheses).

     The first line defines a map for `tokens` with the account-balance key-value pairs. The `account` key is of the [`principal`](https://docs.blockstack.org/core/smart/clarityref#principal-type) type. Principals represent a spending entity and are roughly equivalent to a Stacks address. The `balance` value is an unsigned integer ([`uint`](https://docs.blockstack.org/core/smart/clarityref#uint-type)). Along with principals and signed/unsigned integers, Clarity supports the following types:
    * [booleans](https://docs.blockstack.org/core/smart/clarityref#bool-type)
    * [fixed-length buffers](https://docs.blockstack.org/core/smart/clarityref#buffer-type)
    * [tuples](https://docs.blockstack.org/core/smart/clarityref#tuple-type)

     On line 2 and 3, a `get-balance` function is declared as a private function. To create public functions, you would use the `define-public` function. Public functions can be called from DApps, CLIs, or other contracts.
     The `get-balance` function returns the value (`balance`) for the key provided (`account`). Using `default-to`, an unsigned integer `0` (note the `u0` literal to differentiate between signed integers) is returned if the account key cannot be found in the `tokens` map.

     Let's look at the next method definition. The private method `token-credit!` is defined below. It takes a principal and unsigned integer as input parameters:

    ```cl
    (define-private (token-credit! (account principal) (amount uint))
    (if (<= amount u0)
        (err "must move positive balance")
        (let ((current-amount (get-balance account)))
            (begin
            (map-set tokens (tuple (account account))
                        (tuple (balance (+ amount current-amount))))
            (ok amount)))))
    ```

    On the first line of the method, we see how conditions can be used with the [`if`](https://docs.blockstack.org/core/smart/clarityref#if) statement. The if statement takes a boolean argument and two expressions. The first expression is executed when the boolean argument evalutes true. In this case, [`err`](https://docs.blockstack.org/core/smart/clarityref#int-type) is used to return an error response type with the error message provided.

    --> TODO

    Variables are created via [`let`](https://docs.blockstack.org/core/smart/clarityref#let) binding, but there is no support for mutating functions like `set`.
    - Describe: `token-credit`, `!`, `current-amount`, `begin`, `ok`

    Every smart contract has both a data space and code. The data space of a contract may only interact with that contract. This particular function is interacting with a map named `tokens` (defined on line 1). The [`map-set`](https://docs.blockstack.org/core/smart/clarityref#map-set) function sets the value associated with the input key to the inputted value in the `tokens` data map.

    In the first `token-transfer` public function, you see that it calls the private `get-balance` function and passes it `tx-sender`. The `tx-sender` is a globally defined variable that represents the current principal.

```cl
(define-public (token-transfer (to principal) (amount uint))
  (let ((balance (get-balance tx-sender)))
    (if (or (> amount balance) (<= amount u0))
        (err "must transfer positive balance and possess funds")
        (begin
          (map-set tokens (tuple (account tx-sender))
                      (tuple (balance (- balance amount))))
          (token-credit! to amount)))))

(define-public (mint! (amount uint))
   (let ((balance (get-balance tx-sender)))
     (token-credit! tx-sender amount)))

(token-credit! 'SZ2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQ9H6DPR u10000)
(token-credit! 'SM2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQVX8X0G u300)
```

The final two lines of the program pass a principal, represented by a Stacks address, and an amount to the private user-defined `token-credit` function.

Smart contracts may call other smart contracts using a `contract-call!` function. This ability means that if a transaction invokes a function in a given smart contract, that function is able to make calls into other smart contracts on your behalf. The ability to read and do a static analysis of Clarity code allows clients to learn which functions a given smart contract will ever call. Good clients should always warn users about any potential side effects of a given transaction.

Take a moment to `cat` the contents of the `sample-contracts/names.clar` file.

```bash
cat sample-contracts/names.clar
````

Which `tokens.clar` function is being called? 

## Task 3: Initialize data-space and launch contracts

In this task, you interact with the the contracts using the `clarity-cli` command line. 

1. Initialize a new `db` database in the `/data/` directory

    ```bash
    #  clarity-cli initialize /data/db
    Database created
    ```

    You should see a message saying `Database created`. The command creates an SQLlite database.  The database is available in the container and also in your workstation. In this tutorial, your workstation mount should, at this point, contain the `$HOME/blockstack-dev-data/db`  directory.

2. Type check the `names.clar` contract.

    ```bash
    #  clarity-cli check sample-contracts/names.clar /data/db
    ```
    
    You should get an error:

    ```
    Error (line 11, column 1): use of unresolved contract ''S1G2081040G2081040G2081040G208105NK8PE5.tokens'.
    ```

    This happens because the `names.clar` contract _calls_ the `tokens.clar` contract, and that contract has not been created on the blockchain.

3. Type check the `tokens.clar` contract, it should pass a check as it does not use the `contract-call` function:

    ```bash
    # clarity-cli check sample-contracts/tokens.clar /data/db
    Checks passed.
    ```

    When the `check` command executes successfully and exits with the stand UNIX `0` exit code.

4. Generate a demo Stacks address for testing your contract.

   This address is used to name your contract at launch time. You can use any existing Stacks address. For this sample, you are going to use the `generate_address` command to create one.

   ```bash
   # clarity-cli generate_address
   SP28Z69HE5H70BVRG4VGKN4SYNVJ1J0417WVCKZWM
   ```

   The demo address you generate will be different than the one that appears in this example. 

5. Add the address to your environment.

    ```bash
    # DEMO_ADDRESS=SP28Z69HE5H70BVRG4VGKN4SYNVJ1J0417WVCKZWM
    ```

6. Launch the `tokens.clar` contract and assign it to your `DEMO_ADDRESS` address.

   You use the `launch` command to instantiate a contract on the Stacks blockchain. If you have dependencies between contracts, for example `names.clar` is dependent on `tokens.clar`, you must launch the dependency first.

    ```bash
    # clarity-cli launch $DEMO_ADDRESS.tokens sample-contracts/tokens.clar /data/db
    Contract initialized!
    ```

    Once launched, you can execute the contract or a public method on the contract. Your development database has an instantiated `tokens` contract. If you were to close the container and restart it later with the same mount point and you wouldn't need to relaunch that database; it persists until you remove it from your local drive.
   
7. Instantiate the `names.clar` contract and assign it to your `DEMO_ADDRESS` address. as well.

    ```bash
    # clarity-cli launch $DEMO_ADDRESS.names sample-contracts/names.clar /data/db
    Contract initialized!
    ```

## Task 4. Examine the SQLite database

The test environment uses a SQLite database to represent a virtual blockchain. You initialized this database when you ran this earlier:

```bash
clarity-cli initialize /data/db
```

As you work the contracts, data is added to the `db` database because you pass this database as a parameter, for example:

```bash
clarity-cli launch $DEMO_ADDRESS.tokens sample-contracts/tokens.clar /data/db
```

The database exists on your local workstation and persists through restarts of the container. You can use this database to explore the transactional effects of your Clarity programs. The SQLite database includes a single `data_table` and a set of `marf` structures.

While not required, you can install SQLite in your local environment and use it to examine the data associated with and impacted by your contract. For example, this what the `data_able` contains after you initialize the `tokens` contract.

<img src="../images/sqlite-contract.png" alt="">

The `marf` directory defines a data structure that handles key-value lookups in the presence of blockchain forks. These structures are not intended for use in debugging, they simply support the implementation.


## Task 5: Execute a public function

In this section, you use the public `mint!` function in the  `tokens` contract to mint some new tokens. 

1. Get the current balance of your new address.

   ```bash
    # echo "(get-balance '$DEMO_ADDRESS)" | clarity-cli eval $DEMO_ADDRESS.tokens /data/db
    Program executed successfully! Output: 
    0
    ```

    This command uses the private `get-balance` function in the `tokens` contract and pipes the result to the `eval` subcommand. The `eval` subcommand lets you evaluate both public and _private_ functions of a contract in read-only mode.

2. Try minting some tokens and sending them to an address we'll use for our demo.

    ```bash
    # clarity-cli execute /data/db $DEMO_ADDRESS.tokens mint! $DEMO_ADDRESS u100000
    Transaction executed and committed. Returned: 100000
    ```

    This executes the public `mint!` function defined in the tokens contract, sending 100000 tokens to you `$DEMO_ADDRESS`.

3. Use the `clarity-cli eval` command to check the result of this call.

    ```bash
    # echo "(get-balance '$DEMO_ADDRESS)" | clarity-cli eval $DEMO_ADDRESS.tokens /data/db
    Program executed successfully! Output: 
    100000
    ```

## Task 6: Spend tokens by registering a name 

Now, let's register a name using the `names.clar` contract. Names can _only_ be integers in this sample contract, so you'll register the name 10 in this environment.

1. Compute the hash of the name we want to register. 

   You'll _salt_ the hash with the salt `8888`:

    ```bash
    # echo "(hash160 (xor 10 8888))" | clarity-cli eval $DEMO_ADDRESS.names /data/db
    Program executed successfully! Output: 
    0xb572fb1ce2e9665f1efd0994fe077b50c3a48fde
    ```

    The value of the name hash is:

    ```
    0xb572fb1ce2e9665f1efd0994fe077b50c3a48fde
    ```

2. Preorder the name using the _execute_ command:

    ```bash
    # clarity-cli execute /data/db $DEMO_ADDRESS.names preorder $DEMO_ADDRESS 0xb572fb1ce2e9665f1efd0994fe077b50c3a48fde u1000
    e077b50c3a48fde 1000
    Transaction executed and committed. Returned: 0
    ```

    This executes the public `preorder` function defined in the `names.clar` contract. The function reserves a name by paying the name fee (in this case, 1000 tokens).

3. Check the demo address' new balance:

    ```bash
    # echo "(get-balance '$DEMO_ADDRESS)" | clarity-cli eval $DEMO_ADDRESS.tokens /data/db
    Program executed successfully! Output: 
    99000
    ```

4. Register the name by executing the _register_ function:

    ```bash
    # clarity-cli execute /data/db $DEMO_ADDRESS.names register $DEMO_ADDRESS \'$DEMO_ADDRESS 10 8888
    Transaction executed and committed. Returned: 0
    ```

5. Lookup the "owner address" for the name:

    ```bash
    # echo "(get owner (map-get name-map (tuple (name 10))))" | clarity-cli eval $DEMO_ADDRESS.names /data/db
    Program executed successfully! Output: 
    (some 'SP2Y8T8RWWXFR8S1XBP6K0MHCQF01D552FSWD9M4E)
    ```

## Where to go next
{:.no_toc}

* <a href="clarityRef.html">Clarity Language Reference</a>
* <a href="clarityCLI.html">clarity-cli command line</a>