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 image. Use this tutorial to get a quick introduction to Clarity and the default Blockstack test environment.
- Before you begin (pre-requisites)
- Task 1: Set up the test environment
- Task 2: Review a simple Clarity program
- Task 3: Initialize data-space and launch contracts
- Task 4. Examine the SQLite database
- Task 5: Execute a public function
- Task 6: Spend tokens by registering a name
Clarity is in pre-release
Clarity and its accompanying toolset are in pre-release. If you encounter issues with or have feature requests regarding Clarity, please create an issue on the blockstack/blockstack-core repository. To read previous or join ongoing discussions about smart contracts in general and Clarity in particular, visit the Smart Contracts topic in the Blockstack Forum.
Before you begin (pre-requisites)
The Clarity language goes live in the next Stacks blockchain fork. Until the fork, 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 Docker installed on your workstation.
If, for some reason, you don’t want to run the test environment with Docker, you can build and maintain a local environment. Instructions for downloading and building the environment are available in the
blockstack/blockstack-core repository’s README file.
Task 1: Set up the test environment
Blockstack publishes the
clarity-developer-preview image on Docker hub. A container built from this image contains sample programs, the Blockstack Core, and tools for working with them. In this task, you use Docker to pull and run the image on your local workstation.
Pull the Blockstack core
clarity-developer-previewimage from Docker Hub.
$ docker pull blockstack/blockstack-core:clarity-developer-preview
Start the Blockstack Core test environment with a Bash shell.
$ docker run -it -v $HOME/blockstack-dev-data:/data/ blockstack/blockstack-core:clarity-developer-preview bash
The command launches a container with the Clarity test environment and opens a bash shell into the container. The
-vflag creates a local
$HOME/blockstack-dev-datadirectory in your workstation and mounts it at the
/datadirectory inside the container. The shell opens into the
src/blockstack-coredirectory. This directory contains the source for a core and includes Clarity contract samples you can run.
List the contents of the
root@f88368ba07b2:/src/blockstack-core# ls sample-programs/ names.clar tokens.clar
The sample program’s directory contains two simple Clarity programs. Clarity code files have a
Go ahead and display the contents of the
tokens.clarprogram with the
root@c28600552694:/src/blockstack-core# cat sample-programs/tokens.clar
The next section gives you an introduction to the Clarity language by way of examining this program’s code.
Task 2: Review a simple Clarity program
If you haven’t already done so, use the
more command to display the
tokens.clar file’s code. Clarity is designed for static analysis; it is not a compiled language and is not Turing complete. It language is a LISP-like language. LISP is an acronym for list processing.
The first lines of the
tokens.clar program contains a user-defined
(define-map tokens ((account principal)) ((balance int))) (define-private (get-balance (account principal)) (default-to 0 (get balance (map-get tokens (tuple (account account))))))
get-balance is a private function because it is constructed with the
define-private call. To create public functions, you would use the
define-public function. Public functions can be called from other contracts or even from the command line with the
Notice the program is enclosed in
() (parentheses) and each statement as well. The
get-balance function takes an
account argument of the special type
principal. Principals represent a spending entity and are roughly equivalent to a Stacks address.
Along with the
principal types, Clarity supports booleans, integers, and fixed-length buffers. Variables are created via
let binding, but there is no support for mutating functions like
The next sequence of lines shows an
if statement that allows you to set conditions for execution in the language.
(define-private (token-credit! (account principal) (amount int)) (if (<= amount 0) (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)))))
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
set-entry! function is a native function that sets the value associated with the input key to the inputted value in the
tokens data map. Because
set-entry! mutates data so it has an
! exclamation point; this is by convention in Clarity.
In the first
token-transfer public function, you see that it calls the private
get-balance function and passes it
tx-sender is a globally defined variable that represents the current principal.
(define-public (token-transfer (to principal) (amount int)) (let ((balance (get-balance tx-sender))) (if (or (> amount balance) (<= amount 0)) (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 int)) (let ((balance (get-balance tx-sender))) (token-credit! tx-sender amount))) (token-credit! 'SZ2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQ9H6DPR 10000) (token-credit! 'SM2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQVX8X0G 300)
The final two lines of the program pass a principal, represented by a Stacks address, and an amount to the private user-defined
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
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.
Initialize a new
dbdatabase in the
# 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
Type check the
# clarity-cli check sample-programs/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.clarcontract calls the
tokens.clarcontract, and that contract has not been created on the blockchain.
Type check the
tokens.clarcontract, it should pass a check as it does not use the
# clarity-cli check sample-programs/tokens.clar /data/db Checks passed.
checkcommand executes successfully and exits with the stand UNIX
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_addresscommand to create one.
# clarity-cli generate_address SP28Z69HE5H70BVRG4VGKN4SYNVJ1J0417WVCKZWM
The demo address you generate will be different than the one that appears in this example.
Add the address to your environment.
tokens.clarcontract and assign it to your
You use the
launchcommand to instantiate a contract on the Stacks blockchain. If you have dependencies between contracts, for example
names.claris dependent on
tokens.clar, you must launch the dependency first.
# clarity-cli launch $DEMO_ADDRESS.tokens sample-programs/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
tokenscontract. 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.
names.clarcontract and assign it to your
DEMO_ADDRESSaddress. as well.
# clarity-cli launch $DEMO_ADDRESS.names sample-programs/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:
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:
clarity-cli launch $DEMO_ADDRESS.tokens sample-programs/tokens.clar /data/db
The database exists on your local workstation and persists through restarts of the container. You can use this database to explore the transactional effects of your Clarity programs. The SQLite database includes a single
data_table and a set of
While not required, you can install SQLite in your local environment and use it to examine the data associated with and impacted by your contract. For example, this what the
data_able contains after you initialize the
marf directory defines a data structure that handles key-value lookups in the presence of blockchain forks. These structures are not intended for use in debugging, they simply support the implementation.
Task 5: Execute a public function
In this section, you use the public
mint! function in the
tokens contract to mint some new tokens.
Get the current balance of your new address.
# echo "(get-balance '$DEMO_ADDRESS)" | clarity-cli eval $DEMO_ADDRESS.tokens /data/db Program executed successfully! Output: 0
This command uses the private
get-balancefunction in the
tokenscontract and pipes the result to the
evalsubcommand lets you evaluate both public and private functions of a contract in read-only mode.
Try minting some tokens and sending them to an address we’ll use for our demo.
# clarity-cli execute /data/db $DEMO_ADDRESS.tokens mint! $DEMO_ADDRESS 100000 Transaction executed and committed. Returned: 100000
This executes the public
mint!function defined in the tokens contract, sending 100000 tokens to you
clarity-cli evalcommand to check the result of this call.
# echo "(get-balance '$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.
Compute the hash of the name we want to register.
You’ll salt the hash with the salt
# 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:
Preorder the name using the execute command:
# clarity-cli execute /data/db $DEMO_ADDRESS.names preorder $DEMO_ADDRESS 0xb572fb1ce2e9665f1efd0994fe077b50c3a48fde 1000 e077b50c3a48fde 1000 Transaction executed and committed. Returned: 0
This executes the public
preorderfunction defined in the
names.clarcontract. The function reserves a name by paying the name fee (in this case, 1000 tokens).
Check the demo address’ new balance:
# echo "(get-balance '$DEMO_ADDRESS)" | clarity-cli eval $DEMO_ADDRESS.tokens /data/db Program executed successfully! Output: 99000
Register the name by executing the register function:
# clarity-cli execute /data/db $DEMO_ADDRESS.names register $DEMO_ADDRESS \'$DEMO_ADDRESS 10 8888 Transaction executed and committed. Returned: 0
Lookup the “owner address” for the name:
# echo "(get owner (map-get name-map (tuple (name 10))))" | clarity-cli eval $DEMO_ADDRESS.names /data/db Program executed successfully! Output: (some 'SP2Y8T8RWWXFR8S1XBP6K0MHCQF01D552FSWD9M4E)