Clarity Smart Contract Language


Clarity of Mind Foreword Introduction


Primitive types are the most basic components. These are: signed and unsigned integers, booleans, and principals.

Signed integers

int, short for (signed) integer. These are 128 bits numbers that can either be positive or negative. The minimum value is -2127 and the maximum value is 2127 - 1. Some examples: 0, 5000, -45.

Unsigned integers

uint, short for unsigned integer. These are 128 bits numbers that can only be positive. The minimum value is therefore 0 and the maximum value is 2128 - 1. Unsigned integers are always prefixed by the character u. Some examples: u0, u40935094534.

Clarity has many built-in functions that accept either signed or unsigned integers.


(+ u2 u3)


(- 5 10)


(* u2 u16)


(/ 100 4)

As you might have noticed by now, integers are always whole numbers—there are no decimal points. It is something to keep in mind when writing your code.

(/ u10 u3)

If you punch the above into a calculator, you will likely get 3.3333333333.... Not with integers! The above expression evaluates to u3, the decimals are dropped.

There are many more functions that take integers as inputs. We will get to the rest later in the book.


bool, short for boolean. A boolean value is either true or false. They are used to check if a certain condition is met or unmet (true or false). Some built-in functions that accept booleans:

not (inverts a boolean):

(not true)

and (returns true if all inputs are true):

(and true true true)

or (returns true if at least one input is true):

(or false true false)


A principal is a special type in Clarity and represents a Stacks address on the blockchain. It is a unique identifier you can roughly equate to an email address or bank account number—although definitely not the same! You might have also heard the term wallet address as well. Clarity admits two different kinds of principals: standard principals and contract principals. Standard principals are backed by a corresponding private key whilst contract principals point to a smart contract. Principals follow a specific structure and always start with the characters SP for the Stacks mainnet and ST for the testnet and mocknet1.

A literal principal value is prefixed by a single quote (') in Clarity. Notice there is no closing single quote.


Contract principals are a compound of the standard principal that deployed the contract and the contract name, delimited by a dot:


You will use the principal type often when writing Clarity. It is used to check who is calling the contract, recording information about different principals, function calls across contracts, and much more.

To retrieve the current STX balance of a principal, we can pass it to the stx-get-balance function.

(stx-get-balance 'ST1HTBVD3JG9C05J7HBJTHGR0GGW7KXW28M5JS8QE)

Both kinds of principals can hold tokens, we can thus also check the balance of a contract.

(stx-get-balance '

Zero balances are a little boring, so let us send some some STX to a principal:

(stx-transfer? u500 tx-sender 'ST1HTBVD3JG9C05J7HBJTHGR0GGW7KXW28M5JS8QE)

Knowing about primitives, and the fact that types can never mix, it is now clear why the example in the previous section does not work. Since the first number is a signed integer and the next one is an _unsigned integer_—notice the u—the analyser rejects the code as invalid. We should provide it with two signed or two unsigned integers instead.


(+ 2 u3)


(+ u2 u3)

1: More on the different kinds of networks in a later chapter.