Clarity Smart Contract Language


Clarity of Mind Foreword Introduction

SIP010: the FT standard

(The full SIP can be found here:

Fungible tokens came after for Stacks. Indicative of the numbering, NFTs were ratified first (in SIP009), and FTs followed in SIP010. We discussed what non-fungibility meant in the last section and can thus deduce what a fungible token is supposed to be. These are Bitcoins, STX tokens, US dollars, and so on. If you trade 1 BTC for 1 BTC with somebody else, you end up just where you started (apart from perhaps a loss in transaction fees); in other words, trading the same amount of fungible tokens is an equivalent exchange.

These kinds of tokens are known as ERC-20 tokens in the Ethereum space. Because they are so fundamental, Clarity also features built-in functions to define fungible tokens. The SIP010 standard defines the interface that allows for interoperability, just like how SIP009 does it for NFTs.

The SIP010 FT trait

Creating a SIP010-compliant fungible token also comes down implementing a trait. The trait has a few more features as fungible tokens may be divisible—just like how you have cents to a dollar—and may have a maximum supply.

(define-trait sip010-ft-trait
    ;; Transfer from the caller to a new principal
    (transfer (uint principal principal (optional (buff 34))) (response bool uint))

    ;; the human readable name of the token
    (get-name () (response (string-ascii 32) uint))

    ;; the ticker symbol, or empty if none
    (get-symbol () (response (string-ascii 32) uint))

    ;; the number of decimals used, e.g. 6 would mean 1_000_000 represents 1 token
    (get-decimals () (response uint uint))

    ;; the balance of the passed principal
    (get-balance (principal) (response uint uint))

    ;; the current total supply (which does not need to be a constant)
    (get-total-supply () (response uint uint))

    ;; an optional URI that represents metadata of this token
    (get-token-uri () (response (optional (string-utf8 256)) uint))


A public function that transfers an amount of tokens from one principal to another. If the balance is insufficient, then it must return an err response. The transfer may optionally include a memo which is to be emitted using print. Memos are useful for off-chain indexers and apps like exchanges. If a memo is present, it should be unwrapped and emitted after the token transfer.


A read-only function that returns a human-readable name of the token. The name may then be used in other contracts or off-chain apps.


A read-only functions that returns the ticker symbol for the token. Like how the ticker symbol for Stacks is STX.


Another read-only function that returns the number of decimals for the fungible token. For example, setting it to u6 would indicate that the token is divisible up to six decimal spaces. One unit of the token should then be rendered as 0.000001. This is for display purposes only as the token amount is always represented as an unsigned integer internally.


A read-only functions that returns the the token balance of the provided principal.


A read-only function that returns the current total supply of the token. The total supply need not be a static amount.


A read-only function that returns a valid URI (a link) which resolves to a metadata file for the token. A preliminary structure for the metadata file defined in the SIP is as follows:

  "title": "Asset Metadata",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "description": "Identifies the asset to which this token represents"
    "description": {
      "type": "string",
      "description": "Describes the asset to which this token represents"
    "image": {
      "type": "string",
      "description": "A URI pointing to a resource with mime type image/* representing the asset to which this token represents. Consider making any images at a width between 320 and 1080 pixels and aspect ratio between 1.91:1 and 4:5 inclusive."

The function should return none if the token does not have any off-chain metadata. It is also important to note that the return type for the function contains a (string-utf8 256) as opposed to a (string-ascii 256) string type like we saw in SIP009.