Contract post-conditions

Source: SP2PABAF9FTAJYNFZH93XENAJ8FVY99RRM50D2JG9.xbtc-sbtc-swap-v2

.xbtc-sbtc-swap-v2

;; --snip--

(define-private (transfer-sbtc-to
    (amount uint)
    (sbtc-recipient principal)
  )
  (as-contract?
    ((with-ft 'SM3VDXK3WZZSA84XXFKAFAF15NNZX32CTSG82JFQ4.sbtc-token "sbtc-token"
      amount
    ))
    (try! (contract-call? 'SM3VDXK3WZZSA84XXFKAFAF15NNZX32CTSG82JFQ4.sbtc-token
      transfer amount current-contract sbtc-recipient none
    ))
  )
)

;; --snip--

Description

The as-contract? function resolves the expression caller to the principal of the current contract. Most importantly it allows a contract to protect its own assets during the execution of the expression with a set of specified allowances.

Use Cases

• A contract invoking a body expression that potentially may have unwanted and unexpected asset transfers.
• Protecting a contract invoking another external contract's function.

Key Concepts

• Contract caller context - Switches the current context's tx-sender and contract-caller values to the contract's principal and executes the body expressions within that context, then checks the asset outflows from the contract against the granted allowances, in declaration order.
• Clarity post-conditions - Accepts a set of allowances that limit what the body can do with the contract's assets. with-stx, with-ft and with-nft grant asset outflows. with-staking caps how much STX can be locked through the active PoX contract, and with-pox permits position-altering PoX calls (unstake, unstake-sbtc, update-bond-registration, announce-l1-early-exit). Contracts written in Clarity 4 or 5 use with-stacking instead of with-staking.