Clarity Smart Contract Language


Clarity of Mind Foreword Introduction

Unwrap flavours

The other unwrap functions are all variations that exit the current control flow in a slightly different manner.

unwrap! takes an optional or response as the first input and a throw value as the second input. It follows the same unwrapping behaviour of try! , but instead of propagating the the none or the err it will return the the throw value instead.

(unwrap! (some "wrapped string") (err "unwrap failed"))

unwrap-panic takes a single input which is either an optional or response. If it fails to unwrap the input, it throws a runtime error and exits the current flow.

(unwrap-panic (ok true))

unwrap-err! takes a response input and a throw value. If the first input is an err, it will return the wrapped value. Otherwise it returns the throw value and exit. It is the counterpart to unwrap!.

(unwrap-err! (err false) (err "unwrap failed"))

unwrap-err-panic is the counterpart to unwrap-panic, the input is unwrapped if it is an err, or else a runtime error is thrown.

(unwrap-err-panic (err false))

You should ideally not use the -panic variants unless you absolutely have to, because they confer no meaningful information when they fail. A transaction will revert with a vague "runtime error" and users as well as developers are left to figure out exactly what went wrong.

Unpacking assignments

The unwrap functions are particularly useful when assigning local variables using let. You can unwrap and assign a value if it exists or exit if it does not. It makes working with maps and lists a breeze.

;; Some error constants
(define-constant err-unknown-listing (err u100))
(define-constant err-not-the-maker (err u101))

;; Define an example map called listings, identified by a uint.
(define-map listings
    {id: uint}
    {name: (string-ascii 50), maker: principal}

;; Insert some sample data
(map-set listings {id: u1} {name: "First Listing", maker: tx-sender})
(map-set listings {id: u2} {name: "Second Listing", maker: tx-sender})

;; Simple function to get a listing
(define-read-only (get-listing (id uint))
    (map-get? listings {id: id})

;; Update name function that only the maker for a specific listing
;; can call.
(define-public (update-name (id uint) (new-name (string-ascii 50)))
            ;; The magic happens here.
            (listing (unwrap! (get-listing id) err-unknown-listing))
        (asserts! (is-eq tx-sender (get maker listing)) err-not-the-maker)
        (map-set listings {id: id} (merge listing {name: new-name}))
        (ok true)

;; Two test calls
(print (update-name u1 "New name!"))
(print (update-name u9999 "Nonexistent listing..."))

Find the comment ;; The magic happens here. inside the update-name function and study the next line closely. Here is what happens:

  • It defines a variable called listing.
  • The value will be equal to the unwrapped result of the get-listing function.
  • get-listing returns the result of map-get?, which is either some listing or none.
  • If the unwrap fails, unwap! exits with err-unknown-listing.

The first test call will therefore succeed and return (ok true) while the second call will error out with (err u100) (err-unknown-listing).