Private functions are defined in the same manner as public functions. The difference is that they can only be called by the current contract. They cannot be called from other smart contracts, nor can they be called directly by sending a transaction. Private functions are useful to create utility or helper functions to cut down on code repetition. If you find yourself repeating the similar expressions in multiple locations, then it is worth considering turning those expressions into a separate private function.
The contract bellow allows only the contract owner to update the
recipients map via two public functions. Instead of having to repeat the
tx-sender check, it is abstracted away to its own private function called
(define-constant contract-owner tx-sender) ;; Try removing the contract-owner constant above and using a different ;; one to see the example calls error out: ;; (define-constant contract-owner 'ST20ATRN26N9P05V2F1RHFRV24X8C8M3W54E427B2) (define-constant err-invalid-caller (err u1)) (define-map recipients principal uint) (define-private (is-valid-caller) (is-eq contract-owner tx-sender) ) (define-public (add-recipient (recipient principal) (amount uint)) (if (is-valid-caller) (ok (map-set recipients recipient amount)) err-invalid-caller ) ) (define-public (delete-recipient (recipient principal)) (if (is-valid-caller) (ok (map-delete recipients recipient)) err-invalid-caller ) ) ;; Two example calls to the public functions: (print (add-recipient 'ST1J4G6RR643BCG8G8SR6M2D9Z9KXT2NJDRK3FBTK u500)) (print (delete-recipient 'ST1J4G6RR643BCG8G8SR6M2D9Z9KXT2NJDRK3FBTK))
Another good reason to define private functions it to reduce overall function complexity. Large public functions can be harder to maintain and are more prone to developer error. Splitting such functions up into a public function and a number of smaller private functions can alleviate these issues.
Private functions may return any type, including responses, although returning an
ok or an
err will have no effect on the materialised state of the chain.
(define-constant err-invalid-caller (err u1)) (define-map authorised-callers principal bool) (define-map recipients principal bool) (map-set recipients tx-sender true) (map-set authorised-callers 'ST20ATRN26N9P05V2F1RHFRV24X8C8M3W54E427B2 true) (define-private (is-valid-caller (caller principal)) ;; Implement. ) (define-public (delete-recipient (recipient principal)) (if (is-valid-caller tx-sender) (ok (map-delete recipients recipient)) err-invalid-caller ) ) (print (delete-recipient tx-sender))