err

Function Signature

(err value)

• Input: value - Any type A
• Output: (response A B) where A is the type of the input value

Why it matters

The err function is crucial for:

1. Creating standardized error responses in public functions.
2. Indicating that a function execution has failed.
3. Providing meaningful error information to callers.
4. Triggering automatic rollback of any database changes during function execution.

When to use it

Use err when you need to:

• Return an error state from a public function.
• Indicate that a condition or operation has failed.
• Provide specific error information or codes to the caller.
• Ensure that any state changes are reverted due to a failure condition.

Best Practices

• Use descriptive error values that help diagnose the issue.
• Consider using standardized error codes across your contract.
• Pair err with ok to create comprehensive response handling.
• Remember that returning an err will cause all state changes in the current function to be rolled back.

Practical Example: Token Transfer with Error Handling

Let's implement a simple token transfer function with error handling:

(define-map balances principal uint)

(define-public (transfer (amount uint) (recipient principal))
  (let ((sender-balance (default-to u0 (map-get? balances tx-sender))))
    (if (>= sender-balance amount)
        (begin
          (map-set balances tx-sender (- sender-balance amount))
          (map-set balances recipient (+ (default-to u0 (map-get? balances recipient)) amount))
          (ok true))
        (err u1))))  ;; Error code 1 for insufficient balance

;; Usage
(transfer u100 'ST1PQHQKV0RJXZFY1DGX8MNSNYVE3VGZJSRTPGZGM) ;; Returns (err u1) if balance is insufficient

This example demonstrates:

1. Using err to return an error when the sender has insufficient balance.
2. Pairing err with ok to handle both success and failure cases.
3. Using a simple error code (u1) to indicate the type of error.

Common Pitfalls

1. Forgetting that returning an err will revert all state changes in the current function.
2. Using non-descriptive error values that make debugging difficult.
3. Inconsistent error handling across different functions in the contract.

Related Functions

• ok: Used to construct successful responses in public functions.
• asserts!: Often used with err for condition checking and error reporting.
• try!: Used to propagate errors up the call stack.

Conclusion

The err function is a fundamental tool for error handling and response construction in Clarity smart contracts. By providing a standardized way to indicate and communicate errors, it enables robust and predictable contract behavior. When used effectively in combination with ok and other error-handling mechanisms, err contributes to creating more reliable and maintainable smart contracts.