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. 1Creating standardized error responses in public functions.
2. 2Indicating that a function execution has failed.
3. 3Providing meaningful error information to callers.
4. 4Triggering 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
    (
      (senderBalance (default-to u0 (map-get? Balances tx-sender)))
    )
    (if (>= senderBalance amount)
      (begin
        (map-set Balances tx-sender (- senderBalance amount))
        (map-set Balances recipient (+ (default-to u0 (map-get? Balances recipient)) amount))
        (ok true)
      )
      (err u1)
    )
  )
)

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

This example demonstrates:

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

Common Pitfalls

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