is-err

Function Signature

(is-err value)

• Input: (response A B)
• Output: bool

Why it matters

The is-err function is crucial for:

1. Determining if a response value represents an error.
2. Implementing error handling and validation logic.
3. Ensuring robust contract behavior by checking for errors.
4. Simplifying conditional checks based on response types.

When to use it

Use is-err when you need to:

• Check if a response value is an error.
• Implement conditional logic based on the success or failure of operations.
• Validate the results of function calls that return response types.
• Handle errors gracefully in your contract logic.

Best Practices

• Use is-err in combination with match or if for comprehensive error handling.
• Ensure that the response value being checked is of the correct type.
• Use meaningful variable names for better readability.
• Combine with other response handling functions like is-ok for complete validation.

Practical Example: Validating a Token Transfer

Let's implement a function that validates a token transfer and checks for errors:

(define-map userBalances { userId: principal } { balance: uint })

(define-public (transfer-tokens (amount uint) (recipient principal))
  (let
    (
      (senderBalance (default-to u0 (map-get? userBalances { userId: tx-sender })))
      (recipientBalance (default-to u0 (map-get? userBalances { userId: recipient })))
      (transferResult (if (>= senderBalance amount)
        (begin
          (map-set userBalances { userId: tx-sender } { balance: (- senderBalance amount) })
          (map-set userBalances { userId: recipient } { balance: (+ recipientBalance amount) })
          (ok true)
        )
        (err u1)
      ))
    )
    (if (is-err transferResult)
      (err u2)
      (ok true)
    )
  )
)

;; Usage
(map-set userBalances { userId: tx-sender } { balance: u100 })
(transfer-tokens u50 'SPAXYA5XS51713FDTQ8H94EJ4V579CXMTRNBZKSF) ;; Returns (ok true)
(transfer-tokens u200 'SPAXYA5XS51713FDTQ8H94EJ4V579CXMTRNBZKSF) ;; Returns (err u2)

This example demonstrates:

1. Using is-err to check if the token transfer operation resulted in an error.
2. Implementing conditional logic based on the success or failure of the transfer.
3. Handling both the success and error cases appropriately.

Common Pitfalls

1. Assuming the response value is always of the correct type, leading to type errors.
2. Not handling all possible error cases, resulting in incomplete error handling.
3. Overlooking the need for comprehensive validation and error checking.
4. Using is-err without meaningful error codes or messages, making debugging harder.

Related Functions

• is-ok: Checks if a response value is ok.
• err: Constructs an error response.
• ok: Constructs a success response.

Conclusion

The is-err function is a fundamental tool for error handling in Clarity smart contracts. It allows developers to check if a response value represents an error, enabling robust and comprehensive error handling and validation logic. When used effectively, is-err enhances the reliability and maintainability of your smart contract code by ensuring that errors are detected and handled appropriately.