is-standard

Function Signature

(is-standard value)

• Input: principal
• Output: bool

Why it matters

The is-standard function is crucial for:

1. Determining if a principal is a standard principal (i.e., a user address).
2. Implementing conditional logic based on the type of principal.
3. Ensuring that certain operations are only performed by standard principals.
4. Simplifying checks for principal types in smart contract code.

When to use it

Use is-standard when you need to:

• Check if a principal is a standard user address.
• Implement logic that depends on the type of principal.
• Validate the type of principal before performing certain operations.
• Handle cases where only standard principals are allowed.

Best Practices

• Use is-standard in combination with if or match for comprehensive principal type handling.
• Ensure that the value being checked is of the correct principal type.
• Use meaningful variable names for better readability.
• Note that you can pass in a valid contract principal as well, not just a standard principal (e.g., 'SP12 or 'SP12.contract).

Practical Example: Restricting Access to Standard Principals

Let's implement a function that restricts access to standard principals:

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

(define-public (transfer-tokens (amount uint) (recipient principal))
  (if (is-standard tx-sender)
    (let
      (
        (senderBalance (default-to u0 (map-get? UserBalances { userId: tx-sender })))
        (recipientBalance (default-to u0 (map-get? UserBalances { userId: recipient })))
      )
      (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)
      )
    )
    (err u2)
  )
)

;; Usage
(map-set UserBalances { userId: tx-sender } { balance: u100 })
(transfer-tokens u50 'SPAXYA5XS51713FDTQ8H94EJ4V579CXMTRNBZKSF) ;; Returns (ok true) if tx-sender is a standard principal

This example demonstrates:

1. Using is-standard to check if the transaction sender is a standard principal.
2. Implementing conditional logic based on the type of principal.
3. Handling both the case where the principal is standard and where it is not.

Common Pitfalls

1. Assuming the principal will always be standard, leading to unhandled cases.
2. Using is-standard on non-principal types, causing type errors.
3. Not handling all possible conditions, resulting in incomplete principal type checks.
4. Overlooking the need for comprehensive validation and error checking.

Related Functions

• tx-sender: Returns the principal that initiated the transaction.
• contract-caller: Returns the caller of the current contract context.

Conclusion

The is-standard function is a fundamental tool for checking principal types in Clarity smart contracts. It allows developers to determine if a principal is a standard user address, enabling robust and comprehensive principal type handling and validation logic. When used effectively, is-standard enhances the reliability and maintainability of your smart contract code by ensuring that operations are performed by the correct type of principal.