define-read-only

Function Signature

(define-read-only (function-name (arg-name-0 arg-type-0) ...) function-body)

• Input:
  • function-name: The name of the read-only function
  • arg-name-N: The name of each argument
  • arg-type-N: The type of each argument
  • function-body: The code to be executed when the function is called
• Output: Not applicable (definition statement)

Why it matters

The define-read-only function is crucial for:

1. Creating public functions that can query contract state without modifying it.
2. Enabling efficient and gas-optimized read operations on the contract.
3. Implementing view functions that other contracts or off-chain applications can call.
4. Providing a safe way to expose contract data without risking state changes.

When to use it

Use define-read-only when you need to:

• Create functions that return contract state or computed values.
• Implement getter functions for contract variables or complex data structures.
• Provide public interfaces for querying contract data without modifying state.
• Create helper functions that other contracts can call without incurring state change costs.

Best Practices

• Use descriptive names for read-only functions to clearly indicate their purpose.
• Ensure that read-only functions do not attempt to modify contract state.
• Utilize read-only functions for complex calculations that don't require state changes.
• Consider using read-only functions in combination with public functions to separate read and write operations.

Practical Example: Token Balance Checker

Let's implement a read-only function for checking token balances:

(define-map balances principal uint)

(define-read-only (get-balance (account principal))
  (default-to u0 (map-get? balances account)))

(define-read-only (get-total-supply)
  (fold + (map-values balances) u0))

This example demonstrates:

1. Using define-read-only to create functions for querying token balances.
2. Implementing a getter function for individual account balances.
3. Creating a function to calculate the total token supply without modifying state.

Common Pitfalls

1. Attempting to modify contract state within a read-only function, which will cause an error.
2. Overusing read-only functions for complex calculations that might be better suited for off-chain computation.
3. Forgetting that read-only functions can still consume gas when called, even though they don't modify state.

Related Functions

• define-public: Used to define public functions that can modify contract state.
• define-private: Used to define private functions that can only be called within the contract.
• map-get?: Often used within read-only functions to retrieve data from maps.

Conclusion

The define-read-only function is an essential tool for creating efficient and safe query interfaces in Clarity smart contracts. By providing a way to expose contract data and perform calculations without modifying state, read-only functions enable developers to create more transparent and gas-efficient contracts. When used effectively, they can significantly enhance the usability and accessibility of smart contract data for both on-chain and off-chain applications.