What is a view function and a pure function in Solidity and what are their gas implications?

Understanding View Functions in Solidity

1. A view function in Solidity is a type of function that promises not to modify the state of the blockchain. It can read from the contract’s storage variables but cannot alter them, emit events, or call functions that do change state.

2. Because these functions do not alter state, they can be executed locally on an Ethereum node without broadcasting a transaction to the network. This means users don’t have to pay gas fees when calling them externally.
3. The keyword view must be explicitly declared in the function signature. If omitted and the function attempts to modify state, the compiler will throw an error.
4. Examples include retrieving a user’s balance, checking allowances in ERC-20 tokens, or reading configuration parameters stored in the contract.
5. Despite being free for external callers, view functions still consume computational resources on nodes. Therefore, excessively complex logic inside a view function can lead to timeouts or rejections by some providers.

Exploring Pure Functions in Solidity

1. A pure function goes one step further than a view function by promising not to read or write any state variables. It operates solely on the input parameters passed to it.

2. These functions are typically used for mathematical calculations, encoding/decoding operations, or logical checks that don't depend on stored data.
3. The pure keyword enforces this restriction at compile time. Attempting to access even a single state variable will result in a compilation failure.
4. Like view functions, pure functions incur no gas cost when called externally because they don’t require a transaction. They execute off-chain using local node resources.
5. An example would be a function that calculates compound interest based on input arguments or validates a cryptographic signature without referencing contract storage.

Gas Implications of Non-Modifying Functions

1. When a smart contract function is marked as view or pure, external calls to it do not generate transactions. As a result, no gas is charged to the caller.

2. Internally, however, if a view or pure function is called by another function within a transaction, its execution still consumes gas. The EVM must process every operation, regardless of whether state changes occur.
3. Complex computations inside such functions—like looping over large arrays or performing multiple arithmetic operations—increase the gas cost when invoked internally.
4. Developers often optimize performance by minimizing computation in view/pure functions, especially when they are likely to be called during state-changing transactions.
5. Mislabeling a function as view or pure when it actually modifies state leads to runtime exceptions or deployment failures, potentially wasting development time and testnet gas.

Common Questions About View and Pure Functions

Q: Can a view function call a pure function?A: Yes, a view function can safely call a pure function. Since pure functions do not read or modify state, they are fully compatible with the constraints of view functions.

Q: What happens if I try to modify a state variable inside a pure function?A: The Solidity compiler will produce an error. Pure functions are strictly prohibited from accessing any state variables, whether for reading or writing.

Q: Do view and pure functions use gas when called from web3.js or ethers.js?A: No, calling these functions via JavaScript libraries does not require a transaction and therefore incurs no gas cost. The execution happens locally on the connected Ethereum node.

Q: Is there a performance difference between view and pure functions?A: From a gas cost and execution speed perspective, there is no inherent difference when called externally. Both execute locally without state changes. Internally, performance depends on computational complexity rather than the modifier used.