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Call Types

What is a Call​

We say that a smart contract is called when one of its functions is invoked and its code is run. This means there'll be:

  • a caller
  • arguments
  • return values
  • a call status (successful or failed)

There are multiple types of calls, and some of the naming can make things very confusing. This page lists the different call types and execution modes, pointing out key differences between them.

Ethereum Call Types​

Even though we're discussing Aztec, its design is heavily influenced by Ethereum and many of the APIs and concepts are quite similar. It is therefore worthwhile to briefly review how things work there and what naming conventions are used to provide context to the Aztec-specific concepts.

Broadly speaking, Ethereum contracts can be thought of as executing as a result of three different things: running certain EVM opcodes, running Solidity code (which compiles to EVM opcodes), or via the node JSON-RPC interface (e.g. when executing transactions).

EVM​

Certain opcodes allow contracts to make calls to other contracts, each with different semantics. We're particularly interested in CALL and STATICCALL, and how those relate to contract programming languages and client APIs.

CALL​

This is the most common and basic type of call. It grants execution control to the caller until it eventually returns. No special semantics are in play here. Most Ethereum transactions spend the majority of their time in CALL contexts.

STATICCALL​

This behaves almost exactly the same as CALL, with one key difference: any state-changing operations are forbidden and will immediately cause the call to fail. This includes writing to storage, emitting logs, or deploying new contracts. This call is used to query state on an external contract, e.g. to get data from a price oracle, check for access control permissions, etc.

Others​

The CREATE and CREATE2 opcodes (for contract deployment) also result in something similar to a CALL context, but all that's special about them has to do with how deployments work. DELEGATECALL (and CALLCODE) are somewhat complicated to understand but don't have any Aztec equivalents, so they are not worth covering.

Solidity​

Solidity (and other contract programming languages such as Vyper) compile down to EVM opcodes, but it is useful to understand how they map language concepts to the different call types.

Mutating External Functions​

These are functions marked payable (which can receive ETH, which is a state change) or with no mutability declaration (sometimes called nonpayable). When one of these functions is called on a contract, the CALL opcode is emitted, meaning the callee can perform state changes, make further CALLs, etc.

It is also possible to call such a function with STATICCALL manually (e.g. using assembly), but the execution will fail as soon as a state-changing opcode is executed.

view​

An external function marked view will not be able to mutate state (write to storage, etc.), it can only view the state. Solidity will emit the STATICCALL opcode when calling these functions, since its restrictions provide added safety to the caller (e.g. no risk of reentrancy).

Note that it is entirely possible to use CALL to call a view function, and the result will be the exact same as if STATICCALL had been used. The reason why STATICCALL exists is so that untrusted or unknown contracts can be called while still being able to reason about correctness. From the EIP:

'STATICCALL adds a way to call other contracts and restrict what they can do in the simplest way. It can be safely assumed that the state of all accounts is the same before and after a static call.'

JSON-RPC​

From outside the EVM, calls to contracts are made via JSON-RPC methods, typically from some client library that is aware of contract ABIs, such as ethers.js or viem.

eth_sendTransaction​

This method is how transactions are sent to a node to get them to be broadcast and eventually included in a block. The specified to address will be called in a CALL context, with some notable properties:

  • there are no return values, even if the contract function invoked does return some data
  • there is no explicit caller: it is instead derived from a provided signature

Some client libraries choose to automatically issue eth_sendTransaction when calling functions from a contract ABI that are not marked as view - ethers is a good example. Notably, this means that any return value is lost and not available to the calling client - the library typically returns a transaction receipt instead. If the return value is required, then the only option is to simulate the call eth_call.

Note that it is possible to call non state-changing functions (i.e. view) with eth_sendTransaction - this is always meaningless. What transactions do is change the blockchain state, so all calling such a function achieves is for the caller to lose funds by paying for gas fees. The sole purpose of a view function is to return data, and eth_sendTransaction does not make the return value available.

eth_call​

This method is the largest culprit of confusion around calls, but unfortunately requires understanding of all previous concepts in order to be explained. Its name is also quite unhelpful.

What eth_call does is simulate a transaction (a call to a contract) given the current blockchain state. The behavior will be the exact same as eth_sendTransaction, except:

  • no actual transaction will be created
  • while gas will be measured, there'll be no transaction fees of any kind
  • no signature is required: the from address is passed directly, and can be set to any value (even if the private key is unknown, or if they are contract addresses!)
  • the return value of the called contract is available

eth_call is typically used for one of the following:

  • query blockchain data, e.g. read token balances
  • preview the state changes produced by a transaction, e.g. the transaction cost, token balance changes, etc

Because some libraries (such as ethers) automatically use eth_call for view functions (which when called via Solidity result in the STATICCALL opcode), these concepts can be hard to tell apart. The following bears repeating: an eth_call's call context is the same as eth_sendTransaction, and it is a CALL context, not STATICCALL.

Aztec Call Types​

Large parts of the Aztec Network's design are still not finalized, and the nitty-gritty of contract calls is no exception. This section won't therefore contain a thorough review of these, but rather list some of the main ways contracts can currently be interacted with, with analogies to Ethereum call types when applicable.

While Ethereum contracts are defined by bytecode that runs on the EVM, Aztec contracts have multiple modes of execution depending on the function that is invoked.

Private Execution​

Contract functions marked with #[private] can only be called privately, and as such 'run' in the user's device. Since they're circuits, their 'execution' is actually the generation of a zk-SNARK proof that'll later be sent to the sequencer for verification.

Private Calls​

Private functions from other contracts can be called either regularly or statically by using the .call() and .static_call functions. They will also be 'executed' (i.e. proved) in the user's device, and static_call will fail if any state changes are attempted (like the EVM's STATICCALL).

private_call
let _ = Token::at(stable_coin).burn_private(from, amount, nonce).call(&mut context);
Source code: noir-projects/noir-contracts/contracts/lending_contract/src/main.nr#L267-L269

Unlike the EVM however, private execution doesn't revert in the traditional way: in case of error (e.g. a failed assertion, a state changing operation in a static context, etc.) the proof generation simply fails and no transaction request is generated, spending no network gas or user funds.

Public Calls​

Since public execution can only be performed by the sequencer, public functions cannot be executed in a private context. It is possible however to enqueue a public function call during private execution, requesting the sequencer to run it during inclusion of the transaction. It will be executed in public normally, including the possibility to enqueue static public calls.

Since the public call is made asynchronously, any return values or side effects are not available during private execution. If the public function fails once executed, the entire transaction is reverted including state changes caused by the private part, such as new notes or nullifiers. Note that this does result in gas being spent, like in the case of the EVM.

enqueue_public
Lending::at(context.this_address())
    ._deposit(AztecAddress::from_field(on_behalf_of), amount, collateral_asset)
    .enqueue(&mut context);
Source code: noir-projects/noir-contracts/contracts/lending_contract/src/main.nr#L117-L121

It is also possible to create public functions that can only be invoked by privately enqueueing a call from the same contract, which can very useful to update public state after private execution (e.g. update a token's supply after privately minting). This is achieved by annotating functions with #[internal].

A common pattern is to enqueue public calls to check some validity condition on public state, e.g. that a deadline has not expired or that some public value is set.

enqueueing
Router::at(ROUTER_ADDRESS).check_block_number(operation, value).call(context);
Source code: noir-projects/noir-contracts/contracts/router_contract/src/utils.nr#L17-L19

Note that this reveals what public function is being called on what contract, and perhaps more importantly which contract enqueued the call during private execution. For this reason we've created a canonical router contract which implements some of the checks commonly performed: this conceals the calling contract, as the context.msg_sender() in the public function will be the router itself (since it is the router that enqueues the public call).

An example of how a deadline can be checked using the router contract follows:

call-check-deadline
let deadline = storage.deadline.read();
privately_check_timestamp(Comparator.LT, deadline, &mut context);
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L71-L74

privately_check_timestamp and privately_check_block_number are helper functions around the call to the router contract:

helper_router_functions
/// Asserts that the current timestamp in the enqueued public call enqueued by `check_timestamp` satisfies
/// the `operation` with respect to the `value. Preserves privacy by performing the check via the router contract.
/// This conceals an address of the calling contract by setting `context.msg_sender` to the router contract address.
pub fn privately_check_timestamp(operation: u8, value: u64, context: &mut PrivateContext) {
    Router::at(ROUTER_ADDRESS).check_timestamp(operation, value).call(context);
}

/// Asserts that the current block number in the enqueued public call enqueued by `check_block_number` satisfies
/// the `operation` with respect to the `value. Preserves privacy by performing the check via the router contract.
/// This conceals an address of the calling contract by setting `context.msg_sender` to the router contract address.
pub fn privately_check_block_number(operation: u8, value: Field, context: &mut PrivateContext) {
    Router::at(ROUTER_ADDRESS).check_block_number(operation, value).call(context);
}
Source code: noir-projects/noir-contracts/contracts/router_contract/src/utils.nr#L5-L21

This is what the implementation of the check timestamp functionality looks like:

check_timestamp
/// Asserts that the current timestamp in the enqueued public call satisfies the `operation` with respect
/// to the `value.
#[private]
fn check_timestamp(operation: u8, value: u64) {
    Router::at(context.this_address())._check_timestamp(operation, value).enqueue_view(
        &mut context,
    );
}

#[public]
#[internal]
#[view]
fn _check_timestamp(operation: u8, value: u64) {
    let lhs_field = context.timestamp() as Field;
    let rhs_field = value as Field;
    assert(compare(lhs_field, operation, rhs_field), "Timestamp mismatch.");
}
Source code: noir-projects/noir-contracts/contracts/router_contract/src/main.nr#L13-L31
note

Note that the router contract is not currently part of the aztec-nr repository. To add it as a dependency point to the aztec-packages repository instead:

[dependencies]
aztec = { git = "https://github.com/AztecProtocol/aztec-packages/", tag = "aztec-packages-v0.68.0", directory = "noir-projects/noir-contracts/contracts/router_contract/src" }

Even with the router contract achieving good privacy is hard. For example, if the value being checked against is unique and stored in the contract's public storage, it's then simple to find private transactions that are using that value in the enqueued public reads, and therefore link them to this contract. For this reason it is encouraged to try to avoid public function calls and instead privately read Shared State when possible.

Public Execution​

Contract functions marked with #[public] can only be called publicly, and are executed by the sequencer. The computation model is very similar to the EVM: all state, parameters, etc. are known to the entire network, and no data is private. Static execution like the EVM's STATICCALL is possible too, with similar semantics (state can be accessed but not modified, etc.).

Since private calls are always run in a user's device, it is not possible to perform any private execution from a public context. A reasonably good mental model for public execution is that of an EVM in which some work has already been done privately, and all that is know about it is its correctness and side-effects (new notes and nullifiers, enqueued public calls, etc.). A reverted public execution will also revert the private side-effects.

Public functions in other contracts can be called both regularly and statically, just like on the EVM.

public_call
Token::at(config.accepted_asset)
    .transfer_in_public(context.msg_sender(), context.this_address(), max_fee, nonce)
    .enqueue(&mut context);
Source code: noir-projects/noir-contracts/contracts/fpc_contract/src/main.nr#L118-L122
note

This is the same function that was called by privately enqueuing a call to it! Public functions can be called either directly in a public context, or asynchronously by enqueuing in a private context.

Top-level Unconstrained​

Contract functions with the unconstrained Noir keyword are a special type of function still under development, and their semantics will likely change in the near future. They are used to perform state queries from an off-chain client (from both private and public state!), and are never included in any transaction. No guarantees are made on the correctness of the result since the entire execution is unconstrained and heavily reliant on oracle calls.

Any programming language could be used to construct these queries, since all they do is perform arbitrary computation on data that is either publicly available from any node, or locally available from the PXE. Top-level unconstrained functions exist because they let developers utilize the rest of the contract code directly by being part of the same Noir contract, and e.g. use the same libraries, structs, etc. instead of having to rely on manual computation of storage slots, struct layout and padding, and so on.

A reasonable mental model for them is that of a Solidity view function that can never be called in any transaction, and is only ever invoked via eth_call. Note that in these the caller assumes that the node is acting honestly by executing the true contract bytecode with correct blockchain state, the same way the Aztec version assumes the oracles are returning legitimate data.

aztec.js​

There are three different ways to execute an Aztec contract function using the aztec.js library, with close similarities to their JSON-RPC counterparts.

simulate​

This is used to get a result out of an execution, either private or public. It creates no transaction and spends no gas. The mental model is fairly close to that of eth_call, in that it can be used to call any type of function, simulate its execution and get a result out of it. simulate is also the only way to run top-level unconstrained functions.

public_getter
#[public]
#[view]
fn get_authorized() -> AztecAddress {
    storage.authorized.get_current_value()
}
Source code: noir-projects/noir-contracts/contracts/auth_contract/src/main.nr#L42-L50
simulate_function
const balance = await contract.methods.balance_of_public(wallet.getAddress()).simulate();
expect(balance).toEqual(1n);
Source code: yarn-project/end-to-end/src/composed/docs_examples.test.ts#L49-L52
warning

No correctness is guaranteed on the result of simulate! Correct execution is entirely optional and left up to the client that handles this request.

prove​

This creates and returns a transaction request, which includes proof of correct private execution and side-effects. The request is not broadcast however, and no gas is spent. It is typically used in testing contexts to inspect transaction parameters or to check for execution failure.

local-tx-fails
const call = token.methods.transfer(recipient.getAddress(), 200n);
await expect(call.prove()).rejects.toThrow(/Balance too low/);
Source code: yarn-project/end-to-end/src/guides/dapp_testing.test.ts#L148-L151

Like most Ethereum libraries, prove also simulates public execution to try to detect runtime errors that would only occur once the transaction is picked up by the sequencer. This makes prove very useful in testing environments, but users should be wary of both false positives and negatives in production environments, particularly if the node's data is stale. Public simulation can be skipped by setting the skipPublicSimulation flag.

send​

This is the same as prove except it also broadcasts the transaction and returns a receipt. This is how transactions are sent, getting them to be included in blocks and spending gas. It is similar to eth_sendTransaction, except it also performs some work on the user's device, namely the production of the proof for the private part of the transaction.

send_tx
await contract.methods.buy_pack(seed).send().wait();
Source code: yarn-project/end-to-end/src/e2e_card_game.test.ts#L153-L155