Logs and Events

Logs are Ora's equivalent of Solidity events. They emit EVM log entries for off-chain indexing and observability.

Declaring logs

Logs are declared at contract scope with typed fields:

contract Ledger {
    log Transfer(indexed from: address, indexed to: address, amount: u256);
    log Approval(indexed owner: address, indexed spender: address, amount: u256);
    log StatusChange(account: address, old_status: u8, new_status: u8);
    log Paused();
}

Emitting logs

Use the log statement with positional arguments matching the declaration:

contract Ledger {
    log Transfer(indexed from: address, indexed to: address, amount: u256);

    pub fn send(to: address, amount: u256) {
        let from: address = std.msg.sender();
        log Transfer(from, to, amount);
    }
}

The compiler checks:

• The log name is declared in the current contract
• Argument count matches the declaration
• Argument types match the declared field types

Indexed fields

Mark fields with indexed to make them filterable by off-chain indexers:

log Transfer(indexed from: address, indexed to: address, amount: u256);

Limits

A log can have at most 3 indexed fields. This is an EVM constraint — the LOG opcodes support up to 4 topics, and topic 0 is reserved for the event signature hash. Four or more indexed fields is a compile error:

// Compile error: too many indexed fields
log TooMany(indexed a: address, indexed b: address, indexed c: address, indexed d: address);

Indexed field types

Indexed fields must be scalar types. The compiler rejects these types for indexed fields:

• struct
• tuple
• array
• slice
• map

Scalar types (bool, integers, address, bytes) are supported.

EVM lowering

Logs compile to EVM LOG0–LOG4 opcodes:

Indexed fields	EVM opcode	Topics
0	LOG1	signature only
1	LOG2	signature + 1 indexed
2	LOG3	signature + 2 indexed
3	LOG4	signature + 3 indexed

Topic 0 is the keccak256 hash of the log signature (e.g., keccak256("Transfer(address,address,uint256)")), computed at compile time. Non-indexed fields are ABI-encoded into the log data payload.

Supported field types

contract Events {
    log MixedEvent(id: u256, name: string, active: bool, data: bytes);
}

Fields can be any type the ABI encoder supports: integers, bool, address, string, bytes.