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Standard Library

Ora's standard library is imported as a comptime constant:

comptime const std = @import("std");

This gives access to EVM environment data and useful constants.

Message context​

All examples in this chapter assume std is imported. Inside a function:

let sender: address = std.msg.sender();     // Transaction caller (CALLER opcode)
let value: u256 = std.msg.value();          // Attached ETH value (CALLVALUE opcode)

std.msg.sender() returns the address of the account that called the current function (the EVM CALLER opcode).

std.msg.value() returns the amount of ETH sent with the call.

Transaction context​

let origin: address = std.tx.origin;        // Transaction origin (ORIGIN opcode)
let gas_price: u256 = std.transaction.gasprice;  // Gas price (GASPRICE opcode)

std.transaction.sender() is an alias for std.msg.sender().

Block information​

let ts: u256 = std.block.timestamp;          // Current block timestamp (TIMESTAMP)
let num: u256 = std.block.number;            // Current block number (NUMBER)
let miner: address = std.block.coinbase();   // Block coinbase address (COINBASE)

Constants​

let zero: address = std.constants.ZERO_ADDRESS;    // 0x0000...0000
let max: u256 = std.constants.U256_MAX;             // 2^256 - 1
let max128: u128 = std.constants.U128_MAX;          // 2^128 - 1

These are compile-time constants — using them costs no gas beyond the PUSH opcode.

Usage in specifications​

Standard library constants are commonly used in requires clauses to prevent overflow. Inside a contract:

pub fn deposit(amount: MinValue<u256, 1>)
    requires(totalDeposits <= std.constants.U256_MAX - amount)
{
    totalDeposits += amount;
}

This ensures totalDeposits + amount cannot overflow a u256.

Further reading​