Exact — finance math that can't lose a cent

Rust Users Forum2026-10-15 08:030 views

Hi! I'd like feedback on exact, a small set of Rust crates for financial math.

The problem

0.1 + 0.2 isn't 0.3 with floats, and in a market those errors add up.
Splitting $100 three ways leaves one cent, and someone has to decide who gets it.
Prices, quantities and money are all "just numbers", so they get mixed up by mistake.

What exact does

Exact: stores whole cents/satoshis, so 0.1 + 0.2 is exactly 0.3.
Safe: money, quantity and price are different types; mixing them doesn't compile.
Checked: checked operations report overflow instead of wrapping, and a quantity can never go below zero.
No lost cents: you choose where a split's leftover goes, and an audit verifies that recorded transfers net to zero for each asset.
Snap to the market grid: prices round onto the tick size and quantities onto the lot size (tick 0.05: 0.17 → 0.15).
Safe to send and store: a fixed 10-byte format that carries the amount, its precision and its kind, so a cent amount can't be read back as satoshis.
Lightweight: 8-byte values, allocation-free core arithmetic, no external dependencies.

exact 0.1 + 0.2 = 0.3 f64 0.1 + 0.2 = 0.30000000000000004 split 100.00 into 3 -> [33.34, 33.33, 33.33] (sum is exactly 100.00)

Compared to existing crates

Crates like rust_decimal already do exact decimal arithmetic well. The goal is to combine them in one small crate family: separate money/quantity/price types, a leftover-cent policy, a zero-sum audit and tick/lot snapping.