money

The one exact money type for Go — an Amount in a Currency, backed by hanzoai/decimal (big.Int fixed-point).

Martin Fowler's Money pattern (never floats, integer-exact, Add/Sub/Split/Allocate, same-currency safety) without the precision ceiling. int64/uint64 money libraries cap around 19 digits (~$9.20 at 18 decimals); this holds a fiat cent and an 18-decimal on-chain balance exactly.

price, _ := money.Parse("6.60", money.USD)
cost := price.Mul(decimal.New(200, 0)).Mul(decimal.MustParse("0.000001")) // "0.00132", never 0
bal, _ := money.FromCents(100).Sub(cost)                                  // "0.99868"

money.FromCents(123456).Display()             // "$1,234.56"
parts, _ := money.FromCents(10000).Allocate(50, 30, 20) // $50 / $30 / $20, nothing lost
a, _ := money.ParseAmount("$1,234.56")        // symbol/code/separator aware
money.FromHUSD(wei)                            // 18-dec on-chain stablecoin

Features

• Exact, uncapped: big.Int core — fiat cents to 18-decimal crypto wei, no float, no ceiling
• Arithmetic: Add Sub Mul Neg Cmp Sign, same-currency safety (zero-value agnostic)
• Allocation: Split(n) and Allocate(ratios…) — no sub-unit ever lost
• Aggregation: Sum Min Max Avg
• Exchange: rate table + Convert / ConvertWithRate
• Parsing: ParseAmount ($1,234.56, EUR 250,75, £99.99), European decimal-comma, ParseDecimal
• Ledger edge: ParseMinor/ParseCents (decimal string → integer minor units), its exact inverse MajorString (integer minor units → fixed-scale decimal string), and ScaleMinor (minor units × rate → minor units, rounded half away from zero) — the set every billing API needs, none of which touches a float. RateFromFloat makes a legacy float64 rate exact first. ScaleMinorCeil/ScaleMinorFloor are the same scaling for amounts whose direction is a policy (a fee rounded up, a commission rounded down) — exact, so math.Ceil cannot invent a cent on a product that was already whole
• Formatting: MajorString ("10.99", "1.00", "-19.99" — the wire), Display (symbol + thousands separators — the human), AsMajorUnits (float, display only)
• Currencies: ISO-4217 set (symbol, numeric code) + crypto (USDC/USDT/BTC/ETH/HUSD) + Register custom
• Integration: json.Marshaler/Unmarshaler, sql.Scanner/driver.Valuer

Packages

Single package github.com/hanzoai/money (Amount, Currency, Exchange, parse/format/aggregate), on github.com/hanzoai/decimal.

Original work (big.Int core written from scratch, not a fork). MIT OR Apache-2.0, at your option — per HIP-0137.