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Money is not a float

13 July 2025·2 min read

0.1 + 0.2 is not 0.3 in most languages, and every shop finds out at checkout. A web language should have exact decimals built in, with a literal of their own.

Every web developer learns this once, usually from an invoice: 0.1 + 0.2 is 0.30000000000000004. Floats are binary fractions, and most decimal amounts have no exact binary form. The fixes are well known: count in cents, or pull in a decimal library and remember to use it everywhere. Both are easy to get wrong in one place.

A decimal with its own literal

let price = 19.99mlet total = price * 3 + 4.50m          // 64.47mlet vat = round(total * 0.21m, 2)      // 13.54m, rounded half to even0.1m + 0.2m == 0.3m                    // true

The m suffix (borrowed from C#) makes a dec: an exact decimal of up to 28 digits. It keeps its digits, so 12.50m shows as "12.50", and rounding is something you ask for, with the mode you want.

Mixing is an error, not a guess

1.5m + 0.5        // error: convert one with dec(x) or float(x)

A decimal and a float never quietly become a float. The moment exact money meets an inexact number, the program says so, which is the moment you want to know.

From the outside

Amounts arrive as text, from forms and JSON. dec("12.50") reads them exactly, and bad text is an error value, so dec(form.amount) ?? reply 400 is the whole check.

Exact decimals are a small feature. But a web language without them leaves every shop to rebuild them, and some will not.

Since then: decimals with the m literal, and a money type built on them, are in Quillo. See Money.