money.add

Add, subtract and negate monetary amounts, refusing to mix currencies.

1.0.0 (not the latest) · published 2026-10-03 by charlie · Anterra

Pinned by 4 tests, run in TypeScript, Python and Rust · fewer than the registry now requires. 1.0.1 adds them.

What it does

Also exports subtractMoney and negateMoney: they are the same group operation and splitting them across three packages would only make them easier to get wrong separately.

For example

add_money(£19.99, £0.01) → £20.00 adds pence
add_money(£25.00, £0.00) → £25.00 adds zero
add_money(£25.00, -£10.00) → £15.00 adds a negative (a credit)

The function

The same function in TypeScript, Python and Rust, pinned by the same tests. Pick your language; the choice follows you around the registry.

def add_money(a: Money, b: Money) -> Money

a	Money
b	Money
returns	Money

Your code names it in one line, in the file that uses it

from fune.money.add import add_money  # money.add@^1

impl/python.py · 15 lines · open · raw

Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.

from .money_amount import Money, assert_same_currency, money  ← from money.amount ^1.0.0 · built alongside by fune


def add_money(a: Money, b: Money) -> Money:
    assert_same_currency(a, b)
    return money(a.minor + b.minor, a.currency)


def subtract_money(a: Money, b: Money) -> Money:
    assert_same_currency(a, b)
    return money(a.minor - b.minor, a.currency)


def negate_money(a: Money) -> Money:
    return money(-a.minor, a.currency)

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add money.add

Download for Python money.add-1.0.0-python.fune · 2,509 bytes sha256 b89fec11349e6659981c5a0ed499c131528889982c363161aff92f8e2c8f270e

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./money.add-1.0.0-python.fune, or fetch it from a terminal with fune pull money.add@1.0.0:python.

The whole function, every language, is one file too: money.add-1.0.0.fune, 3,782 bytes, sha256 05a3306e1d263396987387bcee089ab451fdcc5789e97fb0e110889e5def31c9. It installs into a project of any language.

Customise it in your app

The seams this capability offers. Put a marker directly above a function of your own and fune build wires it into the built code; the package on the registry is not changed, the built file’s header lists it under CUSTOMISED, and fune hooks lists every hook in the project. How hooks work.

before — your function gets the arguments and returns them, changed or not, or throws to refuse the call.

# fune: before money.add

after — your function gets the result and the arguments, and returns the final result.

# fune: after money.add

replace — inside this capability’s code only, calls to a dependency go to your function, with the same signature. Other capabilities that use it are unaffected; write in * to replace it everywhere.

# fune: replace money.amount in money.add

step — your function runs at a numbered point inside the function’s body, receives the in-scope values it names as parameters, and may return replacements. List the points with fune show money.add --steps.

# fune: step money.add after <n|label>

Tests

A version published now needs at least 8 tests for every function, and one that expects the error for each function that throws; the registry refuses it otherwise. fune verify --all runs each case in TypeScript, Python and Rust, and a project runs them again with fune verify. This page lists the cases; it does not run them. The exact JSON is vectors.json.

This version has fewer tests than the registry now requires. It was published before every function had to have 8. 1.0.1 meets it, and a project on ^1.0.0 installs that or newer.

addMoney has 4 tests; every function needs at least 8. Add 4 more to vectors.json: the ordinary case, the boundaries (zero, negative, the largest values), the rounding edge and every error it documents

Case	Arguments		Expected
adds pence	£19.99, £0.01	→	£20.00
adds zero	£25.00, £0.00	→	£25.00
adds a negative (a credit)	£25.00, -£10.00	→	£15.00
mixing currencies is an error	£1.00, €1.00	→	error: currency mismatch

Files

Path	Bytes
README.md	185
impl/python.py	394
impl/rust.rs	767
impl/typescript.ts	432
vectors.json	666