money.sum

Total a list of monetary amounts, with an explicit currency for the empty case.

1.0.1 · published 2026-10-03 by charlie · Anterra

Pinned by 11 tests, run in TypeScript, Python and Rust.

What it does

The currency argument looks redundant until the list is empty, which is exactly when `reduce` without an initial value throws in production.

1.0.1 adds tests; behaviour unchanged.

For example

sum_money(£10.00, £25.50, £0.99, GBP) → £36.49 totals three lines
sum_money(, GBP) → £0.00 empty list is zero in the stated currency
sum_money(£50.00, -£12.00, GBP) → £38.00 credits reduce the total

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 sum_money(amounts: Sequence[Money], currency: str) -> Money

amounts	Money[]
currency	string	the currency of the result, required so an empty list still has one
returns	Money

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

from fune.money.sum import sum_money  # money.sum@^1

impl/python.py · 11 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 typing import Iterable

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


def sum_money(amounts: Iterable[Money], currency: str) -> Money:
    total = money(0, currency)
    for amount in amounts:
        total = add_money(total, amount)
    return total

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 2 dependencies, 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.sum

Download for Python money.sum-1.0.1-python.fune · 4,381 bytes sha256 4d7f05ca998ea60d4bf4e62c168330d64acc8e21f1496ff15c9f225c3dbdf191

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./money.sum-1.0.1-python.fune, or fetch it from a terminal with fune pull money.sum@1.0.1:python.

The whole function, every language, is one file too: money.sum-1.0.1.fune, 5,305 bytes, sha256 b10d69d8451e8785f2c24dd1321240ee289aef491bedae6c2419b38984e03a02. 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.sum

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

# fune: after money.sum

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.add in money.sum
# fune: replace money.amount in money.sum

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.sum --steps.

# fune: step money.sum 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.

Case	Arguments		Expected
totals three lines	£10.00, £25.50, £0.99, GBP	→	£36.49
empty list is zero in the stated currency	, GBP	→	£0.00
credits reduce the total	£50.00, -£12.00, GBP	→	£38.00
a foreign amount in the list is an error	£1.00, $1.00, GBP	→	error: currency mismatch
one amount is its own total	€2.50, EUR	→	€2.50
an empty list in a zero-decimal currency	, JPY	→	¥0
credits can take the total below zero	£10.00, -£15.00, GBP	→	-£5.00
ten 10p amounts make exactly one pound	£0.10, £0.10, £0.10, £0.10, £0.10, £0.10, £0.10, £0.10, £0.10, £0.10, GBP	→	£1.00
large totals stay exact up to the largest safe integer	£90,071,992,547,400.00, £9.90, £0.01, GBP	→	£90,071,992,547,409.91
every amount must be in the stated currency, not the first one's	€1.00, €2.00, GBP	→	error: currency mismatch

Show the other 1 test

Case	Arguments		Expected
an invalid currency is refused even for an empty list	, gbp	→	error: ISO 4217

Files

Path	Bytes
README.md	194
impl/python.py	284
impl/rust.rs	590
impl/typescript.ts	276
vectors.json	2,272