Ledger
A double-entry bookkeeping engine a spreadsheet can read.
- Date
- Role
- Design and build
- Client
- Self-directed
- Duration
- Four months
Accounting software fails in one of two ways: it is a black box, or it is a spreadsheet. Ledger is an attempt at neither. Entries go in as plain text, come out as a queryable database, and export as a sheet somebody’s auditor can open without asking a question.
The rule it enforces
One rule, checked on write: every transaction sums to zero. Everything else the engine does is reporting, and reporting is easy once the numbers cannot be wrong.
fn post(entries: &[Entry]) -> Result<(), Unbalanced> {
let sum: i64 = entries.iter().map(|e| e.minor_units).sum();
(sum == 0).then_some(()).ok_or(Unbalanced(sum))
}Money is integer minor units, never a float, which is the other half of not being wrong.
What it refuses to do
No currency conversion at write time. A rate is a fact about a day, and baking one into a stored entry makes yesterday’s books change when today’s rate does.