finance.tax.add-vat Unreviewed

pub fn add_vat(net: &Money, jurisdiction: &str, category: &str, on_date: &str) -> VatBreakdown

/// Net, tax and gross together, so a caller never derives one from the other two.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct VatBreakdown {
    pub net: Money,
    pub tax: Money,
    pub gross: Money,
    pub basis_points: i64,
}

fune!(finance.tax.add-vat@^1);  // then call add_vat(…)

use super::funejson::Value;  ← the fune runtime: the JSON value the test vectors use; fune build keeps it only where a signature takes one
use super::finance_tax_vat_rate::vat_rate;  ← from finance.tax.vat-rate ^1.0.0 · built alongside by fune
use super::money_add::add_money;  ← from money.add ^1.0.0 · built alongside by fune
use super::money_amount::{money_from_value, money_to_value, Money};  ← from money.amount ^1.0.0 · built alongside by fune
use super::money_apply_rate::apply_rate;  ← from money.apply-rate ^1.0.0 · built alongside by fune

/// Add VAT to a net amount at the rate in force on the date of supply.
///
/// Returns the whole breakdown rather than just the tax: an invoice needs net,
/// tax and gross together, and deriving the third from the other two in caller
/// code is where rounding bugs are born.
pub fn add_vat(net: &Money, jurisdiction: &str, category: &str, on_date: &str) -> VatBreakdown {
    let rate = vat_rate(jurisdiction, category, on_date);
    let tax = apply_rate(net, rate.basis_points, "half-up");
    VatBreakdown {
        net: net.clone(),
        gross: add_money(net, &tax),
        tax,
        basis_points: rate.basis_points,
    }
}

pub fn vat_breakdown_to_value(breakdown: &VatBreakdown) -> Value {
    Value::obj(vec![
        ("net", money_to_value(&breakdown.net)),
        ("tax", money_to_value(&breakdown.tax)),
        ("gross", money_to_value(&breakdown.gross)),
        ("basisPoints", Value::Int(breakdown.basis_points)),
    ])
}

pub fn fune_vector(args: &[Value]) -> Value {
    vat_breakdown_to_value(&add_vat(
        &money_from_value(&args[0]),
        args[1].as_str(),
        args[2].as_str(),
        args[3].as_str(),
    ))
}

Install

fune build

With that line in your source, in a Rust project (language rust in fune.project), fune build resolves it and its 4 dependencies, pins them in fune.lock, downloads only the Rust 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. A crate’s build.rs runs it before every compile. Or pin a range in fune.project and build in one step:

fune add finance.tax.add-vat

The manifest, vectors and README with only the Rust implementation. Install it without the registry with fune add ./finance.tax.add-vat-1.0.1-rust.fune, or fetch it from a terminal with fune pull finance.tax.add-vat@1.0.1:rust.

The whole function, every language, is one file too: finance.tax.add-vat-1.0.1.fune, 9,696 bytes, sha256 51e2215962b87cfc7de9ebfdc142e005b3f7b780a6faf91244486bb57a406763. It installs into a project of any language.

// fune: before finance.tax.add-vat

// fune: after finance.tax.add-vat

// fune: replace finance.tax.vat-rate in finance.tax.add-vat
// fune: replace money.add in finance.tax.add-vat
// fune: replace money.amount in finance.tax.add-vat
// fune: replace money.apply-rate in finance.tax.add-vat

// fune: step finance.tax.add-vat after <n|label>

More from the author

## Before you rely on this

**Not professional advice.** This capability calculates tax figures from published rules. It is a software component for developers, not tax advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have a chartered accountant or tax adviser review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified chartered accountant or tax adviser has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

1.0.1 marks it unreviewed. The code and the tests are unchanged.

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