{
  "id": "subscriptions",
  "title": "Subscriptions and SaaS billing",
  "summary": "Recurring billing for a SaaS or subscription business: trials, renewal dates, prorated plan changes, metered usage, renewal invoices with coupons, dunning and MRR, churn and LTV.",
  "synonyms": [
    "saas",
    "saas billing",
    "subscriptions",
    "subscription billing",
    "recurring billing",
    "recurring payments",
    "memberships",
    "plans and pricing",
    "metered billing",
    "usage based billing",
    "stripe billing",
    "chargebee",
    "recurly",
    "trials",
    "proration"
  ],
  "apps": [
    "saas billing system",
    "subscription management app",
    "membership site",
    "subscription box service",
    "usage-based billing platform",
    "saas metrics dashboard"
  ],
  "language": "rust",
  "includes": [
    "money-basics",
    "dates-basics",
    "uk-tax"
  ],
  "includedBy": [],
  "caveats": [
    "Calculates tax from published rules; it is not tax advice. Have an accountant check how the app uses it before anyone files a return."
  ],
  "sections": [
    {
      "key": "plans",
      "title": "Plans and trials",
      "purpose": "Start a subscription and know when it is first charged.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.trial-end",
          "tier": "core",
          "why": "When a free trial ends and the first payment is due.",
          "section": "plans",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "When a free trial ends and the first payment is due, from the trial's start date and length in days.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "trialEnd"
          ],
          "signatures": [
            "pub fn trial_end(start_date: &str, trial_days: i64) -> TrialPeriod"
          ],
          "reference": "fune!(subscriptions.trial-end@^1);  // then call trial_end(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.trial-end"
        },
        {
          "id": "subscriptions.next-billing-date",
          "tier": "core",
          "why": "The next renewal date from the billing anchor and interval, month-end safe.",
          "section": "plans",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "The next renewal date after a given day, from a billing anchor and interval; month-end safe.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "nextBillingDate"
          ],
          "signatures": [
            "pub fn next_billing_date(anchor_date: &str, interval: &str, interval_count: i64, after_date: &str) -> String"
          ],
          "reference": "fune!(subscriptions.next-billing-date@^1);  // then call next_billing_date(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.next-billing-date"
        }
      ],
      "gaps": [
        {
          "key": "plan-catalogue",
          "text": "Plans, prices, add-ons and feature entitlements: the product catalogue and which features each plan unlocks are yours.",
          "section": "plans",
          "from": "subscriptions"
        }
      ]
    },
    {
      "key": "changes",
      "title": "Plan changes",
      "purpose": "Upgrade, downgrade or cancel mid-cycle without over- or under-charging.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.proration",
          "tier": "core",
          "why": "Credit for unused time on the old plan and the charge for the rest of the period on the new one.",
          "section": "changes",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Mid-cycle plan change: credit for unused time on the old plan and charge for the rest of the period on the new one.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "prorateChange"
          ],
          "signatures": [
            "pub fn prorate_change(old_price: &Money, new_price: &Money, period_start: &str, period_end: &str, change_date: &str, mode: &RoundingMode) -> PlanChangeProration"
          ],
          "reference": "fune!(subscriptions.proration@^1);  // then call prorate_change(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.proration"
        },
        {
          "id": "finance.proration",
          "tier": "optional",
          "why": "Splits any amount into used and unused parts of a billing period, for cancellations and refunds.",
          "section": "changes",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Split an amount into the part used and the part unused over a billing period, losing nothing.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "prorate"
          ],
          "signatures": [
            "pub fn prorate(amount: &Money, total_days: i64, used_days: i64) -> ProrationSplit"
          ],
          "reference": "fune!(finance.proration@^1);  // then call prorate(…)",
          "url": "http://www.functionalweave.com/functions/finance.proration"
        }
      ],
      "gaps": []
    },
    {
      "key": "usage",
      "title": "Metered usage",
      "purpose": "Price what a customer used in the period.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.usage-tiered",
          "tier": "optional",
          "why": "Prices metered usage against graduated or volume tiers, with flat fees per tier.",
          "section": "usage",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Price metered usage against tiers, graduated or volume, with an optional flat fee per tier.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "priceUsage"
          ],
          "signatures": [
            "pub fn price_usage(quantity: i64, tiers: &[UsageTier], mode: &str) -> UsageCharge"
          ],
          "reference": "fune!(subscriptions.usage-tiered@^1);  // then call price_usage(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.usage-tiered"
        }
      ],
      "gaps": [
        {
          "key": "usage-metering",
          "text": "Recording usage events reliably and aggregating them per customer per period.",
          "section": "usage",
          "from": "subscriptions"
        }
      ]
    },
    {
      "key": "renewal",
      "title": "Renewal invoices",
      "purpose": "Bill each renewal with its add-ons, usage, coupons and VAT.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.invoice",
          "tier": "core",
          "why": "The renewal invoice: plan, add-ons and usage lines, coupon discounts by billing period, and VAT.",
          "section": "renewal",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "A subscription renewal invoice: plan, add-ons and usage, coupon discounts by billing period, and VAT.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "renewalInvoice"
          ],
          "signatures": [
            "pub fn renewal_invoice(plan: &InvoiceLine, add_ons: &[InvoiceLine], usage: &[InvoiceLine], discounts: &[Discount], period_number: i64, jurisdiction: &str, invoice_date: &str) -> Invoice"
          ],
          "reference": "fune!(subscriptions.invoice@^1);  // then call renewal_invoice(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.invoice"
        },
        {
          "id": "finance.invoice.calculate",
          "tier": "core",
          "why": "Totals the invoice lines and VAT by rate (the renewal invoice builds on it).",
          "section": "renewal",
          "from": "subscriptions",
          "version": "1.0.1",
          "summary": "Calculate a complete invoice: line nets, VAT per line, totals and a VAT summary grouped by rate.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "calculateInvoice"
          ],
          "signatures": [
            "pub fn calculate_invoice(lines: &[InvoiceLine], jurisdiction: &str, invoice_date: &str) -> Invoice"
          ],
          "reference": "fune!(finance.invoice.calculate@^1);  // then call calculate_invoice(…)",
          "url": "http://www.functionalweave.com/functions/finance.invoice.calculate"
        },
        {
          "id": "retail.oss-vat",
          "tier": "optional",
          "why": "Which EU country's VAT applies when selling to consumers across the EU.",
          "section": "renewal",
          "from": "subscriptions",
          "version": "1.0.1",
          "summary": "EU distance-selling VAT: which member state's standard rate applies under OSS and the EUR 10,000 threshold, and the VAT.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "ossVat"
          ],
          "signatures": [
            "pub fn oss_vat(net: &Money, seller_country: &str, customer_country: &str, eu_sales_previous_year: &Money, eu_sales_this_year: &Money, opted_in: bool, on_date: &str) -> OssVat"
          ],
          "reference": "fune!(retail.oss-vat@^1);  // then call oss_vat(…)",
          "url": "http://www.functionalweave.com/functions/retail.oss-vat"
        }
      ],
      "gaps": [
        {
          "key": "invoice-delivery",
          "text": "Numbering, PDF rendering and emailing the invoices.",
          "section": "renewal",
          "from": "subscriptions"
        }
      ]
    },
    {
      "key": "collection",
      "title": "Payment and dunning",
      "purpose": "Take payment and retry when it fails.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.dunning-schedule",
          "tier": "core",
          "why": "Retry dates for a failed payment from a retry policy, in calendar or business days.",
          "section": "collection",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Retry dates for a failed subscription payment from a retry policy, in calendar or business days.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "dunningSchedule"
          ],
          "signatures": [
            "pub fn dunning_schedule(failed_on: &str, retry_offsets_days: &[i64], business_days: bool, holidays: &[String]) -> Vec<String>"
          ],
          "reference": "fune!(subscriptions.dunning-schedule@^1);  // then call dunning_schedule(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.dunning-schedule"
        }
      ],
      "gaps": [
        {
          "key": "payment-provider",
          "text": "Charging cards or Direct Debit through Stripe, GoCardless or similar, storing payment methods and handling webhooks.",
          "section": "collection",
          "from": "subscriptions"
        },
        {
          "key": "dunning-emails",
          "text": "The emails and in-app notices that go with each retry.",
          "section": "collection",
          "from": "subscriptions"
        }
      ]
    },
    {
      "key": "metrics",
      "title": "SaaS metrics",
      "purpose": "Report the numbers a subscription business is run by.",
      "from": "subscriptions",
      "members": [
        {
          "id": "subscriptions.mrr-arr",
          "tier": "core",
          "why": "MRR and ARR from the subscriptions, normalising intervals and leaving out trials.",
          "section": "metrics",
          "from": "subscriptions",
          "version": "1.1.0",
          "summary": "Monthly and annual recurring revenue from a list of subscriptions, normalising intervals and leaving out trials.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "mrrArr"
          ],
          "signatures": [
            "pub fn mrr_arr(subscriptions: &[RecurringCharge], currency: &str) -> MrrArr"
          ],
          "reference": "fune!(subscriptions.mrr-arr@^1);  // then call mrr_arr(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.mrr-arr"
        },
        {
          "id": "subscriptions.churn",
          "tier": "optional",
          "why": "Logo and revenue churn and net revenue retention for a period.",
          "section": "metrics",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Logo churn, gross and net revenue churn, and gross and net revenue retention for one period, in basis points.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "churnMetrics"
          ],
          "signatures": [
            "pub fn churn_metrics(customers_at_start: i64, customers_lost: i64, mrr_at_start: &Money, expansion_mrr: &Money, contraction_mrr: &Money, churned_mrr: &Money) -> ChurnMetrics"
          ],
          "reference": "fune!(subscriptions.churn@^1);  // then call churn_metrics(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.churn"
        },
        {
          "id": "subscriptions.ltv-cac",
          "tier": "optional",
          "why": "Customer lifetime value, LTV:CAC and CAC payback months.",
          "section": "metrics",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Customer lifetime value, LTV to CAC ratio and CAC payback months from ARPA, gross margin, churn and CAC.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "ltvCac"
          ],
          "signatures": [
            "pub fn ltv_cac(arpa: &Money, gross_margin_basis_points: i64, monthly_churn_basis_points: i64, cac: &Money) -> LtvCac"
          ],
          "reference": "fune!(subscriptions.ltv-cac@^1);  // then call ltv_cac(…)",
          "url": "http://www.functionalweave.com/functions/subscriptions.ltv-cac"
        },
        {
          "id": "charts.line-chart",
          "tier": "optional",
          "why": "Line chart geometry for MRR over time.",
          "section": "metrics",
          "from": "subscriptions",
          "version": "1.0.0",
          "summary": "Complete line chart geometry as data: plot area, number or date axes, gridlines, series paths, colours, legend.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "lineChart"
          ],
          "signatures": [
            "pub fn line_chart(spec: &LineChartSpec) -> LineChart"
          ],
          "reference": "fune!(charts.line-chart@^1);  // then call line_chart(…)",
          "url": "http://www.functionalweave.com/functions/charts.line-chart"
        }
      ],
      "gaps": []
    },
    {
      "key": "amounts",
      "title": "Amounts",
      "purpose": "Hold money as currency-tagged integer minor units and do plain arithmetic on it safely.",
      "from": "money-basics",
      "members": [
        {
          "id": "money.amount",
          "tier": "core",
          "why": "The Money type ({ minor, currency }) every other money capability takes and returns.",
          "section": "amounts",
          "from": "money-basics",
          "version": "1.0.2",
          "summary": "A currency-tagged monetary amount held in integer minor units.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "money"
          ],
          "signatures": [
            "pub fn money(minor: i64, currency: &str) -> Money"
          ],
          "reference": "fune!(money.amount@^1);  // then call money(…)",
          "url": "http://www.functionalweave.com/functions/money.amount"
        },
        {
          "id": "money.add",
          "tier": "core",
          "why": "Adds and subtracts amounts, refusing to mix currencies by accident.",
          "section": "amounts",
          "from": "money-basics",
          "version": "1.0.1",
          "summary": "Add, subtract and negate monetary amounts, refusing to mix currencies.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "addMoney"
          ],
          "signatures": [
            "pub fn add_money(a: &Money, b: &Money) -> Money"
          ],
          "reference": "fune!(money.add@^1);  // then call add_money(…)",
          "url": "http://www.functionalweave.com/functions/money.add"
        },
        {
          "id": "money.sum",
          "tier": "core",
          "why": "Totals a list of amounts, with an explicit currency for an empty list.",
          "section": "amounts",
          "from": "money-basics",
          "version": "1.0.1",
          "summary": "Total a list of monetary amounts, with an explicit currency for the empty case.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "sumMoney"
          ],
          "signatures": [
            "pub fn sum_money(amounts: &[Money], currency: &str) -> Money"
          ],
          "reference": "fune!(money.sum@^1);  // then call sum_money(…)",
          "url": "http://www.functionalweave.com/functions/money.sum"
        },
        {
          "id": "money.compare",
          "tier": "core",
          "why": "Compares, mins, maxes and clamps amounts of one currency without unwrapping them.",
          "section": "amounts",
          "from": "money-basics",
          "version": "2.0.0",
          "summary": "Compare two amounts of one currency, refusing to compare across currencies; min, max and clamp too.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": true,
          "functions": [
            "compareMoney",
            "minMoney",
            "maxMoney",
            "clampMoney"
          ],
          "signatures": [
            "pub fn compare_money(a: &Money, b: &Money) -> i64",
            "pub fn min_money(a: &Money, b: &Money) -> Money",
            "pub fn max_money(a: &Money, b: &Money) -> Money",
            "pub fn clamp_money(amount: &Money, min: &Money, max: &Money) -> Money"
          ],
          "reference": "fune!(money.compare@^2);  // then call compare_money, min_money, max_money, clamp_money",
          "url": "http://www.functionalweave.com/functions/money.compare"
        }
      ],
      "gaps": []
    },
    {
      "key": "rates",
      "title": "Rates and rounding",
      "purpose": "Apply percentages and divide amounts with the rounding stated, never a float.",
      "from": "money-basics",
      "members": [
        {
          "id": "money.apply-rate",
          "tier": "core",
          "why": "Applies a basis-point rate (VAT, discount, commission) to an amount with explicit rounding.",
          "section": "rates",
          "from": "money-basics",
          "version": "1.0.1",
          "summary": "Apply a rate expressed in basis points to a monetary amount, with an explicit rounding mode.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "applyRate"
          ],
          "signatures": [
            "pub fn apply_rate(amount: &Money, basis_points: i64, mode: &RoundingMode) -> Money"
          ],
          "reference": "fune!(money.apply-rate@^1);  // then call apply_rate(…)",
          "url": "http://www.functionalweave.com/functions/money.apply-rate"
        },
        {
          "id": "math.round-div",
          "tier": "core",
          "why": "Integer division with a named rounding mode, for any money maths the others do not cover.",
          "section": "rates",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Integer division with an explicit rounding mode, for money arithmetic that must not drift.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "roundDiv"
          ],
          "signatures": [
            "pub fn round_div(numerator: i64, denominator: i64, mode: &str) -> i64"
          ],
          "reference": "fune!(math.round-div@^1);  // then call round_div(…)",
          "url": "http://www.functionalweave.com/functions/math.round-div"
        },
        {
          "id": "math.basis-points",
          "tier": "optional",
          "why": "Converts \"12.5%\" to and from basis points exactly, for rates typed in by users.",
          "section": "rates",
          "from": "money-basics",
          "version": "2.0.0",
          "summary": "Convert a rate between percent, basis points and a plain ratio exactly, as decimal text or integer basis points.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": true,
          "functions": [
            "convertRate",
            "toBasisPoints",
            "fromBasisPoints"
          ],
          "signatures": [
            "pub fn convert_rate(value: &str, from_unit: &str, to_unit: &str) -> String",
            "pub fn to_basis_points(value: &str, unit: &str) -> i64",
            "pub fn from_basis_points(basis_points: i64, unit: &str) -> String"
          ],
          "reference": "fune!(math.basis-points@^2);  // then call convert_rate, to_basis_points, from_basis_points",
          "url": "http://www.functionalweave.com/functions/math.basis-points"
        },
        {
          "id": "math.percent-change",
          "tier": "optional",
          "why": "Percentage change between two figures in basis points, for \"up 4.2% on last month\".",
          "section": "rates",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Percentage change from one value to another, in basis points, with an explicit rounding mode.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "percentChange"
          ],
          "signatures": [
            "pub fn percent_change(from_value: i64, to_value: i64, mode: &RoundingMode) -> Option<i64>"
          ],
          "reference": "fune!(math.percent-change@^1);  // then call percent_change(…)",
          "url": "http://www.functionalweave.com/functions/math.percent-change"
        },
        {
          "id": "math.rational",
          "tier": "optional",
          "why": "Exact fractions for rates that must not drift across many steps.",
          "section": "rates",
          "from": "money-basics",
          "version": "2.0.0",
          "summary": "Exact fraction arithmetic, always reduced, for rates and ratios that must not drift.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": true,
          "functions": [
            "calculateRational",
            "rational",
            "addRational",
            "subtractRational",
            "multiplyRational",
            "divideRational",
            "compareRational",
            "rationalToInteger"
          ],
          "signatures": [
            "pub fn calculate_rational(a: &Rational, op: &str, b: &Rational) -> Rational",
            "pub fn rational(numerator: i64, denominator: i64) -> Rational",
            "pub fn add_rational(a: &Rational, b: &Rational) -> Rational",
            "pub fn subtract_rational(a: &Rational, b: &Rational) -> Rational",
            "pub fn multiply_rational(a: &Rational, b: &Rational) -> Rational",
            "pub fn divide_rational(a: &Rational, b: &Rational) -> Rational",
            "pub fn compare_rational(a: &Rational, b: &Rational) -> i64",
            "pub fn rational_to_integer(r: &Rational, mode: &RoundingMode) -> i64"
          ],
          "reference": "fune!(math.rational@^2);  // then call calculate_rational, rational, add_rational, subtract_rational, multiply_rational, divide_rational, compare_rational, rational_to_integer",
          "url": "http://www.functionalweave.com/functions/math.rational"
        },
        {
          "id": "math.round-div-big",
          "tier": "optional",
          "why": "Rounded division on numbers past 2^53, for very large totals.",
          "section": "rates",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Divide two integers of any size with an explicit rounding mode, on decimal strings, for exact money sums past 2^53.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "roundDivBig"
          ],
          "signatures": [
            "pub fn round_div_big(numerator: &str, denominator: &str, mode: &RoundingMode) -> String"
          ],
          "reference": "fune!(math.round-div-big@^1);  // then call round_div_big(…)",
          "url": "http://www.functionalweave.com/functions/math.round-div-big"
        }
      ],
      "gaps": []
    },
    {
      "key": "splitting",
      "title": "Splitting",
      "purpose": "Divide an amount between people, periods or lines so the parts add back to the whole.",
      "from": "money-basics",
      "members": [
        {
          "id": "money.allocate",
          "tier": "core",
          "why": "Splits an amount by ratios (60/40, by quantity) without losing or inventing a penny.",
          "section": "splitting",
          "from": "money-basics",
          "version": "1.0.1",
          "summary": "Split an amount across ratios without losing or inventing a single minor unit.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "allocate"
          ],
          "signatures": [
            "pub fn allocate(amount: &Money, ratios: &[i64]) -> Vec<Money>"
          ],
          "reference": "fune!(money.allocate@^1);  // then call allocate(…)",
          "url": "http://www.functionalweave.com/functions/money.allocate"
        },
        {
          "id": "money.split-even",
          "tier": "optional",
          "why": "Splits an amount into n near-equal parts that sum exactly, for instalments or shares.",
          "section": "splitting",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Split an amount into n near-equal parts that add back up to exactly the whole.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "splitEven"
          ],
          "signatures": [
            "pub fn split_even(amount: &Money, parts: i64) -> Vec<Money>"
          ],
          "reference": "fune!(money.split-even@^1);  // then call split_even(…)",
          "url": "http://www.functionalweave.com/functions/money.split-even"
        }
      ],
      "gaps": []
    },
    {
      "key": "display",
      "title": "Display and input",
      "purpose": "Show amounts to people and read what they type.",
      "from": "money-basics",
      "members": [
        {
          "id": "money.format",
          "tier": "core",
          "why": "Renders an amount as text with the currency's real number of decimal places.",
          "section": "display",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Render a monetary amount as text, using the currency's real minor-unit precision.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "formatMoney"
          ],
          "signatures": [
            "pub fn format_money(amount: &Money) -> String"
          ],
          "reference": "fune!(money.format@^1);  // then call format_money(…)",
          "url": "http://www.functionalweave.com/functions/money.format"
        },
        {
          "id": "money.parse",
          "tier": "optional",
          "why": "Reads \"£1,234.50\" or \"1.234,50 €\" typed by a user into minor units, strictly.",
          "section": "display",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Parse \"£1,234.50\" or \"1.234,50 €\" into integer minor units, strictly: no guessing, no rounding.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "parseMoney"
          ],
          "signatures": [
            "pub fn parse_money(text: &str, currency: &str, style: &str) -> Money"
          ],
          "reference": "fune!(money.parse@^1);  // then call parse_money(…)",
          "url": "http://www.functionalweave.com/functions/money.parse"
        },
        {
          "id": "money.currency-digits",
          "tier": "optional",
          "why": "How many decimal places each ISO 4217 currency has, for input boxes and validation.",
          "section": "display",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "How many decimal places a currency's minor unit has, from the ISO 4217 list.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "currencyDigits"
          ],
          "signatures": [
            "pub fn currency_digits(currency: &str) -> i64"
          ],
          "reference": "fune!(money.currency-digits@^1);  // then call currency_digits(…)",
          "url": "http://www.functionalweave.com/functions/money.currency-digits"
        }
      ],
      "gaps": []
    },
    {
      "key": "currencies",
      "title": "Other currencies",
      "purpose": "Convert between currencies at a rate you hold.",
      "from": "money-basics",
      "members": [
        {
          "id": "money.convert",
          "tier": "optional",
          "why": "Converts an amount at a supplied exact rate with explicit rounding.",
          "section": "currencies",
          "from": "money-basics",
          "version": "1.0.0",
          "summary": "Convert an amount to another currency at a supplied exact rate, with an explicit rounding mode.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "convertMoney"
          ],
          "signatures": [
            "pub fn convert_money(amount: &Money, rate: &ExchangeRate, mode: &RoundingMode) -> Money"
          ],
          "reference": "fune!(money.convert@^1);  // then call convert_money(…)",
          "url": "http://www.functionalweave.com/functions/money.convert"
        }
      ],
      "gaps": []
    },
    {
      "key": "date-arithmetic",
      "title": "Calendar arithmetic",
      "purpose": "Move and measure ISO dates exactly, with no date library and no clock reads.",
      "from": "dates-basics",
      "members": [
        {
          "id": "dates.add-days",
          "tier": "core",
          "why": "Shifts a date by whole days, for due dates and windows.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Shift an ISO date by a whole number of days, forwards or backwards, with exact calendar arithmetic.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "addDays"
          ],
          "signatures": [
            "pub fn add_days(iso: &str, days: i64) -> String"
          ],
          "reference": "fune!(dates.add-days@^1);  // then call add_days(…)",
          "url": "http://www.functionalweave.com/functions/dates.add-days"
        },
        {
          "id": "dates.add-months",
          "tier": "core",
          "why": "Adds months clamping to month end, so 31 January plus a month is the end of February.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Add calendar months to an ISO date, clamping to the month end: 31 Jan + 1 month is 28 or 29 Feb.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "addMonths"
          ],
          "signatures": [
            "pub fn add_months(iso: &str, months: i64) -> String"
          ],
          "reference": "fune!(dates.add-months@^1);  // then call add_months(…)",
          "url": "http://www.functionalweave.com/functions/dates.add-months"
        },
        {
          "id": "dates.days-between",
          "tier": "core",
          "why": "Signed calendar days between two dates, for ages of invoices and stays.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Calendar days from one ISO date to another, signed: negative when the second date is earlier.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "daysBetween"
          ],
          "signatures": [
            "pub fn days_between(start_iso: &str, end_iso: &str) -> i64"
          ],
          "reference": "fune!(dates.days-between@^1);  // then call days_between(…)",
          "url": "http://www.functionalweave.com/functions/dates.days-between"
        },
        {
          "id": "dates.month-boundaries",
          "tier": "core",
          "why": "First and last day of a date's month, for monthly periods and statements.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "The first day, last day and length of the calendar month containing a date.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "monthBoundaries"
          ],
          "signatures": [
            "pub fn month_boundaries(iso: &str) -> MonthBoundaries"
          ],
          "reference": "fune!(dates.month-boundaries@^1);  // then call month_boundaries(…)",
          "url": "http://www.functionalweave.com/functions/dates.month-boundaries"
        },
        {
          "id": "dates.day-of-week",
          "tier": "core",
          "why": "ISO weekday of a date, for weekend rules and opening hours.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "The ISO 8601 day of the week for a date: 1 is Monday through 7 is Sunday.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "dayOfWeek"
          ],
          "signatures": [
            "pub fn day_of_week(iso: &str) -> i64"
          ],
          "reference": "fune!(dates.day-of-week@^1);  // then call day_of_week(…)",
          "url": "http://www.functionalweave.com/functions/dates.day-of-week"
        },
        {
          "id": "dates.is-leap-year",
          "tier": "optional",
          "why": "Whether a year is a leap year, for 365/366-day calculations.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Whether a year is a Gregorian leap year: every 4th, except centuries, except every 400th.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "isLeapYear"
          ],
          "signatures": [
            "pub fn is_leap_year(year: i64) -> bool"
          ],
          "reference": "fune!(dates.is-leap-year@^1);  // then call is_leap_year(…)",
          "url": "http://www.functionalweave.com/functions/dates.is-leap-year"
        },
        {
          "id": "dates.overlap",
          "tier": "optional",
          "why": "Days two date ranges share, for prorating and clash checks.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Days two date ranges have in common, with the end of each range stated as inclusive or exclusive.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "overlapDays"
          ],
          "signatures": [
            "pub fn overlap_days(a_start: &str, a_end: &str, b_start: &str, b_end: &str, end_inclusive: bool) -> i64"
          ],
          "reference": "fune!(dates.overlap@^1);  // then call overlap_days(…)",
          "url": "http://www.functionalweave.com/functions/dates.overlap"
        },
        {
          "id": "dates.age",
          "tier": "optional",
          "why": "Age in whole years on a date, handling 29 February birthdays.",
          "section": "date-arithmetic",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Age in whole years on a given date, with 29 February birthdays ageing on 1 March in other years.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "age"
          ],
          "signatures": [
            "pub fn age(birth_date: &str, on_date: &str) -> i64"
          ],
          "reference": "fune!(dates.age@^1);  // then call age(…)",
          "url": "http://www.functionalweave.com/functions/dates.age"
        }
      ],
      "gaps": []
    },
    {
      "key": "reporting-periods",
      "title": "Periods and reporting dates",
      "purpose": "Put dates into the periods reports are grouped by.",
      "from": "dates-basics",
      "members": [
        {
          "id": "dates.quarter",
          "tier": "core",
          "why": "Calendar and fiscal quarter of a date for any year-start month.",
          "section": "reporting-periods",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Calendar quarter and fiscal quarter of a date, with the fiscal year and quarter date ranges, for any start month.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "quarter"
          ],
          "signatures": [
            "pub fn quarter(iso: &str, fiscal_year_start_month: i64) -> Quarter"
          ],
          "reference": "fune!(dates.quarter@^1);  // then call quarter(…)",
          "url": "http://www.functionalweave.com/functions/dates.quarter"
        },
        {
          "id": "dates.week-number",
          "tier": "optional",
          "why": "ISO week number and week-year, for weekly reports.",
          "section": "reporting-periods",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "ISO 8601 week number and week-numbering year of a date: weeks start Monday, week 1 holds the first Thursday.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "weekNumber"
          ],
          "signatures": [
            "pub fn week_number(iso: &str) -> IsoWeek"
          ],
          "reference": "fune!(dates.week-number@^1);  // then call week_number(…)",
          "url": "http://www.functionalweave.com/functions/dates.week-number"
        },
        {
          "id": "dates.day-count-fraction",
          "tier": "optional",
          "why": "Year fraction by ACT/365F, ACT/360, 30/360 or ACT/ACT, for interest periods.",
          "section": "reporting-periods",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Year fraction between two dates by ACT/365F, ACT/360, 30/360, 30E/360 or ACT/ACT ISDA, as an exact fraction.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "dayCountFraction"
          ],
          "signatures": [
            "pub fn day_count_fraction(start_iso: &str, end_iso: &str, convention: &str) -> YearFraction"
          ],
          "reference": "fune!(dates.day-count-fraction@^1);  // then call day_count_fraction(…)",
          "url": "http://www.functionalweave.com/functions/dates.day-count-fraction"
        }
      ],
      "gaps": []
    },
    {
      "key": "working-days",
      "title": "Working days and holidays",
      "purpose": "Skip weekends and public holidays when counting or adding days.",
      "from": "dates-basics",
      "members": [
        {
          "id": "dates.bank-holidays",
          "tier": "core",
          "why": "UK bank holidays for a region and year, as gov.uk publishes them.",
          "section": "working-days",
          "from": "dates-basics",
          "version": "1.0.1",
          "summary": "UK bank and public holidays for a region and year, as published by gov.uk, including one-off holidays.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "bankHolidays"
          ],
          "signatures": [
            "pub fn bank_holidays(region: &str, year: i64) -> Vec<BankHoliday>"
          ],
          "reference": "fune!(dates.bank-holidays@^1);  // then call bank_holidays(…)",
          "url": "http://www.functionalweave.com/functions/dates.bank-holidays"
        },
        {
          "id": "dates.add-business-days",
          "tier": "core",
          "why": "Moves a date by working days, skipping weekends and a holiday list.",
          "section": "working-days",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Move a date forward or back by working days, skipping weekends and a caller-supplied holiday list.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "addBusinessDays"
          ],
          "signatures": [
            "pub fn add_business_days(iso: &str, days: i64, holidays: &[String]) -> String"
          ],
          "reference": "fune!(dates.add-business-days@^1);  // then call add_business_days(…)",
          "url": "http://www.functionalweave.com/functions/dates.add-business-days"
        },
        {
          "id": "dates.business-days-between",
          "tier": "core",
          "why": "Working days between two dates, for SLAs and notice periods.",
          "section": "working-days",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Working days between two dates, excluding weekends and a caller-supplied holiday list.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "businessDaysBetween"
          ],
          "signatures": [
            "pub fn business_days_between(start_iso: &str, end_iso: &str, holidays: &[String]) -> i64"
          ],
          "reference": "fune!(dates.business-days-between@^1);  // then call business_days_between(…)",
          "url": "http://www.functionalweave.com/functions/dates.business-days-between"
        }
      ],
      "gaps": []
    },
    {
      "key": "repeating-dates",
      "title": "Repeating schedules",
      "purpose": "Generate the dates of something that repeats.",
      "from": "dates-basics",
      "members": [
        {
          "id": "dates.recurrence",
          "tier": "optional",
          "why": "The next n dates of a rule: monthly on the 31st, last Friday, every two weeks.",
          "section": "repeating-dates",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "The next n dates of a repeating rule: monthly on a day, last weekday of the month, every k days or weeks, yearly.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "recurrence"
          ],
          "signatures": [
            "pub fn recurrence(rule: &RecurrenceRule, start_iso: &str, count: i64) -> Vec<String>"
          ],
          "reference": "fune!(dates.recurrence@^1);  // then call recurrence(…)",
          "url": "http://www.functionalweave.com/functions/dates.recurrence"
        }
      ],
      "gaps": []
    },
    {
      "key": "times",
      "title": "Times and durations",
      "purpose": "Work with times of day, durations and timestamps.",
      "from": "dates-basics",
      "members": [
        {
          "id": "time.duration",
          "tier": "optional",
          "why": "Parses and adds durations like 1h30m in whole minutes, and formats the total.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Parse durations written as 1h30m, 90m or 01:30, add them up in whole minutes, and format the total.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "duration"
          ],
          "signatures": [
            "pub fn duration(parts: &[String]) -> Duration"
          ],
          "reference": "fune!(time.duration@^1);  // then call duration(…)",
          "url": "http://www.functionalweave.com/functions/time.duration"
        },
        {
          "id": "time.minutes-between",
          "tier": "optional",
          "why": "Minutes between two local date-times, across midnight.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Minutes from one local date and time to another, correct across midnight; wall-clock time, no time zones.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "minutesBetween"
          ],
          "signatures": [
            "pub fn minutes_between(start_date: &str, start_time: &str, end_date: &str, end_time: &str) -> i64"
          ],
          "reference": "fune!(time.minutes-between@^1);  // then call minutes_between(…)",
          "url": "http://www.functionalweave.com/functions/time.minutes-between"
        },
        {
          "id": "time.round-to-increment",
          "tier": "optional",
          "why": "Rounds minutes to a billing increment such as 6 or 15.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Round a duration in minutes to a billing increment such as 6 or 15 minutes, up, down or to nearest.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "roundToIncrement"
          ],
          "signatures": [
            "pub fn round_to_increment(minutes: i64, increment: i64, mode: &RoundingMode) -> i64"
          ],
          "reference": "fune!(time.round-to-increment@^1);  // then call round_to_increment(…)",
          "url": "http://www.functionalweave.com/functions/time.round-to-increment"
        },
        {
          "id": "time.iso-to-unix",
          "tier": "optional",
          "why": "RFC 3339 timestamps to Unix seconds.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "An RFC 3339 timestamp with a Z or offset, such as 2026-09-26T13:00:00+01:00, to Unix time in whole seconds.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "isoToUnix"
          ],
          "signatures": [
            "pub fn iso_to_unix(timestamp: &str) -> i64"
          ],
          "reference": "fune!(time.iso-to-unix@^1);  // then call iso_to_unix(…)",
          "url": "http://www.functionalweave.com/functions/time.iso-to-unix"
        },
        {
          "id": "time.unix-to-iso",
          "tier": "optional",
          "why": "Unix seconds to an ISO 8601 UTC timestamp.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Unix time in seconds to an ISO 8601 UTC timestamp such as 2026-09-26T12:00:00Z, for years 0001 to 9999.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "unixToIso"
          ],
          "signatures": [
            "pub fn unix_to_iso(seconds: i64) -> String"
          ],
          "reference": "fune!(time.unix-to-iso@^1);  // then call unix_to_iso(…)",
          "url": "http://www.functionalweave.com/functions/time.unix-to-iso"
        },
        {
          "id": "time.countdown",
          "tier": "optional",
          "why": "Seconds left until a moment and a 4:05 style timer text.",
          "section": "times",
          "from": "dates-basics",
          "version": "1.0.0",
          "summary": "Seconds left until a Unix time, whether it has passed, and a timer text like 4:05, for tokens or retry waits.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "countdown"
          ],
          "signatures": [
            "pub fn countdown(until: i64, now: i64) -> Countdown"
          ],
          "reference": "fune!(time.countdown@^1);  // then call countdown(…)",
          "url": "http://www.functionalweave.com/functions/time.countdown"
        }
      ],
      "gaps": []
    },
    {
      "key": "vat",
      "title": "VAT",
      "purpose": "Work out VAT from the dated rates HMRC publishes.",
      "from": "uk-tax",
      "members": [
        {
          "id": "finance.tax.vat-rate",
          "tier": "core",
          "why": "The VAT rate in force for a jurisdiction, category and date; the others look it up.",
          "section": "vat",
          "from": "uk-tax",
          "version": "1.2.2",
          "summary": "Look up the VAT rate in force for a jurisdiction, category and date.",
          "status": "unreviewed",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "vatRate"
          ],
          "signatures": [
            "pub fn vat_rate(jurisdiction: &str, category: &str, on_date: &str) -> VatRate"
          ],
          "reference": "fune!(finance.tax.vat-rate@^1);  // then call vat_rate(…)",
          "url": "http://www.functionalweave.com/functions/finance.tax.vat-rate"
        },
        {
          "id": "finance.tax.add-vat",
          "tier": "core",
          "why": "Net to gross: VAT on a net amount at the rate on the date of supply.",
          "section": "vat",
          "from": "uk-tax",
          "version": "1.0.1",
          "summary": "Add VAT to a net amount using the rate in force on the date of supply.",
          "status": "unreviewed",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "addVat"
          ],
          "signatures": [
            "pub fn add_vat(net: &Money, jurisdiction: &str, category: &str, on_date: &str) -> VatBreakdown"
          ],
          "reference": "fune!(finance.tax.add-vat@^1);  // then call add_vat(…)",
          "url": "http://www.functionalweave.com/functions/finance.tax.add-vat"
        },
        {
          "id": "finance.tax.remove-vat",
          "tier": "core",
          "why": "Gross to net: the VAT already inside a VAT-inclusive price.",
          "section": "vat",
          "from": "uk-tax",
          "version": "1.0.2",
          "summary": "Extract the VAT already included in a gross amount, using the rate in force on the date of supply.",
          "status": "unreviewed",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "removeVat"
          ],
          "signatures": [
            "pub fn remove_vat(gross: &Money, jurisdiction: &str, category: &str, on_date: &str) -> VatBreakdown"
          ],
          "reference": "fune!(finance.tax.remove-vat@^1);  // then call remove_vat(…)",
          "url": "http://www.functionalweave.com/functions/finance.tax.remove-vat"
        },
        {
          "id": "finance.tax.vat-return",
          "tier": "optional",
          "why": "The nine-box VAT return totals for a period from categorised sales and purchases.",
          "section": "vat",
          "from": "uk-tax",
          "version": "1.0.1",
          "summary": "UK nine-box VAT return totals for a period from categorised sales and purchases (VAT Notice 700/12).",
          "status": "unreviewed",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "vatReturn"
          ],
          "signatures": [
            "pub fn vat_return(transactions: &[VatTransaction], period_start: &str, period_end: &str) -> VatReturn"
          ],
          "reference": "fune!(finance.tax.vat-return@^1);  // then call vat_return(…)",
          "url": "http://www.functionalweave.com/functions/finance.tax.vat-return"
        },
        {
          "id": "finance.tax.vat-flat-rate",
          "tier": "optional",
          "why": "VAT due under the Flat Rate Scheme by business sector, for small businesses on it.",
          "section": "vat",
          "from": "uk-tax",
          "version": "1.0.1",
          "summary": "UK VAT Flat Rate Scheme: VAT due on gross turnover at the sector's dated rate, limited cost rate and first-year 1% off.",
          "status": "unreviewed",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "flatRateVat"
          ],
          "signatures": [
            "pub fn flat_rate_vat(turnover: &Money, sector: &str, limited_cost_business: bool, registration_date: Option<&str>, supply_date: &str) -> FlatRateVat"
          ],
          "reference": "fune!(finance.tax.vat-flat-rate@^1);  // then call flat_rate_vat(…)",
          "url": "http://www.functionalweave.com/functions/finance.tax.vat-flat-rate"
        }
      ],
      "gaps": []
    },
    {
      "key": "tax-year",
      "title": "Tax periods",
      "purpose": "Put dates into the UK periods tax is worked out over.",
      "from": "uk-tax",
      "members": [
        {
          "id": "dates.uk-tax-year",
          "tier": "core",
          "why": "Which UK tax year (6 April to 5 April) a date falls in.",
          "section": "tax-year",
          "from": "uk-tax",
          "version": "1.0.0",
          "summary": "The UK personal tax year containing a date: 6 April to 5 April, with HMRC's label.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "ukTaxYear"
          ],
          "signatures": [
            "pub fn uk_tax_year(iso: &str) -> UkTaxYear"
          ],
          "reference": "fune!(dates.uk-tax-year@^1);  // then call uk_tax_year(…)",
          "url": "http://www.functionalweave.com/functions/dates.uk-tax-year"
        }
      ],
      "gaps": []
    },
    {
      "key": "identifiers",
      "title": "Tax identifiers",
      "purpose": "Check the numbers HMRC and Companies House issue before storing them.",
      "from": "uk-tax",
      "members": [
        {
          "id": "validation.uk-vat-number",
          "tier": "core",
          "why": "Checks a UK VAT registration number's checksum.",
          "section": "identifiers",
          "from": "uk-tax",
          "version": "1.0.0",
          "summary": "Check a UK VAT registration number against HMRC's mod-97 checksum, including the post-2010 9755 variant.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "isUkVatNumber"
          ],
          "signatures": [
            "pub fn is_uk_vat_number(value: &str) -> bool"
          ],
          "reference": "fune!(validation.uk-vat-number@^1);  // then call is_uk_vat_number(…)",
          "url": "http://www.functionalweave.com/functions/validation.uk-vat-number"
        },
        {
          "id": "validation.uk-utr",
          "tier": "optional",
          "why": "Checks a Unique Taxpayer Reference's check digit.",
          "section": "identifiers",
          "from": "uk-tax",
          "version": "2.0.1",
          "summary": "Check an HMRC Unique Taxpayer Reference's modulus 11 check digit, accepting the K and spacing people type.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "validateUkUtr"
          ],
          "signatures": [
            "pub fn validate_uk_utr(value: &str) -> UkUtr"
          ],
          "reference": "fune!(validation.uk-utr@^2);  // then call validate_uk_utr(…)",
          "url": "http://www.functionalweave.com/functions/validation.uk-utr"
        },
        {
          "id": "validation.uk-company-number",
          "tier": "optional",
          "why": "Checks and pads a Companies House company number.",
          "section": "identifiers",
          "from": "uk-tax",
          "version": "2.0.1",
          "summary": "Check a Companies House company number's format and prefix (SC, NI, OC, LP...) and zero-pad it to eight characters.",
          "status": "stable",
          "license": "NOASSERTION",
          "languages": [
            "typescript",
            "python",
            "rust"
          ],
          "available": true,
          "group": false,
          "functions": [
            "validateUkCompanyNumber"
          ],
          "signatures": [
            "pub fn validate_uk_company_number(value: &str) -> UkCompanyNumber"
          ],
          "reference": "fune!(validation.uk-company-number@^2);  // then call validate_uk_company_number(…)",
          "url": "http://www.functionalweave.com/functions/validation.uk-company-number"
        }
      ],
      "gaps": []
    }
  ],
  "gaps": [
    {
      "key": "plan-catalogue",
      "text": "Plans, prices, add-ons and feature entitlements: the product catalogue and which features each plan unlocks are yours.",
      "section": "plans",
      "from": "subscriptions"
    },
    {
      "key": "usage-metering",
      "text": "Recording usage events reliably and aggregating them per customer per period.",
      "section": "usage",
      "from": "subscriptions"
    },
    {
      "key": "invoice-delivery",
      "text": "Numbering, PDF rendering and emailing the invoices.",
      "section": "renewal",
      "from": "subscriptions"
    },
    {
      "key": "payment-provider",
      "text": "Charging cards or Direct Debit through Stripe, GoCardless or similar, storing payment methods and handling webhooks.",
      "section": "collection",
      "from": "subscriptions"
    },
    {
      "key": "dunning-emails",
      "text": "The emails and in-app notices that go with each retry.",
      "section": "collection",
      "from": "subscriptions"
    },
    {
      "key": "customer-portal",
      "text": "A self-serve portal to change plan, update a card and download invoices.",
      "section": null,
      "from": "subscriptions"
    },
    {
      "key": "fx-rate-source",
      "text": "Where exchange rates come from: money.convert takes a rate you supply; fetching and storing daily rates (ECB, a provider's API) is yours.",
      "section": null,
      "from": "money-basics"
    },
    {
      "key": "money-storage",
      "text": "Storing amounts: the database columns (integer minor units plus a currency code) and their migrations are yours.",
      "section": null,
      "from": "money-basics"
    },
    {
      "key": "time-zones",
      "text": "Time zones and daylight saving: caps works in ISO dates, UTC and wall-clock times; converting with the IANA tz rules is yours.",
      "section": null,
      "from": "dates-basics"
    },
    {
      "key": "locale-formatting",
      "text": "Showing dates in a user's language and format (\"3 octobre 2026\"): caps returns ISO dates, not localised text.",
      "section": null,
      "from": "dates-basics"
    },
    {
      "key": "non-uk-holidays",
      "text": "Public holidays outside the UK: dates.bank-holidays is UK only; pass another country's list to the working-day functions yourself.",
      "section": null,
      "from": "dates-basics"
    },
    {
      "key": "mtd-submission",
      "text": "Making Tax Digital: submitting the return to HMRC's VAT API (OAuth, fraud-prevention headers, obligations) is yours; caps gives the nine boxes.",
      "section": null,
      "from": "uk-tax"
    },
    {
      "key": "corporation-tax",
      "text": "Corporation tax and self-assessment income tax computations are not in caps.",
      "section": null,
      "from": "uk-tax"
    },
    {
      "key": "vat-number-lookup",
      "text": "Checking that a VAT number is registered (HMRC's lookup API) is network work; caps checks the checksum only.",
      "section": null,
      "from": "uk-tax"
    }
  ],
  "counts": {
    "members": 59,
    "core": 29,
    "optional": 30,
    "unreviewed": 5,
    "deprecated": [],
    "unavailable": [],
    "missing": [],
    "gaps": 14
  },
  "warnings": [
    "3 core capabilities are unreviewed - they pass their tests but await an expert's sign-off; tell the user before relying on them, and never use one silently for tax, payroll, legal, clinical or other regulated rules: finance.tax.vat-rate, finance.tax.add-vat, finance.tax.remove-vat",
    "2 optional capabilities are unreviewed too: finance.tax.vat-return, finance.tax.vat-flat-rate",
    "Calculates tax from published rules; it is not tax advice. Have an accountant check how the app uses it before anyone files a return."
  ],
  "url": "http://www.functionalweave.com/suites/subscriptions",
  "owner": null
}