Suites · subscriptions
Subscriptions and SaaS billing
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.
59 capabilities (29 core, 30 optional) and 14 gaps, in build order. Put each line in the file that calls it, then fune build.
Calculates tax from published rules; it is not tax advice. Have an accountant check how the app uses it before anyone files a return.
TypeScript Python Rust
1. Plans and trials
Start a subscription and know when it is first charged.
subscriptions.trial-end core
When a free trial ends and the first payment is due.
When a free trial ends and the first payment is due, from the trial's start date and length in days.
import { trialEnd } from "#fune/subscriptions.trial-end@^1" ;
1.0.0
subscriptions.next-billing-date core
The next renewal date from the billing anchor and interval, month-end safe.
The next renewal date after a given day, from a billing anchor and interval; month-end safe.
import { nextBillingDate } from "#fune/subscriptions.next-billing-date@^1" ;
1.0.0
plan-catalogue Plans, prices, add-ons and feature entitlements: the product catalogue and which features each plan unlocks are yours.
2. Plan changes
Upgrade, downgrade or cancel mid-cycle without over- or under-charging.
subscriptions.proration core
Credit for unused time on the old plan and the charge for the rest of the period on the new one.
Mid-cycle plan change: credit for unused time on the old plan and charge for the rest of the period on the new one.
import { prorateChange } from "#fune/subscriptions.proration@^1" ;
1.0.0
finance.proration optional
Splits any amount into used and unused parts of a billing period, for cancellations and refunds.
Split an amount into the part used and the part unused over a billing period, losing nothing.
import { prorate } from "#fune/finance.proration@^1" ;
1.0.0
3. Metered usage
Price what a customer used in the period.
subscriptions.usage-tiered optional
Prices metered usage against graduated or volume tiers, with flat fees per tier.
Price metered usage against tiers, graduated or volume, with an optional flat fee per tier.
import { priceUsage } from "#fune/subscriptions.usage-tiered@^1" ;
1.0.0
usage-metering Recording usage events reliably and aggregating them per customer per period.
4. Renewal invoices
Bill each renewal with its add-ons, usage, coupons and VAT.
subscriptions.invoice core
The renewal invoice: plan, add-ons and usage lines, coupon discounts by billing period, and VAT.
A subscription renewal invoice: plan, add-ons and usage, coupon discounts by billing period, and VAT.
import { renewalInvoice } from "#fune/subscriptions.invoice@^1" ;
1.0.0
finance.invoice.calculate core
Totals the invoice lines and VAT by rate (the renewal invoice builds on it).
Calculate a complete invoice: line nets, VAT per line, totals and a VAT summary grouped by rate.
import { calculateInvoice } from "#fune/finance.invoice.calculate@^1" ;
1.0.1
retail.oss-vat optional
Which EU country's VAT applies when selling to consumers across the EU.
EU distance-selling VAT: which member state's standard rate applies under OSS and the EUR 10,000 threshold, and the VAT.
import { ossVat } from "#fune/retail.oss-vat@^1" ;
1.0.1
invoice-delivery Numbering, PDF rendering and emailing the invoices.
5. Payment and dunning
Take payment and retry when it fails.
subscriptions.dunning-schedule core
Retry dates for a failed payment from a retry policy, in calendar or business days.
Retry dates for a failed subscription payment from a retry policy, in calendar or business days.
import { dunningSchedule } from "#fune/subscriptions.dunning-schedule@^1" ;
1.0.0
payment-provider Charging cards or Direct Debit through Stripe, GoCardless or similar, storing payment methods and handling webhooks.dunning-emails The emails and in-app notices that go with each retry.
6. SaaS metrics
Report the numbers a subscription business is run by.
subscriptions.mrr-arr core
MRR and ARR from the subscriptions, normalising intervals and leaving out trials.
Monthly and annual recurring revenue from a list of subscriptions, normalising intervals and leaving out trials.
import { mrrArr } from "#fune/subscriptions.mrr-arr@^1" ;
1.1.0
subscriptions.churn optional
Logo and revenue churn and net revenue retention for a period.
Logo churn, gross and net revenue churn, and gross and net revenue retention for one period, in basis points.
import { churnMetrics } from "#fune/subscriptions.churn@^1" ;
1.0.0
subscriptions.ltv-cac optional
Customer lifetime value, LTV:CAC and CAC payback months.
Customer lifetime value, LTV to CAC ratio and CAC payback months from ARPA, gross margin, churn and CAC.
import { ltvCac } from "#fune/subscriptions.ltv-cac@^1" ;
1.0.0
charts.line-chart optional
Line chart geometry for MRR over time.
Complete line chart geometry as data: plot area, number or date axes, gridlines, series paths, colours, legend.
import { lineChart } from "#fune/charts.line-chart@^1" ;
1.0.0
Hold money as currency-tagged integer minor units and do plain arithmetic on it safely.
money.amount core
The Money type ({ minor, currency }) every other money capability takes and returns.
A currency-tagged monetary amount held in integer minor units.
import { money } from "#fune/money.amount@^1" ;
1.0.2
money.add core
Adds and subtracts amounts, refusing to mix currencies by accident.
Add, subtract and negate monetary amounts, refusing to mix currencies.
import { addMoney } from "#fune/money.add@^1" ;
1.0.1
money.sum core
Totals a list of amounts, with an explicit currency for an empty list.
Total a list of monetary amounts, with an explicit currency for the empty case.
import { sumMoney } from "#fune/money.sum@^1" ;
1.0.1
money.compare core
Compares, mins, maxes and clamps amounts of one currency without unwrapping them.
Compare two amounts of one currency, refusing to compare across currencies; min, max and clamp too.
import { compareMoney, minMoney, maxMoney, clampMoney } from "#fune/money.compare@^2" ;
2.0.04 functions
8. Rates and rounding from money-basics
Apply percentages and divide amounts with the rounding stated, never a float.
money.apply-rate core
Applies a basis-point rate (VAT, discount, commission) to an amount with explicit rounding.
Apply a rate expressed in basis points to a monetary amount, with an explicit rounding mode.
import { applyRate } from "#fune/money.apply-rate@^1" ;
1.0.1
math.round-div core
Integer division with a named rounding mode, for any money maths the others do not cover.
Integer division with an explicit rounding mode, for money arithmetic that must not drift.
import { roundDiv } from "#fune/math.round-div@^1" ;
1.0.0
math.basis-points optional
Converts "12.5%" to and from basis points exactly, for rates typed in by users.
Convert a rate between percent, basis points and a plain ratio exactly, as decimal text or integer basis points.
import { convertRate, toBasisPoints, fromBasisPoints } from "#fune/math.basis-points@^2" ;
2.0.03 functions
math.percent-change optional
Percentage change between two figures in basis points, for "up 4.2% on last month".
Percentage change from one value to another, in basis points, with an explicit rounding mode.
import { percentChange } from "#fune/math.percent-change@^1" ;
1.0.0
math.rational optional
Exact fractions for rates that must not drift across many steps.
Exact fraction arithmetic, always reduced, for rates and ratios that must not drift.
import { calculateRational, rational, addRational, subtractRational, multiplyRational, divideRational, compareRational, rationalToInteger } from "#fune/math.rational@^2" ;
2.0.08 functions
math.round-div-big optional
Rounded division on numbers past 2^53, for very large totals.
Divide two integers of any size with an explicit rounding mode, on decimal strings, for exact money sums past 2^53.
import { roundDivBig } from "#fune/math.round-div-big@^1" ;
1.0.0
Divide an amount between people, periods or lines so the parts add back to the whole.
money.allocate core
Splits an amount by ratios (60/40, by quantity) without losing or inventing a penny.
Split an amount across ratios without losing or inventing a single minor unit.
import { allocate } from "#fune/money.allocate@^1" ;
1.0.1
money.split-even optional
Splits an amount into n near-equal parts that sum exactly, for instalments or shares.
Split an amount into n near-equal parts that add back up to exactly the whole.
import { splitEven } from "#fune/money.split-even@^1" ;
1.0.0
10. Display and input from money-basics
Show amounts to people and read what they type.
money.format core
Renders an amount as text with the currency's real number of decimal places.
Render a monetary amount as text, using the currency's real minor-unit precision.
import { formatMoney } from "#fune/money.format@^1" ;
1.0.0
money.parse optional
Reads "£1,234.50" or "1.234,50 €" typed by a user into minor units, strictly.
Parse "£1,234.50" or "1.234,50 €" into integer minor units, strictly: no guessing, no rounding.
import { parseMoney } from "#fune/money.parse@^1" ;
1.0.0
money.currency-digits optional
How many decimal places each ISO 4217 currency has, for input boxes and validation.
How many decimal places a currency's minor unit has, from the ISO 4217 list.
import { currencyDigits } from "#fune/money.currency-digits@^1" ;
1.0.0
11. Other currencies from money-basics
Convert between currencies at a rate you hold.
money.convert optional
Converts an amount at a supplied exact rate with explicit rounding.
Convert an amount to another currency at a supplied exact rate, with an explicit rounding mode.
import { convertMoney } from "#fune/money.convert@^1" ;
1.0.0
12. Calendar arithmetic from dates-basics
Move and measure ISO dates exactly, with no date library and no clock reads.
dates.add-days core
Shifts a date by whole days, for due dates and windows.
Shift an ISO date by a whole number of days, forwards or backwards, with exact calendar arithmetic.
import { addDays } from "#fune/dates.add-days@^1" ;
1.0.0
dates.add-months core
Adds months clamping to month end, so 31 January plus a month is the end of February.
Add calendar months to an ISO date, clamping to the month end: 31 Jan + 1 month is 28 or 29 Feb.
import { addMonths } from "#fune/dates.add-months@^1" ;
1.0.0
dates.days-between core
Signed calendar days between two dates, for ages of invoices and stays.
Calendar days from one ISO date to another, signed: negative when the second date is earlier.
import { daysBetween } from "#fune/dates.days-between@^1" ;
1.0.0
dates.month-boundaries core
First and last day of a date's month, for monthly periods and statements.
The first day, last day and length of the calendar month containing a date.
import { monthBoundaries } from "#fune/dates.month-boundaries@^1" ;
1.0.0
dates.day-of-week core
ISO weekday of a date, for weekend rules and opening hours.
The ISO 8601 day of the week for a date: 1 is Monday through 7 is Sunday.
import { dayOfWeek } from "#fune/dates.day-of-week@^1" ;
1.0.0
dates.is-leap-year optional
Whether a year is a leap year, for 365/366-day calculations.
Whether a year is a Gregorian leap year: every 4th, except centuries, except every 400th.
import { isLeapYear } from "#fune/dates.is-leap-year@^1" ;
1.0.0
dates.overlap optional
Days two date ranges share, for prorating and clash checks.
Days two date ranges have in common, with the end of each range stated as inclusive or exclusive.
import { overlapDays } from "#fune/dates.overlap@^1" ;
1.0.0
dates.age optional
Age in whole years on a date, handling 29 February birthdays.
Age in whole years on a given date, with 29 February birthdays ageing on 1 March in other years.
import { age } from "#fune/dates.age@^1" ;
1.0.0
13. Periods and reporting dates from dates-basics
Put dates into the periods reports are grouped by.
dates.quarter core
Calendar and fiscal quarter of a date for any year-start month.
Calendar quarter and fiscal quarter of a date, with the fiscal year and quarter date ranges, for any start month.
import { quarter } from "#fune/dates.quarter@^1" ;
1.0.0
dates.week-number optional
ISO week number and week-year, for weekly reports.
ISO 8601 week number and week-numbering year of a date: weeks start Monday, week 1 holds the first Thursday.
import { weekNumber } from "#fune/dates.week-number@^1" ;
1.0.0
dates.day-count-fraction optional
Year fraction by ACT/365F, ACT/360, 30/360 or ACT/ACT, for interest periods.
Year fraction between two dates by ACT/365F, ACT/360, 30/360, 30E/360 or ACT/ACT ISDA, as an exact fraction.
import { dayCountFraction } from "#fune/dates.day-count-fraction@^1" ;
1.0.0
14. Working days and holidays from dates-basics
Skip weekends and public holidays when counting or adding days.
dates.bank-holidays core
UK bank holidays for a region and year, as gov.uk publishes them.
UK bank and public holidays for a region and year, as published by gov.uk, including one-off holidays.
import { bankHolidays } from "#fune/dates.bank-holidays@^1" ;
1.0.1
dates.add-business-days core
Moves a date by working days, skipping weekends and a holiday list.
Move a date forward or back by working days, skipping weekends and a caller-supplied holiday list.
import { addBusinessDays } from "#fune/dates.add-business-days@^1" ;
1.0.0
dates.business-days-between core
Working days between two dates, for SLAs and notice periods.
Working days between two dates, excluding weekends and a caller-supplied holiday list.
import { businessDaysBetween } from "#fune/dates.business-days-between@^1" ;
1.0.0
15. Repeating schedules from dates-basics
Generate the dates of something that repeats.
dates.recurrence optional
The next n dates of a rule: monthly on the 31st, last Friday, every two weeks.
The next n dates of a repeating rule: monthly on a day, last weekday of the month, every k days or weeks, yearly.
import { recurrence } from "#fune/dates.recurrence@^1" ;
1.0.0
16. Times and durations from dates-basics
Work with times of day, durations and timestamps.
time.duration optional
Parses and adds durations like 1h30m in whole minutes, and formats the total.
Parse durations written as 1h30m, 90m or 01:30, add them up in whole minutes, and format the total.
import { duration } from "#fune/time.duration@^1" ;
1.0.0
time.minutes-between optional
Minutes between two local date-times, across midnight.
Minutes from one local date and time to another, correct across midnight; wall-clock time, no time zones.
import { minutesBetween } from "#fune/time.minutes-between@^1" ;
1.0.0
time.round-to-increment optional
Rounds minutes to a billing increment such as 6 or 15.
Round a duration in minutes to a billing increment such as 6 or 15 minutes, up, down or to nearest.
import { roundToIncrement } from "#fune/time.round-to-increment@^1" ;
1.0.0
time.iso-to-unix optional
RFC 3339 timestamps to Unix seconds.
An RFC 3339 timestamp with a Z or offset, such as 2026-09-26T13:00:00+01:00, to Unix time in whole seconds.
import { isoToUnix } from "#fune/time.iso-to-unix@^1" ;
1.0.0
time.unix-to-iso optional
Unix seconds to an ISO 8601 UTC timestamp.
Unix time in seconds to an ISO 8601 UTC timestamp such as 2026-09-26T12:00:00Z, for years 0001 to 9999.
import { unixToIso } from "#fune/time.unix-to-iso@^1" ;
1.0.0
time.countdown optional
Seconds left until a moment and a 4:05 style timer text.
Seconds left until a Unix time, whether it has passed, and a timer text like 4:05, for tokens or retry waits.
import { countdown } from "#fune/time.countdown@^1" ;
1.0.0
17. VAT from uk-tax
Work out VAT from the dated rates HMRC publishes.
finance.tax.vat-rate core Unreviewed
The VAT rate in force for a jurisdiction, category and date; the others look it up.
Look up the VAT rate in force for a jurisdiction, category and date.
import { vatRate } from "#fune/finance.tax.vat-rate@^1" ;
1.2.2
finance.tax.add-vat core Unreviewed
Net to gross: VAT on a net amount at the rate on the date of supply.
Add VAT to a net amount using the rate in force on the date of supply.
import { addVat } from "#fune/finance.tax.add-vat@^1" ;
1.0.1
finance.tax.remove-vat core Unreviewed
Gross to net: the VAT already inside a VAT-inclusive price.
Extract the VAT already included in a gross amount, using the rate in force on the date of supply.
import { removeVat } from "#fune/finance.tax.remove-vat@^1" ;
1.0.2
finance.tax.vat-return optional Unreviewed
The nine-box VAT return totals for a period from categorised sales and purchases.
UK nine-box VAT return totals for a period from categorised sales and purchases (VAT Notice 700/12).
import { vatReturn } from "#fune/finance.tax.vat-return@^1" ;
1.0.1
finance.tax.vat-flat-rate optional Unreviewed
VAT due under the Flat Rate Scheme by business sector, for small businesses on it.
UK VAT Flat Rate Scheme: VAT due on gross turnover at the sector's dated rate, limited cost rate and first-year 1% off.
import { flatRateVat } from "#fune/finance.tax.vat-flat-rate@^1" ;
1.0.1
18. Tax periods from uk-tax
Put dates into the UK periods tax is worked out over.
dates.uk-tax-year core
Which UK tax year (6 April to 5 April) a date falls in.
The UK personal tax year containing a date: 6 April to 5 April, with HMRC's label.
import { ukTaxYear } from "#fune/dates.uk-tax-year@^1" ;
1.0.0
19. Tax identifiers from uk-tax
Check the numbers HMRC and Companies House issue before storing them.
validation.uk-vat-number core
Checks a UK VAT registration number's checksum.
Check a UK VAT registration number against HMRC's mod-97 checksum, including the post-2010 9755 variant.
import { isUkVatNumber } from "#fune/validation.uk-vat-number@^1" ;
1.0.0
validation.uk-utr optional
Checks a Unique Taxpayer Reference's check digit.
Check an HMRC Unique Taxpayer Reference's modulus 11 check digit, accepting the K and spacing people type.
import { validateUkUtr } from "#fune/validation.uk-utr@^2" ;
2.0.1
validation.uk-company-number optional
Checks and pads a Companies House company number.
Check a Companies House company number's format and prefix (SC, NI, OC, LP...) and zero-pad it to eight characters.
import { validateUkCompanyNumber } from "#fune/validation.uk-company-number@^2" ;
2.0.1
Gaps What this kind of app usually needs that Functional Weave does not have yet: write these yourself, or use a service.
customer-portal A self-serve portal to change plan, update a card and download invoices.fx-rate-source Where exchange rates come from: money.convert takes a rate you supply; fetching and storing daily rates (ECB, a provider's API) is yours.money-storage Storing amounts: the database columns (integer minor units plus a currency code) and their migrations are yours.time-zones Time zones and daylight saving: caps works in ISO dates, UTC and wall-clock times; converting with the IANA tz rules is yours.locale-formatting Showing dates in a user's language and format ("3 octobre 2026"): caps returns ISO dates, not localised text.non-uk-holidays Public holidays outside the UK: dates.bank-holidays is UK only; pass another country's list to the working-day functions yourself.mtd-submission Making Tax Digital: submitting the return to HMRC's VAT API (OAuth, fraud-prevention headers, obligations) is yours; caps gives the nine boxes.corporation-tax Corporation tax and self-assessment income tax computations are not in caps.vat-number-lookup Checking that a VAT number is registered (HMRC's lookup API) is network work; caps checks the checksum only.