A tax combinator library could be useful for Rules-as-Code [lightning talk]

Lightning talk at Everything Open 2023.

I demonstrate the Haskell /tax/ library and introduce /tax-ato/,
a library for Australian personal income tax calculations.

Transcript

1. A tax combinator library could be useful for Rules as Code or: surely you didn’t think you’d escape without hearing me talk about Haskell? Fraser Tweedale @hackuador@functional.cafe March 16, 2023
2. Rates Rebate Act 1973 (NZ) s 3(1) A ratepayer who, at the commencement of a rating year, was the ratepayer of a residential property is entitled, on application in that year, to a rebate of— (a) so much of the rates payable for that rating year in respect of the property as represents— (i) two-thirds of the amount by which those rates exceed $160, reduced by— (ii) $1 for each $8 by which the ratepayer’s income for the preceding tax year exceeded $28,080, that last-mentioned amount being increased by $500 in respect of each person who was a dependant of the ratepayer at the commencement of the rating year in respect of which the application is made; or (b) $700,— whichever amount is smaller.
3. incomeThreshold :: Natural -> Money Rational incomeThreshold numDependants = Money (28080 + fromIntegral numDependants * 500) reduction :: Natural -> Tax (Money Rational) (Money Rational) reduction numDependants = above (incomeThreshold numDependants ) ( -1/8) rebate :: Natural -> Money Rational -> Tax (Money Rational) (Money Rational) rebate numDependants income = lesserOf ( lump (Money 700)) ( greaterOf ( lump (Money 0)) ( above (Money 160) (2/3) <> lump ( getTax (reduction numDependants ) income )))
4. Combinators: functions that compose smaller computations into a more complex computation. . . that (typically) follow some laws. . . such that our ability to reason about the computation can scale as it grows.
5. medicareLevy loThreshold = lesserOf ( above loThreshold 0.1) ( flat 0.02)
6. lowIncomeTaxOffset = limit (Money 0) ( lump (Money ( -445)) <> above (Money 37000) 0.015 )
7. individualIncomeTax = marginal’ [ ( 18200 , 0.19 ) , ( 45000 , 0.325 - 0.19 ) , (120000 , 0.37 - 0.325) , (180000 , 0.45 - 0.37 ) ]
8. data Tax b a = Tax { getTax :: b -> a } deriving ( Semigroup, Monoid, Functor, Profunctor )
9. allTheTaxes = individualIncomeTax <> medicareLevy (Money 23226) <> lowIncomeTaxOffset weeklyWithholding = dimap ($* 52) ($/ 52) allTheTaxes fnlyWithholding = dimap ($* 26) ($/ 26) allTheTaxes monthlyWithholding = dimap ($* 12) ($/ 12) allTheTaxes
10. https://hackage.haskell.org/package/tax https://github.com/frasertweedale/hs-tax blog post: https://is.gd/tax_combinators useful for "Rules as Code" calculations relating to tax, levies, rates, rebates, payroll...
11. What do /I/ use it for? hs-tax-ato - personal income tax library https://hackage.haskell.org/package/tax-ato https://github.com/frasertweedale/hs-tax-ato
12. hs-tax-ato: stuff that’s implemented individual income tax, employee share schemes Medicare levy, Medicare levy surcharge HELP (HECS/SFSS student loans) deductions offsets: low income, LMITO, spouse contribution private health insurance rebate adjustments dividends, CGT, foreign income, foreign income tax offset rates/rules for previous financial years (back to 2017)
13. hs-tax-ato: stuff that’s NOT implemented PAYG instalments (coming soon) some adjustments/variations based on family income or dependents partnership/trust/personal services income super income streams and lump payments some "grandfathering" rules (e.g. CGT indexation method) lots of other esoteric (to me) features and quirks
14. hs-tax-ato: known issues needs updates for 2022–23 FY (coming soon) missing some rounding steps lack of usage examples
15. Maybe these libraries will be useful or interesting to someone...
16. © 2023 Fraser Tweedale Except where otherwise noted this work is licensed under http://creativecommons.org/licenses/by/4.0/ Code https://github.com/frasertweedale Blog frasertweedale.github.io/blog-fp Fediverse @hackuador@functional.cafe