Pure functional tax bands

tax_bands.hs

infinity = 1.0/0
savingsIncomeTax200910 = [(0, 6475), (0.1, 8915), (0.2, 43875), (0.4, infinity)]
incomeTax200910        = [(0, 6475), (0.2, 43875), (0.4, infinity)]

taxWithBands bands income
  = if income <= threshold then
      rate * income
    else 
      rate * threshold + taxWithBands loweredRemainingBands remainingIncome
  where
    (rate, threshold) : remainingBands = bands
    remainingIncome         = income - threshold
    loweredRemainingBands   = map (\(r,t) -> (r, t-threshold)) remainingBands

-- *Main> taxWithBands incomeTax200910 1000000
-- 389930.0