endomorphism2.hs

{-# LANGUAGE RecordWildCards #-}

import Data.Monoid

data SalaryConfig = SalaryConfig
    { sAllowances :: Bool
    , sBonus      :: Bool
    , sTax        :: Bool
    , sSurcharge  :: Bool
    }

defaultConfig = SalaryConfig True True True True 

salary :: SalaryConfig -> Double -> Double
salary SalaryConfig{..} = appEndo fns 
  where fns = mconcat $ zipWith (\p fn -> if p then Endo fn else mempty)
                                [sAllowances, sBonus, sTax, sSurcharge]
                                [(*1.2), (*1.1), (*0.7), (*0.9)]

-- salary defaultConfig 20 -- 16.632