# Haskell Quiz/Internal Rate of Return/Solution Dolio

### From HaskellWiki

My solution for this quiz uses the secant method, which is quite easy to implement.

import Data.Function import Numeric import System.Environment secant :: (Double -> Double) -> Double -> Double secant f delta = fst . head . dropWhile err . iterate update $ (0,1) where update (x,y) = (x - (x - y)*(f x)/(f x - f y), x) err (x,y) = abs (x - y) > delta npv :: Double -> [Double] -> Double npv i = sum . zipWith (\t c -> c / (1 + i)**t) [0..] main = do (s:t) <- getArgs let sig = read s cs = map read t putStrLn . ($"") . showFFloat (Just sig) $ secant (flip npv cs) (0.1^sig)

The resulting program expects the first argument to be the number of digits to be displayed after the decimal point, while the rest are the yearly income. For instance:

./IRR 4 -100 30 35 40 45 0.1709