GPA計算するやつ

calcGPA.hs

-- <example>
-- $runghc calcGPA.hs
-- 3
-- A 2 
-- B 2
-- C 1
-- 2.2

import Control.Monad
import Control.Applicative

calcGPA :: [(String, Double)] -> Double
calcGPA xs = grades xs / addUnits xs

grades :: [(String, Double)] -> Double
grades [] = 0
grades (x:xs)
    | fst x == "AA" = 4 * snd x + grades xs
    | fst x == "A"  = 3 * snd x + grades xs
    | fst x == "B"  = 2 * snd x + grades xs
    | fst x == "C"  = 1 * snd x + grades xs
    | fst x == "D"  = grades xs
    | otherwise     = 0

addUnits :: [(String, Double)] -> Double
addUnits = foldl (\acc x -> acc + snd x) 0

main = do
        n <- getLine
        s <- replicateM (read n) $ (\(x:y) -> (x, read (head y) :: Double)) . words <$> getLine
        putStrLn . show $ calcGPA s