愛しの Lazy I/O ref: http://qiita.com/nobsun/items/a0fc3b987f013e9649f0

file0.txt

type Score = Int
data Grade = A | B | C | D

instance Show Grade where
  show A = "優"
  show B = "良"
  show C = "可"
  show D = "不可"

grade :: Score -> Maybe Grade
grade x
  |  outRange x = Nothing
  |  x < 60     = Just D
  |  x < 70     = Just C
  |  x < 80     = Just B
  |  otherwise  = Just A

outRange :: Score -> Bool
outRange = not. inRange

inRange :: Score -> Bool
inRange x =  0 <= x && x <= 100

file1.txt

data Count = Count { nA, nB, nC, nD :: Int }

instance Show Count where
  show (Count a b c d) = printf "優:%4d名  良:%4d名  可:%4d名  不可:%4d名" a b c d

zero :: Count
zero = Count 0 0 0 0

incA, incB, incC, incD :: Count -> Count
incA c = c { nA = nA c + 1 }
incB c = c { nB = nB c + 1 }
incC c = c { nC = nC c + 1 }
incD c = c { nD = nD c + 1 }

file2.txt

main :: IO ()
main = loop zero

file3.txt

loop :: Count -> IO ()
loop c = do
  { prompt "得点？ "
  ; input <- getLine
  ; let score = read input
  ; let judge = grade score
  ; maybe (print c)
          (printCountUpLoop c)
          judge
  }

printCountUpLoop :: Count -> Grade -> IO ()
printCountUpLoop c g = print g >> loop (countUp g c)

countUp :: Grade -> (Count -> Count)
countUp A = incA
countUp B = incB
countUp C = incC
countUp D = incD

prompt :: String -> IO ()
prompt s = putStr s >> hFlush stdout

file4.txt

countAndGrades :: [Score] -> (Count, [Grade])
countAndGrades = mapAccumL countGrade zero . unfoldr phi
  where
    phi []     = Nothing
    phi (x:xs) = bool Nothing (Just (x,xs)) (inRange x)
    countGrade c s = case grade s of
      Just g  -> (countUp g c, g)

file5.txt

serialize :: (Count, [Grade]) -> String
serialize ~(c,gs)
  = promptStr ++ concatMap (followedByPrompt . shows) gs ++ shows c "\n\n"

followedByPrompt :: ShowS -> String
followedByPrompt ss = ss promptStr

promptStr :: String
promptStr = "\n得点？ "

file6.txt

main :: IO ()
main =  hSetBuffering stdout NoBuffering
     >> interact (serialize . countAndGrades . map read . lines)

separatedIO.hs

module Main where

import Data.Bool
import Data.List
import System.IO
import Text.Printf

type Score = Int
data Grade = A | B | C | D

instance Show Grade where
  show A = "優"
  show B = "良"
  show C = "可"
  show D = "不可"

grade :: Score -> Maybe Grade
grade x
  |  outRange x = Nothing
  |  x < 60     = Just D
  |  x < 70     = Just C
  |  x < 80     = Just B
  |  otherwise  = Just A

outRange :: Score -> Bool
outRange = not . inRange

inRange :: Score -> Bool
inRange x = 0 <= x && x <= 100

data Count = Count { nA, nB, nC, nD :: Int }

instance Show Count where
  show (Count a b c d) = printf "｜ 優:%4d名 ｜ 良:%4d名 ｜ 可:%4d名 ｜ 不可:%4d名 ｜" a b c d

zero :: Count
zero = Count 0 0 0 0

incA, incB, incC, incD :: Count -> Count
incA c = c { nA = nA c + 1 }
incB c = c { nB = nB c + 1 }
incC c = c { nC = nC c + 1 }
incD c = c { nD = nD c + 1 }

countUp :: Grade -> (Count -> Count)
countUp A = incA
countUp B = incB
countUp C = incC
countUp D = incD

countAndGrades :: [Score] -> (Count, [Grade])
countAndGrades = mapAccumL countGrade zero . unfoldr phi
  where
    phi []     = Nothing
    phi (x:xs) = bool Nothing (Just (x,xs)) (inRange x)
    countGrade c s = case grade s of Just g  -> (countUp g c, g)

selialize :: (Count, [Grade]) -> String
selialize ~(c,gs) = promptStr ++ concatMap (followedByPrompt . shows) gs ++ shows c "\n\n"

followedByPrompt :: ShowS -> String
followedByPrompt ss = ss promptStr

promptStr :: String
promptStr = "\n得点？ "

main :: IO ()
main =  hSetBuffering stdout NoBuffering
     >> interact (serialize . countAndGrades . map read . lines)