An answer of http://qiita.com/hiratara/items/169b5cb83b0adbfda764

.gitignore

*~

2-1-hostname.hs

#!/usr/bin/env stack
-- stack --resolver lts-7.0 --install-ghc runghc --package turtle

{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle

main = do
  name <- hostname
  echo name

2-2-modified.hs

#!/usr/bin/env stack
-- stack runghc
{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle

main = do
    args <- arguments
    let fileTxt = head args
        file = fromText fileTxt
    dt <- datefile file
    let dateTxt = repr dt
    echo dateTxt

2-3-modified-multi.hs

#!/usr/bin/env stack
-- stack runghc
{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle

echoModified fname = do
    let file = fromText fname
    dt <- datefile file
    printf (s%"\t"%utc%"\n") fname dt

main = do
    args <- arguments
    mapM echoModified args

2-4-hello-io.hs

#!/usr/bin/env stack
-- stack --resolver lts-7.0 --install-ghc runghc --package turtle

{-# LANGUAGE OverloadedStrings #-}
import Turtle

nestedIO = do
    putStr "Hello, "
    return (putStrLn "I/O!")

main = do
  printIO <- nestedIO
  printIO

3-1-count-import.hs

#!/usr/bin/env stack
-- stack --install-ghc runghc --package turtle

{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle

main = do
    args <- arguments
    let path = head args
    empty & inproc "find" ([path] <> ["-name", "*.hs"])
          & inshell "xargs grep '^import '"
          & shell "wc -l"

3-2-count-import.hs

#!/usr/bin/env stack
-- stack --install-ghc runghc --package turtle

{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle
import qualified Control.Foldl as Fold

main = do
    args <- arguments
    let path' = head args
        path  = fromText path'
    find (suffix ".hs") path
      & grepImport
      & (`fold` Fold.length)
      & view

grepImport upstream = do
    path <- upstream
    input path & grep (prefix "import")

4-1-count-import.hs

#!/usr/bin/env stack
-- stack --install-ghc runghc --package turtle

{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle
import qualified Control.Foldl as Fold

parser = optional $ optText "dir" 'd' "the root of search directories"

main = do
    mPath' <- options "Count imported modules" parser
    let path' = case mPath' of Nothing -> "."
                               Just p  -> p
        path  = fromText path'
    find (suffix ".hs") path
      & grepImport
      & (`fold` Fold.length)
      & view

grepImport upstream = do
    path <- upstream
    input path & grep (prefix "import ")

4-2-count-import.hs

#!/usr/bin/env stack
-- stack --install-ghc runghc --package turtle

{-# OPTIONS_GHC -fwarn-name-shadowing #-}
{-# LANGUAGE OverloadedStrings #-}
import Turtle
import qualified Control.Foldl as Fold

parser = (,) <$> optional (optText "dir" 'd' "the root of search directories")
             <*> switch "show" 's' "Show the name of all imported modules"

main = do
    (mPath', isShow) <- options "Count imported modules" parser
    let path' = case mPath' of Nothing -> "."
                               Just p  -> p
        path  = fromText path'
        sink  = if isShow then dump else wc
    find (suffix ".hs") path
      & grepImport
      & sink

grepImport upstream = do
    path <- upstream
    input path & grep (prefix "import ")

wc src = src & (`fold` Fold.length) & view

dump src = src
         & sed (prefix (importDef *> moduleName))
         & stdout
  where
    importDef = "import" <> spaces1 <> ("qualified" <> spaces1 <|> "")
    moduleName = plus (notChar ' ')

README

[Haskell Day のチュートリアル](http://qiita.com/hiratara/items/169b5cb83b0adbfda764) で出題した問題の回答例をここで公開予定です。