HaskellのRank2Typesがだいぶわかるやつ ref: http://qiita.com/aiya000/items/2d05df630635f345f607

file0.txt

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE Rank2Types #-}

import Data.String (IsString)
import Data.Text (Text)
import Shelly (run_)
import System.EasyFile (doesFileExist)

type FilePath' = forall s. IsString s => s

flagFile :: FilePath'
flagFile = "/tmp/flag"

-- ここに、shellyを使ったほうがいい事情があるとします。
-- 故にshelly使います (※)
createFlagFile :: IO ()
createFlagFile = shelly $ run_ "rm" ["-f", flagFile]

flagFileExists :: IO Bool
flagFileExists = doesFileExist flagFile

file1.txt

type FilePath' = forall s. IsStrings s => s

file2.txt

g :: (a -> a) -> (Bool, Char)
g f = (f True, f 'a')

file3.txt

g :: (forall a. a -> a) -> (Bool, Char)
g f = (f True, f 'a')

file4.txt

type FilePath' = forall s. IsString s => s

file5.txt

flagFile :: FilePath'
flagFile = "/tmp/flag"

-- 1
createFlagFile :: IO ()
createFlagFile = shelly $ run_ "rm" ["-f", flagFile]

-- 2
flagFileExists :: IO Bool
flagFileExists = doesFileExist flagFile