foo1.hs

-- foo1.hs
main = do
  a <- readFile "bar.txt"
  b <- lines a
  return putStrLn b
{-
foo1.hs:3:8:
    Couldn't match type `[]' with `IO'
    Expected type: IO String
      Actual type: [String]
    In the return type of a call of `lines'
    In a stmt of a 'do' block: b <- lines a
    In the expression:
      do { a <- readFile "bar.txt";
           b <- lines a;
           return putStrLn b }

foo.hs:4:3:
    Couldn't match type `String -> IO ()' with `IO b0'
    Expected type: String -> IO b0
      Actual type: String -> String -> IO ()
    The function `return' is applied to two arguments,
    but its type `(String -> IO ()) -> String -> String -> IO ()'
    has only three
    In a stmt of a 'do' block: return putStrLn b
    In the expression:
      do { a <- readFile "bar.txt";
           b <- lines a;
           return putStrLn b }
-}

foo2.hs

-- foo2.hs
main = do
  a <- readFile "bar.txt"
  b <- lines a
  putStrLn b

{-
foo2.hs:3:8:
    Couldn't match type `[]' with `IO'
    Expected type: IO String
      Actual type: [String]
    In the return type of a call of `lines'
    In a stmt of a 'do' block: b <- lines a
    In the expression:
      do { a <- readFile "bar.txt";
           b <- lines a;
           putStrLn b }
-}

foo3.hs

-- foo3.hs
main = do
  a <- readFile "bar.txt"
  let b = lines a
  return putStrLn b

{-
runghc foo.hs

foo3.hs:4:3:
    Couldn't match type `String -> IO ()' with `IO b0'
    Expected type: [String] -> IO b0
      Actual type: [String] -> String -> IO ()
    The function `return' is applied to two arguments,
    but its type `(String -> IO ()) -> [String] -> String -> IO ()'
    has only three
    In a stmt of a 'do' block: return putStrLn b
    In the expression:
      do { a <- readFile "bar.txt";
           let b = lines a;
           return putStrLn b }
-}

foo4.hs

-- foo4.hs
main = do
  a <- readFile "bar.txt"
  let b = lines a
  return putStrLn b

{-
foo4.hs:4:12:
    Couldn't match type `[Char]' with `Char'
    Expected type: String
      Actual type: [String]
    In the first argument of `putStrLn', namely `b'
    In a stmt of a 'do' block: putStrLn b
    In the expression:
      do { a <- readFile "bar.txt";
           let b = lines a;
           putStrLn b }
-}