couldn't match expected type IO t

Could You tell my why I have error 'Couldn't match expected type IO开发者_运维问答 t against inferred type String' - see below to see bad line:

data RectangleType = Rectangle Int Int Int deriving(Show)

menuRectangles :: [RectangleType] -> IO [RectangleType]
menuRectangles rs = do
    putStrLn "Please choose option:"
    putStrLn "3 - Show all rectangles"
    putStrLn "4 - Quit"
    putStr "Number: "
    n <- getLine
    case n of
        "3"         ->  do { showRectangles rs; menuRectangles rs }  -- this line is wrong
        "4"         ->  do { putStrLn "Quitting"; return rs }
        otherwise   ->  do { putStrLn "The End"; return rs }

showRectangles :: [RectangleType] -> String
showRectangles x = showingRectangles x

showingRectangles [] = "Empty"
showingRectangles (x:xs) = do printRectangle x
                              showingRectangles xs

printRectangle :: RectangleType -> String
printRectangle (Rectangle id width height) = "id: " ++ show id ++ "width: " ++ show width ++ "height: " ++ show height ++ "\n";

1. showingRectangles is a pure function returning a string. Why use a do notation here? Change it to printRectangle x ++ showingRectangles xs.

2. Since showRectangles is just a pure function, it won't print to the console. The "statement" showRectangles rs is therefore invalid. You need to putStrLn it.

   "3" -> do { putStrLn (showRectangles rs); menuRectangles rs }
   

(BTW with this simple fix, the showingRectangles will always show Empty in the last line. You need to add one more definition to the function to avoid this.)

Take more out of IO so you can see what is happening in your do block:

  data RectangleType = Rectangle Int Int Int deriving (Show)

  menuRectangles :: [RectangleType] -> IO [RectangleType]
  menuRectangles rs = do
    putStr options
    n <- getLine
    case n of
        "3"        -> putStrLn (showRectangles rs) >> menuRectangles rs  
        "4"        -> putStrLn "Quitting"          >> return rs 
        otherwise  -> putStrLn "The End"           >> return rs 

  showRectangles :: [RectangleType] -> String
  showRectangles [] = "Empty"
  showRectangles xs = concat (map showRectangle xs)

  showRectangle :: RectangleType -> String
  showRectangle (Rectangle id width height) = 
              "id: "      ++ show id ++ 
              " width: "  ++ show width ++ 
              " height: " ++ show height ++ "\n"

  options ::  String
  options =   "Please choose option:\n" ++
              "3 - Show all rectangles\n" ++
              "4 - Quit\n" ++ 
              "Number "

  -- for testing:
  r n = Rectangle n n n
  rectangleList = map r [1..10]
  testlist = showRectangles rectangleList 
  testempty = showRectangles []
  testio = menuRectangles rectangleList