Question

Could You tell my why I have error ‘Couldn’t match expected type IO 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";

Answer 1

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.)