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5695968
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2011-04-17T19:43:48.277
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2011-04-17T19:54:38.927
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The solution is to use an accumulating list in <code>menuRectangles</code>. When you call <code>addRectangles</code> you have to bind the result to a new variable - that is what this expression is doing: <pre><code>rs_new <- addRectangle rs </code></pre> Note - this is special syntax in Haskell for binding within an IO or monadic expression. In Haskell the other binding form is <code>let</code>: <pre><code>let xs = [2,3,4] ys = 1 : xs in length ys </code></pre> Here's a working version of your program - note you now have to run with <code>main</code> so it can seed <code>menuRectangles</code> with the empty list. When <code>menuRectangles</code> is called recursively for a menu option you have to pass on the accumulating list. <pre><code>import IO import Char main :: IO () main = do xs <- menuRectangles [] mapM print xs return () -- A new helper - this will throw an error -- if you don't type a number. There are -- plenty of ways to make it more robust. -- getInt :: IO Int getInt = do xs <- getLine return (read xs) menuRectangles :: [RectangleType] -> IO [RectangleType] menuRectangles rs = do putStrLn "Please choose option:" putStrLn "1 - Add rectangle" putStrLn "2 - Show rectangle" putStrLn "3 - Quit" putStr "Number: " n <- getLine case n of "1" -> do { rs_new <- addRectangle rs; menuRectangles rs_new }; "2" -> do { showRectangle rs; menuRectangles rs } "3" -> do { putStrLn "Quitting"; return rs } otherwise -> do { putStrLn "The End"; return rs } addRectangle :: [RectangleType] -> IO [RectangleType] addRectangle rs = do putStrLn "Id: " id <- getInt putStrLn "Width: " width <- getInt putStrLn "Height: " height <- getInt let new_rs = addingRectangle (Rectangle id width height) rs return new_rs showRectangle :: [RectangleType] -> IO () showRectangle rs = do putStrLn "Choose id rectangle: " id <- getInt -- Need a lookup case findRectangle id rs of Nothing -> putStrLn ("Not found " ++ show id) Just rect1 -> putStrLn (showingRectangle rect1) data RectangleType = Rectangle Int Int Int deriving(Show) addingRectangle :: RectangleType -> [RectangleType] -> [RectangleType] addingRectangle r [] = [r] addingRectangle r rx = r:rx showingRectangle :: RectangleType -> String showingRectangle (Rectangle id width height) = "id: " ++ show id ++ "width: " ++ show width ++ "height: " ++ show height ++ "\n"; -- We have to use Maybe - Just rectangle OR Nothing -- to account for the case where the id does not match -- findRectangle :: Int -> [RectangleType] -> Maybe RectangleType findRectangle _ [] = Nothing findRectangle n ((Rectangle i w h):xs) = if n == i then Just (Rectangle i w h) else findRectangle n xs </code></pre>
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1. This table or related slice is empty.
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1. POadding rectangles to list and showing rectangle
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1. USstephen tetley
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1. USstephen tetley
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1. POadding rectangles to list and showing rectangle
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1. COThx You very much :)) !
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