Question

I can have many Figures on my list. Each Figure can have many Rectangles on its list. I have a problem with my function checkNewRectangleId – this function should ask user about new rectangle id until he write really new id and then it should return this id – but I have an error: couldn’t match expected type IO t against inferred type Maybe figureType line (Figure id width height rectangles) <- findFigure idFigure x in my function – could you help ?

import IO
import Char
import System.Exit
import Maybe
import Data.Time.Calendar
import System.Time


checkNewRectangleId :: Int -> [FigureType] -> IO Int
checkNewRectangleId idFigure x  = do
    idRectangle <- getInt "Give me new rectangle id: "
    (Figure id width height rectangles) <- findFigure idFigure x
    if isJust (findRectangle idRectangle rectangles) then do
            putStrLn ("We have yet rectangle with id " ++ show idRectangle)
            checkNewRectangleId idFigure x
        else return idRectangle


data FigureType = Figure Int Int Int [RectangleType] deriving(Show, Read)

data RectangleType = Rectangle Int CalendarTime deriving(Show, Read)

findFigure :: Int -> [FigureType] -> Maybe FigureType
findFigure _ [] = Nothing
findFigure n ((Figure id width height rectangles) : xs) =
    if n == id then Just (Figure id width height rectangles)
    else findFigure n xs

findRectangle :: Int -> [RectangleType] -> Maybe RectangleType
findRectangle _ [] = Nothing
findRectangle n ((Rectangle id date) : xs) =
    if n == id then Just (Rectangle id date)
    else findRectangle n xs

isInt i = not (null i) && all isDigit i

getInt :: String -> IO Int
getInt q = do
    putStr q;
    i <- getLine
    if isInt i == False then do
            putStrLn "Bad number"
            getInt q
        else return (read i)

Answer 1

Since you say idFigure is guaranteed to exist, you can use fromJust in the Data.Maybe module to convert a Maybe FigureType into a FigureType:

let (Figure id width height rectangles) = fromJust $ findFigure idFigure x