TeachingHaskell-Exercise 1-Answer

Main.hs

data Shape 
    = Circle    Float
    | Square    Float
    | Rectangle Float Float
    | Null
    deriving (Show)

main :: IO ()
main = do
    -- Read contents from shape.txt
    contents <- readFile "shape.txt"

    -- Parse shapes
    let shapes = map toShape (lines contents)

    -- Compute areas
    let areas = map calculateArea shapes

    putStrLn "AREAS:"
    putStrLn (concatMap (\x -> show x ++"\n") areas)

    -- Compute counts
    let (circleCount, squareCount, rectangleCount, nullCount) = 
            foldl 
                (\(c,s,r,n) nextShape -> 
                    case nextShape of 
                        Circle{}    -> (c+1,s,r,n)
                        Square{}    -> (c,s+1,r,n)
                        Rectangle{} -> (c,s,r+1,n)
                        Null{}      -> (c,s,r,n+1))
                (0,0,0,0)
                shapes

    putStrLn "\nCOUNTS:"
    putStrLn ("Circle "    ++ show circleCount)
    putStrLn ("Square "    ++ show squareCount)
    putStrLn ("Rectangle " ++ show rectangleCount)
    putStrLn ("NULL "      ++ show nullCount)


toShape :: String -> Shape
toShape row = 
    case words row of
        ["Circle", radius] ->
            Circle (readFloat radius)

        ["Square", side] ->
            Square (readFloat side)

        ["Rectangle", height, width] ->
            Rectangle (readFloat height) (readFloat width)
        
        ["NULL"] ->
            Null

calculateArea :: Shape -> Float
calculateArea s = case s of
    Circle r      -> 3.142 * (r ^ 2)
    Square s      -> s * s
    Rectangle h w -> h * w
    Null          -> 0.0


readFloat :: String -> Float
readFloat = read