PFP Laboratory 6

Binary Trees

• Create a data type Tree that defines binary tree where values are stored in leaves and also in branches.

data Tree a = Leaf a 
            | Branch a (Tree a) (Tree a) deriving (Show)

• Prepare an example of a binary tree.

testTree1 :: Tree Int            
testTree1 = Branch 12 (Branch 23 (Leaf 34) (Leaf 45)) (Leaf 55)

testTree2 :: Tree Char            
testTree2 = Branch 'a' (Branch 'b' (Leaf 'c') (Leaf 'd')) (Leaf 'e')

• Create a function that sums all values stored in the tree.

sum' :: Tree Int -> Int

sum' :: Tree Int -> Int
sum' (Leaf x) = x
sum' (Branch x l r) = sum' l + x + sum' r

• Create a function that extracts all values from the tree into an list.

toList :: Tree a -> [a]

toList :: Tree a -> [a]
toList (Leaf x) = [x]
toList (Branch x l r) = toList l ++ [x] ++ toList r

• One possibility how to represent a tree in a textual form is a(b(d,e),c(e,f(g,h))). Create functions that are able to read and store a tree in such a notation.

toString :: Show a => Tree a -> String
fromString :: Read a => String -> Tree a

toString :: Show a => Tree a -> String
toString (Leaf x) = show x
toString (Branch x l r) = show x ++ "(" ++ (toString l) ++ "," ++ (toString r) ++ ")"

fromString :: Read a => String -> Tree a
fromString inp = fst (fromString' inp) where
  fromString' :: Read a => String -> (Tree a,String)
  fromString' inp = 
    let
      before = takeWhile (\x ->  x /='(' &&  x /=',' &&  x/=')') inp 
      after = dropWhile (\x ->  x /='(' &&  x /=',' && x/=')') inp
      value = read before
    in if null after || head after /= '(' then (Leaf value, after) else 
        let
          (l,after') = fromString' (tail after)
          (r,after'') = fromString' (tail after') 
        in (Branch value l r, tail after'')

Complex data structure

Consider following data structure representing some kind of GUI.

data Point = Point {column::Int,row::Int} deriving (Show)

data Position = Position {leftTopCorner :: Point, width :: Int, height :: Int} 

data Component
  = TextBox {name :: String, position :: Position, text :: String}
  | Button {name :: String, position :: Position, text :: String}
  | Container {name :: String, children :: [Component]}

As an example, we can use following data structure.

gui :: Component
gui =
  Container "My App"
    [ Container "Menu"
        [ Button "btn_new" (Position (Point 0 0) 100 20) "New",
          Button "btn_open" (Position (Point 100 0) 100 20) "Open",
          Button "btn_close" (Position (Point 200 0) 100 20) "Close"
        ],
      Container "Body" [TextBox "textbox_1" (Position (Point 0 20) 300 500) "Some text goes here"],
      Container "Footer" []
    ]

• Add the data type Component into the type class Show.

The result for our data from previous example should be something like this.

ghci> gui
Container - My App
        Container - Menu
                (0,0)[100,20] Button[btn_new]: New
                (100,0)[100,20] Button[btn_open]: Open
                (200,0)[100,20] Button[btn_close]: Close
        Container - Body
                (0,20)[300,500] TextBox[textbox_1]: Some text goes here
        Container - Footer

instance Show Position where
    show (Position (Point col row) width height) = "(" ++ show col ++ "," ++ show row ++ ")["++ show width++","++ show height++"]"

instance Show Component where
    show :: Component -> String
    show gui = showIndent "" gui where
        showIndent ind (TextBox name position text) = ind ++ show position ++ " TextBox[" ++ name ++ "]: " ++ text ++"\n" 
        showIndent ind (Button name position text) = ind ++ show position ++ " Button[" ++ name ++ "]: " ++ text ++"\n"
        showIndent ind (Container name children) = let 
            inner = concat[showIndent (ind++"\t") c |c<-children]
            in ind ++ "Container - " ++ name ++ "\n" ++ inner

• Cerate a function insertInto, it will insert an element into the existing container from a GUI. The functions parameters will be:
  • first parameter will be the GUI, where we are inserting the new element;
  • second parameter is the name of the container, where we insert the new element, you can safely assume, that it will always exist. The element will be placed as last in the container;
  • last parameter is the inserted element.

insertInto :: Component -> String -> Component -> Component

ghci> insertInto gui "Footer" (TextBox "Done" (Position (Point 0 500) 300 10) "We are done!")
Container - My App
        Container - Menu
                (0,0)[100,20] Button[btn_new]: New
                (100,0)[100,20] Button[btn_open]: Open
                (200,0)[100,20] Button[btn_close]: Close
        Container - Body
                (0,20)[300,500] TextBox[textbox_1]: Some text goes here
        Container - Footer
                (0,500)[300,10] TextBox[Done]: We are done!

insertInto :: Component -> String -> Component -> Component
insertInto (Container cName children ) toName element 
    | cName == toName = Container cName (children++[element]) 
    | otherwise = Container cName [insertInto c toName element  |c<-children]
insertInto x toName element = x

• Extend the definition of a button in our GUI as follows.

data Event = MouseEvent Point
           | KeyEvent {keyPressed::Char} deriving (Show)
...
  | Button {name :: String, position :: Position, text :: String, onClick :: Maybe (Event -> String)}
...

Our onClick is a function, that will be fired when the button is clicked on. The parameter of this function is data describing the firing event.

...
[ Button "btn_new" (Position (Point 0 0) 100 20) "New" (Just (\event -> "Clicked on new button.")),
  Button "btn_open" (Position (Point 100 0) 100 20) "Open" Nothing,
  Button "btn_close" (Position (Point 200 0) 100 20) "Close" (Just (\event -> "Clicked on close button.")) ]
...

• Create a function clickOnButton that will take our GUI and an event. If it is a mouse event, and the position where we have clicked is inside some of the buttons from the gui, then it evaluates the coresponding onClick function and the result will be produced string. In all other cases, the result will be Nothing.

clickOnButton :: Component -> Event -> Maybe String

ghci> clickOnButton gui (MouseEvent (Point 5 5))
Just "Clicked on new button."
ghci> clickOnButton gui (MouseEvent (Point 205 5))
Just "Clicked on close button."
ghci> clickOnButton gui (MouseEvent (Point 205 50))
Nothing

isInside :: Point -> Position -> Bool
isInside (Point pCol pRow) (Position (Point cornerCol cornerRow) width height) =
     cornerCol <= pCol && pCol <= cornerCol + width && cornerRow <= pRow && pRow <= cornerRow + height

getFirstOrNothing :: [Maybe a] -> Maybe a
getFirstOrNothing [] = Nothing
getFirstOrNothing (Nothing:xs) = getFirstOrNothing xs
getFirstOrNothing (Just x: xs) = Just x

clickOnButton :: Component -> Event -> Maybe String
clickOnButton (Button {position = pos, onClick = (Just func)}) (MouseEvent point) | isInside point pos = Just (func (MouseEvent point))
clickOnButton (Container {children=inner}) event = getFirstOrNothing [clickOnButton c event |c<-inner]
clickOnButton _ _ = Nothing