Programming Languages and Techniques (CIS120)

Transcription

1 Programming Languages and Techniques (CIS120) Lecture 18 February 27 th, 2015 Objects

2 I couldn't figure out the last problem :( Difficult, qlookup is hard I had some trouble on this one.. Never knew planets can be so frustrawng... HW 04 feedback I couldn't understand it for a while, so I got so stressed out over it and I'm glad late days exist. WHY DO YOU DO THIS TO US?? This was much much harder than the earlier homeworks. Took me a long Wme to read through everything and to know which of the earlier funcwons I could/ need to use swll have no idea what s wrong with my code. It was hard but sawsfying SO SATISFYING! Great hw Wow... whew While frustrawng at Wmes, it really improved my Ocaml skills and taught me a lot about debugging. (close_enough my_code soluwon) = hopefully true :) omg. god bless the TAs' pawence 2

3 Announcements Midterm 1 reminder SoluWons available on the web site. You may view your exam by going to Levine 308 You may obtain a photocopy by requeswng one Thursday and then picking it up Friday Re- grade requests Due before next midterm, March 25 th Submit form to Levine 308 3

4 What number is printed by this program? type counter_state = { mutable count:int } let = { count = 0 } let incr () : int = ;; let _ = incr() in print_int (incr ()) other CIS120 Answer: 2 How did you answer this queswon? 1. SubsWtuWon model 2. Abstract Stack Machine 3. I just knew the answer 4. I didn t know, so I guessed

5 Objects in Java public class Counter { private int count; public Counter () { count = 0; } class name instance variable constructor class declarawon } public int incr () { count = count + 1; return count; } public int decr () { count = count - 1; return count; } methods public class Main { object creawon and use public static void main (String[] args) { constructor invocawon Counter c = new Counter(); System.out.println( c.inc() ); } } method call

6 What is an Object? Object = Instance variables (fields) + Methods Field = Mutable record Methods = (Immutable) record of first- class funcwons that update the fields Objects encapsulate state when the methods are the only way to mutate the fields. Objects are first- class. Can we get similar behavior in OCaml? 6

7 An incr funcwon How is this code like a Java object? How is it different? type counter_state = { mutable count:int } let = { count = 0 } (* each call to incr will produce the next integer *) let incr () : int = Drawbacks: No abstrac2on: There is only one counter in the world. If we want another, we need another counter_state value and another incr funcwon. No encapsula2on: Any other code can modify count, too. 7

8 Using Hidden State Make a funcwon that creates a counter state and an incr funcwon each Wme a counter is needed. (* More useful: a counter generator: *) let () : unit -> int = (* this is private to the returned function *) let = { count = 0 } in (* make one counter *) let incr1 : unit -> int = () (* make another counter *) let incr2 : unit -> int = () 8

9 What number is printed by this program? let () : unit -> int = let = { count = 0 } in let incr1 = () (* make one counter *) let incr2 = () (* and another *) let _ = incr1 () in print_int (incr2 ()) other CIS120 Answer: 1

10 let () : unit -> int = Running let incr1 : unit -> int = () 10

11 let : unit -> unit -> int = Running let incr1 : unit -> int = () 11

12 let : unit -> unit -> int = Running let incr1 : unit -> int = () 12

13 Running let : unit -> unit -> int = let incr1 : unit -> int = () 13

14 Running let : unit -> unit -> int =. let incr1 : unit -> int = () 14

15 Running let incr1 : unit -> int = () 15

16 Running let incr1 : unit -> int = () 16

17 Running let incr1 : unit -> int = ( ()) 17

18 Running let incr1 : unit -> int = ( ()) 18

19 Running let incr1 : unit -> int = ( ) 19

20 Running let incr1 : unit -> int = ( ) 20

21 Running let = in let incr1 : unit -> int = ( ) count 0 21

22 Running let = in let incr1 : unit -> int = ( ) count 0 22

23 Running let incr1 : unit -> int = ( ) count 0 23

24 Running let incr1 : unit -> int = ( ) count 0 24

25 Local FuncWons let incr1 : unit -> int = ( ) count 0 NOTE: We need one refinement of the ASM model to handle local funcwons. Why? The funcwon menwons, which is on the stack (but about to be popped off) so we save a copy of the needed stack bindings with the funcwon itself. (This is somewmes called a closure ) 25

26 Local FuncWons let incr1 : unit -> int = ( ) count 0 POP! 26

27 Local FuncWons let incr1 : unit -> int = ( ) count 0 27

28 Local FuncWons let incr1 : unit -> int = ( ) count 0 28

29 Local FuncWons incr1 count 0 DONE! Note how the count record is accessible only via the incr1 funcwon. This is the sense in which the state is local to incr1. 29

30 incr1 () Now let s run incr1 () incr1 count 0 30

31 incr1 () Now let s run incr1 () incr1 count 0 31

32 Now let s run incr1 () ( ()) incr1 count 0 32

33 Now let s run incr1 () ( ()) incr1 count 0 33

34 Now let s run incr1 () ( ) incr1 count 0 NOTE: Since the funcwon had some saved stack bindings, we add them to the stack at the same Wme that we put the code in the workspace. 34

35 Now let s run incr1 () ( ) incr1 count 0 35

36 Now let s run incr1 ().count <- + 1; ( ) incr1 count 0 36

37 Now let s run incr1 ().count <- + 1; ( ) incr1 count 0 37

38 Now let s run incr1 ().count <-.count + 1; ( ) incr1 count 0 38

39 Now let s run incr1 ().count <-.count + 1; ( ) incr1 count 0 39

40 Now let s run incr1 ().count < ; ( ) incr1 count 0 40

41 Now let s run incr1 ().count < ; ( ) incr1 count 0 41

42 Now let s run incr1 ().count <- 1; ( ) incr1 count 0 42

43 Now let s run incr1 ().count <- 1; ( ) incr1 count 0 43

44 Now let s run incr1 () (); ( ) incr1 count 1 44

45 Now let s run incr1 () (); ( ) incr1 count 1 45

46 Now let s run incr1 () ( ) incr1 count 1 46

47 Now let s run incr1 () ( ) incr1 count 1 47

48 Now let s run incr1 ().count ( ) incr1 count 1 48

49 Now let s run incr1 ().count ( ) incr1 count 1 49

50 Now let s run incr1 () 1 incr1 ( ) count 1 50

51 Now let s run incr1 () 1 incr1 ( ) count 1 POP! 51

52 Now let s run incr1 () 1 incr1 count 1 DONE! 52

53 Now Let s run again let incr2 : unit -> int = () incr1 count 1 53

54 Wme passes

55 Aner creawng incr2 incr1 incr2 count 1 NOTE: the two different incr funcwons have separate local states because a new count record was created in each call to. count 0 55

56 One step further shows us how to create different instance of local state so that we can have several different counters. What if we want to bundle together several operawons that share the same local state? e.g. incr and decr operawons that work on the same counter 56

57 A Counter Object (* The type of counter objects *) type counter = { get : unit -> int; incr : unit -> unit; decr : unit -> unit; reset : unit -> unit; } (* Create a counter object with hidden state: *) let mk_counter () : counter = { get = ( ) ; incr = ( <- + 1) ; decr = ( <- - 1) ; reset = ( <- 0) ; } 57

58 let c1 = mk_counter () Stack mk_counter c1 Heap { } get incr decr reset <- + 1 count 1 <- 1 <- 0 58

59 Using Counter Objects (* a helper function to create a nice string for printing *) let _string (s:string) (i:int) = s ^ ". = " ^ (string_of_int i) ^ "\n" let c1 = mk_counter () let c2 = mk_counter () ;; print_string (_string "c1" (c1.get ())) ;; c1.incr () ;; c1.incr () ;; print_string (_string "c1" (c1.get ())) ;; c1.decr () ;; print_string (_string "c1" (c1.get ())) ;; c2.incr () ;; print_string (_string "c2" (c2.get ())) ;; c2.decr () ;; print_string (_string "c2" (c2.get ())) 59

60 GUI Design puong objects to work

61 Where we re going HW 6: Build a GUI library and client applicawon from scratch in OCaml Goals: Apply everything we ve seen so far to do some preqy serious programming PracWce with first- class func2ons and hidden state Illustrate the event- driven programming model Give you a feel for how GUI libraries (like Java s Swing) work Bridge to object- oriented programming

62 Where we re going HW 6: Build a GUI library and client applicawon from scratch in OCaml M T W R F Goals: HW 5 out TODAY Apply everything we ve seen so far to do some preqy serious Objects programming GUI HW 5 due HW 6 out GUI Illustrate the event- driven programming model PracWce with first- class func2ons and hidden state Spring Break! Give you a feel for how GUI libraries (like Java s Swing) work Bridge to object- oriented programming OCaml vs. Java Java Java Java HW 6 due Java Aner Spring Break: transiwon Midterm to Java 2 HW 7 out Advice: Finish Task 4 (at least) before break!

63 Building a GUI and GUI ApplicaWons

64 Have you ever used a GUI library (such as Java s Swing) to construct a user interface? 1. Yes 2. No

65 Step #1: Understand the Problem We don t want to build just one graphical applicawon: we want to make sure that our code is reusable. What are the concepts involved in GUI libraries and how do they relate to each other? How can we separate the various concerns on the project?

66 Designing a GUI library OCaml s Graphics library provides very simple primiwves for: CreaWng a window Drawing various shapes: points, lines, text, rectangles, circles, etc. Geong the mouse posiwon, whether the mouse buqon is pressed, what key is pressed, etc. See: hqp://caml.inria.fr/pub/docs/manual- ocaml/libref/graphics.html How do we go from that to a funcwoning, reusable GUI library? 66

67 Step 2, Interfaces: Project Architecture Project Architecture* *Note: Subsequent program snippets are color- coded according to this diagram. ApplicaWon Paint GUI Library Eventloop Widget Gctx NaWve graphics library OCaml s Graphics Module (graphics.cma) Goal of the GUI library: provide a consistent layer of abstracwon between the applicawon (Paint) and the Graphics module. 67

68 Step 2, Interfaces: Project Architecture* *Note: Subsequent program snippets are color- coded according to this diagram. ApplicaWon Paint GUI Library Eventloop Widget Gctx NaWve graphics library OCaml s Graphics Module (graphics.cma) Goal of the GUI library: provide a consistent layer of abstracwon between the applicawon (Paint) and the Graphics module. 68