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4020 COP Programming Languages 1 November 29, 2007 Homework 5: Message Passing Due: exercises 1 2 and 4 6, Monday, November 19, 2007 at 11pm; exercises 7 and 9, Monday, November 26, 2007 at 11pm. In this homework you will learn about the message passing model and basic techniques of programming in that model. The programming techniques include using port objects [Concepts] [UseModels]. A few problems also make comparisons with the other models we have studied, and also with message passing features in other languages [EvaluateModels] [MapToLanguages]. Your code should be written in the message passing model, so you should not use cells and assignment in your Oz solutions. You should use helping functions whenever you nd that useful. Unless we speci cally say how you are to solve a problem, feel free to use any functions from the Oz library (base environment), especially functions like Map and FoldR. For all Oz programing tasks, you must run your code using the Mozart/Oz system. For these you must also provide evidence that your program is correct (for example, test cases). Oz code with tests for various problems is available in a zip le, which you can download from the course resources web page. For testing, you may want to use tests based on our code in the course library le TestingNoStop.oz. Turn in (on WebCT) your code and, if necessary, output of your testing for all questions that require code. Please upload text les with your code that have the suf x .oz (or .java as appropriate), and text les with suf x .txt that contain the output of your testing. Please use the name of the main function as the name of the le. (In any case, don t put any spaces in your le names!) If we provide tests and your code passes all of them, you can just indicate with a comment in the code that your code passes all the tests. Otherwise it is necessary for you to provide us test code and/or test output. If the code does not pass some tests, indicate in your code with a comment what tests did not pass, and try to say why, as this enhances communication and makes commenting on the code easier and more speci c to your problem than just leaving the buggy code without comments. If you re not sure how to use our testing code, ask us for help. Your code should compile with Oz, if it doesn t you probably should keep working on it. If you don t have time, at least tell us that you didn t get it to compile. Be sure to clearly label what problem each area of code solves with a comment. Don t hesitate to contact the staff if you are stuck at some point. Read Chapter 5 of the textbook [RH04]. (See the syllabus for optional readings.) Message Passing Semantics and Expressiveness 1. (30 points) [Concepts] [UseModels] Using Oz s message passing model, implement a data abstraction Box, by writing the following functions and procedures. NewBox: <fun <$ Value>: Box> BoxExchange: <proc <$ Box Value Value>> BoxAssign: <proc <$ Box Value>> BoxAccess: <fun <$ Box>: Value> A Box is like a Cell, in that it holds a value (of any type). The function call {NewBox X} returns a new Box containing the value X. The procedure call {BoxExchange B Old New} atomically binds Old to the value in box B and makes New be the new value contained in Box B. The procedure call {BoxAssign B V} makes the value V be the new value of Box B. The function call {BoxAccess B} returns the value contained in Box B. Your code should pass the tests shown in Figure 1 on the following page 1 % $Id: BoxTest.oz,v 1.1 2007/11/09 21:32:07 leavens Exp leavens $ \insert Box.oz \insert TestingNoStop.oz declare B1 B2 V1old V2old {StartTesting Box operations } B1 = {NewBox 1} B2 = {NewBox 2} {BoxExchange B1 V1old 7} {BoxExchange B2 V2old 99} {Test V1old == 1} {Test V2old == 2} declare V1x V2x {BoxExchange B1 V1x 88} {BoxExchange B2 V2x 333} {Test V2x == 99} {Test V1x == 7} {Test {BoxAccess B1} == 88} {Test {BoxAccess B2} == 333} {Test {BoxAccess B2} == 333} {Test {BoxAccess B1} == 88} {BoxAssign B1 4} {Test {BoxAccess B1} == 4} {Test {BoxAccess B2} == 333} {BoxAssign B1 asymbolliteral} {Test {BoxAccess B1} == asymbolliteral} declare X=1 Y=2 Z=3 B={NewBox Z} {StartTesting Some equations } {Test {BoxAccess {NewBox X}} == X} {BoxAssign B Y} {Test {BoxAccess B} == Y} Figure 1: Testing code for Exercise 1 on the previous page. 2 You should use the NewPortObject.oz le, given in the book and supplied with the test cases for this homework, in your solution. (Hint: represent a Box with a port object, have NewBox return a port, and have the other functions send messages to the port. The state of the port object will be the Box s value.) You are not allowed to use cells in your solution! 2. (20 points) [Concepts] [UseModels] Using Cells, but without using the message passing primitives NewPort and Send, de ne in Oz an ADT PortAsCell, which acts like the built-in port type, but is represented as a Cell. (For more about the imperative model and cells, look back at Section 1.12 of the textbook or forward to chapter 6.) The PortAsCell ADT has two operations: MyNewPort: <fun <$ Stream>: PortAsCell> MySend: <proc <$ PortAsCell Value>>, which are intended to act like NewPort and Send. That is, the function MyNewPort takes an unbound store variable, and returns a PortAsCell, which is a Cell that we want to act like a Port. The procedure MySend takes such a PortAsCell and a Value and adds the Value to the corresponding stream. In other words, the idea behind the ADT PortAsCell is that it should act like the built-in Port ADT of Oz, but be represented using Cells. For this problem, don t worry about the improper uses of the stream argument to MyNewPort (see Exercise 3). Also for the moment, don t worry about potential race conditions when multiple threads are used. Hint: look at the semantics for NewPort and Send in section 5.1 of the textbook, which shows how these work in terms of the mutable memory. Mutable memory is like a collection of cells. Think of the port name as being represented by the cell s identity. The cell holds the unbound store variable that is the end of the list. Note how the semantics of Send manipulates the mutable memory and works with new unbound store variables. Your code should pass the tests shown in Figure 2 on the next page. 3. (20 points; extra credit) [Concepts] [UseModels] Using read-only views (see Sections 3.7.5 and 13.1.14 of our textbook [RH04]), x your solution to Exercise 2 so that your code has the same behavior for improper uses of the stream argument passed to MyNewPort as does Oz s built-in NewPort primitive. You must show by writing your own tests that your code has the required same behavior for such uses. Testing is up to you and an important part of this exercise. 4. [Concepts] [EvaluateModels] This problem concerns the expressive power of the message passing model in comparison to the imperative model. (a) (5 points) Can the Box ADT of Exercise 1 on page 1 do everything that a Cell can do in Oz? Brie y explain. (b) (5 points) the Can PortAsCell ADT of Exercise 2 do everything that a Cell can do in a sequential Oz program? Brie y explain. (c) (10 points) Does your implementation of the PortAsCell ADT of Exercise 2 have any potential race conditions when used in an Oz program with multiple threads? That is, is it possible that MyNewPort or MySend act differently than NewPort and Send when there are multiple threads? Brie y explain your answer. (d) (5 points) Is there any signi cant difference in expressive power between adding Cells or adding NewPort and Send to the kernel language? 3 % $Id: PortAsCellTest.oz,v 1.2 2007/11/18 14:09:59 leavens Exp leavens $ \insert PortAsCell.oz \insert TestingNoStop.oz {StartTesting MyNewPort } % Simulating basic semantics of NewPort and Send declare Strm Port in Port = {MyNewPort Strm} {StartTesting MySend } {MySend Port 3} {MySend Port 4} % Must use List.take, otherwise Test suspends... {Test {List.take Strm 2} == [3 4]} {MySend Port 5} {MySend Port 6} {Test {List.take Strm 4} == [3 4 5 6]} {StartTesting MyNewPort second part } declare S2 P2 U1 U2 in P2 = {MyNewPort S2} {StartTesting MySend second part } {MySend P2 7} {MySend P2 unit} {MySend P2 true} {MySend P2 U1} {MySend P2 hmmm(x:U2)} U1 = 4020 {Test {List.take S2 5} == [7 unit true 4020 hmmm(x:U2)]} {Test {List.take Strm 4} == [3 4 5 6]} Figure 2: Testing code for Exercise 2 on the preceding page. 4 5. (10 points) [EvaluateModels] Would it have been easier to use the message passing model to solve the square root approximation and differentiation exercises (numbers 13, 14, and 16) of homework 4? Brie y explain your answer. (You don t have to actually solve these problems with the message passing model.) 6. (10 points) [EvaluateModels] Suppose you are asked to program a simulation of an agent-based auction system for someone doing research in economics. This system consists of several independent agents, each of which must communicate with a central auction server to evaluate merchandise, place bids, and make payments. Among the programming models we studied this semester, What is the most restrictive (i.e., the least expressive or smallest) programming model that can practically be used program the overall structure of such a system? Brie y justify your answer. Message Passing Programming 7. (60 points) [UseModels] Do part (a) of problem 3 in section 5.9 of the textbook [RH04] (Fault tolerance for the lift control system). You can get the code for the book s gures from the textbook s supplementary web site, which is: http://www.info.ucl.ac.be/~pvr/ds/mitbook.html or more directly from the book s supplementary web directory: http://www.info.ucl.ac.be/~pvr/bookfigures/ or even more directly from WebCT. Note that in part (a) when the oor is called refers to call messages sent to a Floor port object. Feel free to make other changes to the code to make it more easy for you to understand and more sensible. Since deciding when the code works correctly is not easy, you are responsible for your own testing for this problem. In essence, you should set up a situation where a lift gets blocked on some oor, and make sure that the oor s timer is reset and that the blocked lift s schedule is distributed to other lifts and that the oors aren t calling the blocked lift. You can should add extra outputs to see whether the system is functioning as you intend. Now for some hints. To get started, overall it s useful to understand how the state diagrams (like Figure 5.7) relate to the code (like Figure 5.8 s Timer class). Think about the design at the level of the state diagrams, and then make the corresponding changes to the code. You may nd it useful to look at the diagrams to understand the code (and vice versa). Also don t be afraid to introduce new state (or parts of state) and new kinds of messages. To get started coding, look at the Floor component in Figure 5.10. When it s in the doorsopen state and it gets a call message is when the lift is being blocked at a oor. You ll have to change the code in that spot to start with. Get it to work reset the timer, perhaps by having the oor remember how many stoptimer messages it should wait for before telling the lift to close the doors. Then add to the Lift a way to get the schedule, and then gure out what component should ask for it and redistribute the schedule to other lifts. Don t wait until the last minute to start working on this problem. Be sure to mark, with comments, any changes you made to the code to solve the problem. 8. (60 points; extra credit) [UseModels] For extra credit, you can do one of parts (b) (e) of problem 3 in section 5.9 of the textbook [RH04] (Fault tolerance for the lift control system). In part (b), you can design a wrapper port object that takes a disable message and then sends the down messages suggested; this wrapper can take port for the linked component as an argument, and send it the down message when the wrapper instance gets the disable message. That will allow testing. 5 9. (80 points) [Concepts] [UseModels] [MapToLanguages] Do problem 7 in section 5.9 of the textbook [RH04] (Erlang s receive as a control abstraction). We have provided a test script for this that is attached to the homework in the le hw5-tests.zip. Note that, according to the book s errata, when the D argument to Mailbox.receive has the form T#E, then E should be a zero-argument function. Some hints. Be sure to read section 5.7.3 in the textbook. Figure 5.21 in particular explains the semantics of receive, although you are not going to code that directly. In the problem, the name Mailbox is a variable identi er that is bound to a record containing three elds: new, send, and receive. Thus in outline your code should do something like the following. declare local % ... fun {New} ... end proc {MSend C M} ... end fun {MReceive C PGL D} ... end in Mailbox = mailbox(new:New send:MSend receive:MReceive) end Now Mailbox.new denotes the function New, since it does a record selection on the Mailbox record. In essence, your function New will return a port object (or something that contains a port object), which you will make by calling NewPortObject. Then MSend and MReceive will send messages to the port object passed as their rst argument (or contained in their rst argument). However, MSend and MReceive do very little themselves. In particular, MReceive can pass along its arguments PGL and D in a message, so that the port object (which has the appropriate state) can do the work. The port object should contain a list (or queue) of unprocessed messages in its state. It is helpful to think of the port object as having two states, one in which it is receiving (i.e., still processing a receive), and another in which it is not receiving. You can use a timer like the one in Figure 5.8 for timeouts, but you may need to modify it to suit this problem. References [RH04] Peter Van Roy and Seif Haridi. Concepts, Techniques, and Models of Computer Programming. The MIT Press, Cambridge, Mass., 2004. 6

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