Original eed to work: Compiler Construction Programming answers should be written in some notation approximating SML or OCaml. (a) Describe what is meant by tail recursion. [4 marks] (b) Eliminate tail recursion from foldl given below. Explain your answer. (* foldl : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a *) let rec foldl f accu l = match l with [] -> accu | a::l -> foldl f (f accu a) l [8 marks] (c) Eliminate tail recursion from the following mutually tail-recursive functions. Explain your answer. let rec is_even n = if n = 0 then true else is_odd (n - 1) and is_odd n = if n = 0 then false else is_even(n - 1) [8 marks] 4 CST.2016.3.5 4 Compiler Construction Consider writing a compiler for a simple language of expressions given by the following grammar, e ::= n (integer) | ? (read integer input from user) | e + e (addition) | e e (subtraction) | e e (multiplication) | (e, e) (pair) | fst e (first projection) | snd e (second projection) (a) Describe the tasks that should be carried in implementing a front end for this language and any difficulties that might be encountered. [5 marks] (b) Suppose that the target virtual machine is stack-oriented and that the stack elements are integer values, and addresses can be stored as integers. Explain which other features are required in such a virtual machine. Invent a simple language of instructions for such a machine and show how it would be used to implement each of the expressions. [10 marks] (c) Suppose that the following rules are proposed as possible optimizations to be implemented in your compiler. expression simplifies to expression (fst e, snd e) e fst (e1, e2) e1 snd (e1, e2) e2 Describe how you could implement these rules so that the simplifications are made only when the program's semantics is correctly preserved. [5 marks] 5 (TURN OVER) CST.2016.3.6 5 Concepts in Programming Languages (a) Explain what is meant by a monad in a programming language, giving the two fundamental operations of a monad along with their types. [3 marks] (b) Consider the use of a monad for input-output. For the purposes of this question, Consider different versions of an optimising compiler, each of which uses IEEE standard representation for all variables. Give two reasons why they might compile a floating-point program into code that, when run, produces differing results. [2 marks] 6 CST.2016.1.7 6 Numerical Methods A picnicker brings hot black coffee and cold milk in two identical insulated flasks and then mixes them for his drink. His friend claims that the drink would have ended up the same temperature if he had mixed the two at home and brought one flask. Note: The temperature of an object is the heat energy within it divided by its heat capacity. The rate of heat energy flow from a hotter to a cooler object is their temperature difference divided by their insulation resistance. When two fluids are mixed the resultant temperature is the sum of their initial temperatures weighted by their proportions. (a) Give a suitable state vector for a simple, finite-difference, time domain simulation of the drink system. [3 marks] (b) List the initial values and any other parameters that are needed for the simulationtake the IO monad as including two operations readint and writeint which respectively read integers from stdin and write integers to stdout. Explain any differences in behaviour between assignments v1 = w and v2 = w and also between method calls v1.set(4,n) and v2.set(4,n). [4 marks] 6 CST.2016.3.7 6 Further Java (a) Describe the operation of wait() and notifyAll(). Ensure that your answer explains when locks are acquired and released. [5 marks] (b) A future is a mechanism to store the eventual result of a computation done in another thread. The idea is that the computation is run asynchronously and the calling thread only blocks if it tries to use a result that hasn't been computed yet. An example program using a future is shown below. Future f = new Future() { @Override public String execute() { // ...long running computation... return data; }; // ... String result = f.get(); // blocks if execute() unfinished Use wait() and notifyAll() to provide an implementation of the Future class that would work with the example program above. [10 marks] (c) Give one potential advantage and one potential disadvantage of using notify() instead of notifyAll().(d) Would it have been beneficial to use notify() instead of notifyAll() in your implementation? Justify your answer. [3 marks] 7 (TURN OVER) CST.2016.3.8 7 Prolog In this question you should ensure that your predicates behave appropriately with backtracking and avoid over-use of cut. You should provide an implementation of any library predicates used. You may not make use of extra-logical built-in predicates such as findAll. Minor syntactic errors will not be penalised. (a) Rewrite chooseAll without using not and cut (!). [10 marks] chooseAll(N,L,Res) :- chooseAll(N,L,[],Res). chooseAll(N,L,Seen,Res) :- choose(N,L,R), not(member(R,Seen)), !, chooseAll(N,L,[R|Seen],Res). chooseAll(,,Res,Res). (e) What is Last Call Optimisation and why is it beneficial? [3 marks] (f ) Rewrite pos to enable Last Call Optimisation. [2 marks] pos([],[]). pos([H|T],[H|R]) :- H >= 0, pos(T,R). pos([H|T],R) :- H < 0, pos(T,R). 8 CST.2016.3.9 8 Software Engineering Discuss the contribution and the relative value of the following aspects of the modern development environment. Illustrate with examples from your group project, or from experience you gained working for a commercial software developer, or both. In each case, would you discard this feature if your employer let you, or insist on retaining it (even covertly) should your employer not value it? Explain your reasons. (a) Dividing a project into short development episodes or sprints. (b) Project progress visualisation tools such as PERT and GANTT charts. (c) Automated regression testing tools. (d) Source code management tools. (e) Scrumming TLBs and caches are examples of content-addressable memories (CAMs). Data Structures and Algorithms (a) Describe how the Lempel Ziv text compression algorithm works, illustrating your answer by deriving the sequence of numbers and corresponding bit patterns it would generate when applied to a string starting with the following 24 characters: ABCDABCDABCDABCDABCDABCD ... You may assume that the initial table is of size 256 (containing bytes 0 to 255) and that the codes for "A", "B", "C" and "D" are 65, 66, 67 and 68, respectively. [12 marks] (b) Estimate how many bits the algorithm would use to encode a string consisting of 1000 repetitions of the character "A". [8 marks] 4 Comparative Programming Languages (a) Discuss to what extent a programmer can expect a program that conforms to a standard to generate identical results when run under different conforming compilers on different machines. [6 marks] (b) ALGOL 60 provided call by value and call by name, Pascal provided call by value and call by reference, and ALGOL-W provided a variety of calling methods including call by result and call by value-result. Briefly describe the calling mechanisms just mentioned and discuss why most modern programming languages provide only call by value. [8 marks] (c) Discuss the reasons why languages such as Fortran, Algol and PL/I designed in 1950s and 1960s are less widely used than languages designed in the last 20 years. [6 marks] 3 [TURN OVER CST.2004.3.4 5 Operating Systems II (a) What problem do real-time scheduling algorithms try to solve? [2 marks] (b) Describe one static priority and one dynamic priority real-time scheduling algorithm. You should discuss the issue of admission control, and comment on the data structures that an implementation would need to maintain and on how these would be used to make scheduling decisions.are a data structure protected by a mutual exclusion lock. (i) What scheduling problem could arise here? [2 marks] (ii) How could this problem be overcome? [2 marks] (d) The designer also wishes the real-time system to use demand paged virtual memory for efficiency. Consider the Java code: Object n = new Integer(42); ArrayList v1; ArrayList v2; ArrayList w = new ArrayList<>(10); Explain any differences in behaviour between assignments v1 = w and v2 = w and also between method calls v1.set(4,n) and v2.set(4,n). [4 marks] 6 CST.2016.3.7 6 Further Java (a) Describe the operation of wait() and notifyAll(). Ensure that your answer explains when locks are acquired and released. [5 marks] (b) A future is a mechanism to store the eventual result of a computation done in another thread. The idea is that the computation is run asynchronously and the calling thread only blocks if it tries to use a result that hasn't been computed yet. An example program using a future is shown below. Future f = new Future() { @Override public String execute() { // ...long running computation... return data; }; // ... String result = f.get(); // blocks if execute() unfinished Use wait() and notifyAll() to provide an implementation of the Future class that would work with the example program above. 18 CST.2016.3.9 8 Software Engineering Discuss the contribution and the relative value of the following aspects of the modern development environment. Illustrate with examples from your group project, or from experience you gained working for a commercial software developer, or both. In each case, would you discard this feature if your employer let you, or insist on retaining it (even covertly) should your employer not value it? Explain your reasons. (a) Dividing a project into short development episodes or sprints. (b) Project progress visualisation tools such as PERT and GANTT charts. (c) Automated regression testing tools. (d) Source code management tools. Compiler Construction Programming answers should be written in some notation approximating SML or OCaml. (a) Describe what is meant by tail recursion. [4 marks] (b) Eliminate tail recursion from foldl given below. Explain your answer. (* foldl : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a *) let rec foldl f accu l = match l with [] -> accu | a::l -> foldl f (f accu a) l [8 marks] (c) Eliminate tail recursion from the following mutually tail-recursive functions. Explain your answer. let rec is_even n = if n = 0 then true else is_odd (n - 1) and is_odd n = if n = 0 then false else is_even(n - 1) [8 marks] 4 CST.2016.3.5 4 Compiler Construction Consider writing a compiler for a simple language of expressions given by the following grammar, e ::= n (integer) | ? (read integer input from user) | e + e (addition) | e e (subtraction) | e e (multiplication) | (e, e) (pair) | fst e (first projection) | snd e (second projection) (a) Describe the tasks that should be carried in implementing a front end for this language and any difficulties that might be encountered. [5 marks] (b) Suppose that the target virtual machine is stack-oriented and that the stack elements are integer values, and addresses can be stored as integers. Explain which other features are required in such a virtual machine. Invent a simple language of instructions for such a machine and show how it would be used to implement each of the expressions. [10 marks] (c) Suppose that the following rules are proposed as possible optimizations to be implemented in your compiler. expression simplifies to expression (fst e, snd e) e fst (e1, e2) e1 snd (e1, e2) e2 Describe how you could implement these rules so that the simplifications are made only when the program's semantics is correctly preserved. [5 marks] 5 (TURN OVER) CST.2016.3.6 5 Concepts in Programming Languages (a) Explain what is meant by a monad in a programming language, giving the two fundamental operations of a monad along with their types. [3 marks] (b) Consider the use of a monad for input-output. For the purposes of this question, [4 marks]