Concepts of Programming Languages Supervision Work

Chose one of the following options to do

Chose one of the following options to do

Outline the key features that a language must have to be called object-oriented. Further, briefly discuss to what extent the programming languages Simula, Smalltalk, C++, and Java has them. [6 marks]

Define the following parameter-passing mechanisms: pass-by-value, pass-by-reference, pass-by-value/result, and pass-by-name. Briefly comment on their merits and drawbacks. [5 marks]

What is aliasing in the context of programming languages? Explain the contexts in which it arises and provide examples of the phenomenon. In what kinds of languages is it generally considered bad, and in what kinds of languages is it considered useful? [5 marks]

Recall that Algol 60 has a primitive static type system. In particular, in Algol 60, the type of a procedure parameter does not include the types of its parameters. Thus, for instance, in the Algol 60 code

procedure P(procedure F)
begin F(true) end ;

the types of the parameters of the procedure parameter F are not specified. Explain why this piece of code is statically type correct. Explain also why a call P(Q) may produce a run-time type error, and exemplify your answer by exhibiting a declaration for Q with this effect. Why does this problem not arise in Standard ML? [3 marks]

Consider the Simula declarations

CLASS A
A CLASS B;

which have the effect of producing the subtype relation B<:A, and

REF(A) a
REF(B) b;

Recall that Simula uses the semantically incorrect principle that if B<:A then REF(B)<:REF(A) and consider now the following Simula code

PROCEDURE ASSIGNa(REF(A) x)
BEGIN x :- a END ;
ASSIGNa(b);

Does it statically type check? If so, will it cause a run-time type error? Justify your answers. [3 marks]

What is the problem with making the tag optional in Pascal’s variant records? [1 mark]

What is the type of the expression

fn a => fn b => fn c => (a b) (b c)

given by the SML interpreter? Explain how this is inferred. [6 marks]

For each of the following ML declarations, either justify their ability to be soundly used or give a program using them which would violate type safety: [2 marks]

1. exception poly of ’a;
2. val ml = ref [];

You manage two junior programmers and overhear the following conversation:

A: “I don’t know why anyone needs a language other than Java, it provides clean thread-based parallel programming.”

B: “Maybe, but I write my parallel programs in a functional programming language because they are then embarrassingly parallel.”

Discuss the correctness of these statements and the extent to which they cover the range of languages for parallel programming. [6 marks]

What is the difference between internal and external iteration? [1 mark]

Scripting languages and dynamically typed languages are identical; discuss [3 marks]

Discuss the notion of ‘class’ in relation to JavaScript. [2 marks]

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]

Consider the use of a monad for input-output. For the purposes of this question, take the IO monad as including two operations readint and writeint which respectively read integers from stdin and write integers to stdout. Give the types of these operators. [2 marks]

Assume MLreadint and MLwriteint are primitives with side effects for input/output and consider the ML expression add1 of type int:

let val x = MLreadint() in MLwriteint(x+1); x end

Give an equivalent expression which uses the IO monad instead of side-effects, and state its type. [3 marks]

Give a function run2diff which can be applied to the previous answer. When so applied it should give a value in the IO monad which corresponds to ML code that runs add1 twice and returns the difference between the values read. [4 marks]

State what happens when attempting to compile and execute the following Java fragment (explaining the origin of any error messages or exceptions which might arise). [4 marks]

Object n = new Integer(42), o = new String("Whoops");
Object [] v;
Integer [] w = new Integer[10];
v = w;
v[4] = n;
v[5] = o;

Consider the Java code:

Object n = new Integer(42);
ArrayList<? extends Object> v1;
ArrayList<Object> v2;
ArrayList<Integer> 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]

In the programming language Scala, a generic class like the following one

abstract class Stack[A] {
  def push(x : A) : Stack[A];
  def top : A;
  def pop : Stack[A];
}

is non-variant by default. Why? Modify it to make it co-variant. [6 marks]

Consider the declarations

structure Z = struct   type t = int; val z = 0 end;
structure A = Z :  sig type t;       val z : t end;
structure B = Z :> sig type t = int; val z : t end;
structure C = Z :> sig type t;       val z : t end;

in the SML Modules language. Explain the behaviour of the SML interpreter on inputting each of the expressions. [6 marks]

Z.z = A.z;
Z.z = B.z;
Z.z = C.z;

What are the similarities and differences between Haskell type classes, overloading, and dynamic dispatch used in object-oriented languages? [3 marks]