CSC 347 - Concepts of Programming Languages

Exercises

Instructor: Stefan Mitsch

Exercise Topics

• Tail recursion
• Scope and lifetime
• Algebraic datatypes
• Argument passing
• Closures
• Inheritance and delegation
• JavaScript

Tail Recursion

def f (n:Int) = if n<=1 then 1 else n * f (n-1)
def g (n:Int) = f (n) * g (n-1)
def h (n:Int) = h (g (n), f (n))
def i (n:Int) = n * n

• Is f recursive? tail-recursive?
• Is g recursive? tail-recursive?
• Is h recursive? tail-recursive?
• Is i recursive? tail-recursive?

Scope and Lifetime: Dangling Pointers

int* f(int x) { return &x; }
int* g(int x) { 
  int* y = (int*) malloc (sizeof (int)); 
  *y=x; 
  return y; 
}
int main(void) {
  int* p = f(5);
  printf("%d\n", *p);
  int* q = g(5);              
}

Static vs. Dynamic Scope

What is printed in a statically-scoped vs. dynamically-scoped language?

var x:Int = 10
def foo() =
  x = 20                
def bar() = 
  var x:Int = 30
  foo()
  println(x)                
def zee() = 
  var x:Int = 40
  bar()
  println(x)
print("bar: "); bar(); print(" " + x)
print("zee: "); zee(); print(" " + x)
print("foo: "); foo(); print(" " + x)

Algebraic Datatypes

Define an algebraic datatype for arithmetic expressions with literals, variables, and operators +, -, *

• In Scala

  enum Expr:
    case Literal(x:Int)
    case Variable(n:String, v: Option[Expr])
    case Neg(e:Expr)
    case Plus(l:Expr, r:Expr)
    case Minus(l:Expr, r:Expr)
    case Times(l:Expr, r:Expr)
  

Algebraic Datatypes

Define a toInt method for arithmetic expressions

enum Expr:
  case Literal(x:Int)
  case Variable(n:String, v:Option[Expr])     
  case Neg(e:Expr)
  case Plus(l:Expr, r:Expr)
  case Minus(l:Expr, r:Expr)
  case Times(l:Expr, r:Expr)

Call-by-value and Call-by-reference

What is printed in a call-by-value vs. a call-by-reference language?

def f(a:Int, b:Int) = {
  a = b
  b = b + 1
}
var x = 5
var y = 10
f(x,y); println(s"x=$x, y=$y")
f(x,x); println(s"x=$x")
f(x,x+2); println(s"x=$x")

Closures

def f(): String=>String = {
  var s = ""
  (t:String) => { s = s + t; s }
}
val a = f()
println(a("x"))
println(a("y"))
println(a("z"))

Nested Closures

def f(): ()=>String=>String = {
  var s = ""
  () => {
    var i = 0
    (t:String) => { s = s + t + i; i = i+1; s }
  }
  
}
val factory = f()
val a = factory()
val b = factory()
println(a("x"))
println(a("y"))
println(b("z"))

• Do a and b share s?
• Do a and b share i?
• What is printed?

Inheritance

class A {
  def f() = { println("A.f") }
}
class B extends A {
  override def f() = { println("B.f"); super.f() }
           def g() = { println("B.g") }
}
val b:A = new B() // b:B?
b.f()

• Does the program compile? If yes, what is printed?
• Does b.g() compile? If yes, what is printed?
• If no, would it compile with val b:B = new B()?

Parametric Types

class B extends A {}
var as: List[A] = List(new A(), new B())

Which code compiles and why/why not?

• val bs: List[B] = as
• val bs: List[B] = List(new B(), new B())
• as = bs
• val a1: Any = as(0) // a1: A // a1: B

JavaScript

function f() {
  return {
    a: 1,
    b: "hello"
    c: function () { return 2; }
  }
}

• What is the result type of function f?
• What is the result of f().a?
• What is the result of f().d?
• What is the result of f().c()?

JavaScript: Hoisting

function f(n) {
  var fs = [];
  for (var i = 0; i < n; i++) {
    var x = i;
    fs.push(x);
  }
  return fs;
}

• What is the result of f(3) and why?
• How can we make the result of f(3) be [0,1,2]?

JavaScript: Closures

function f(n) {
  var x = n;
  return {
    get: function(y) { x = x+y; return x; }
  }
}

• What is the result of f(3).get(1) and why?
• What are the values of b and c in

  var a = f(3); 
  var b = a.get(1); 
  var c = a.get(2);
  

Scala Lists

val xs = 1 :: 2 :: Nil
val ys = List(1, 2)
val zs = 0 :: xs
val as = List(0) :: xs
val bs = List(0) ::: xs

• Do xs and ys contain the same elements?
• What is zs? Does it contain the same elements as 0 :: ys?
• What is as?
• What is bs?

Missing Types

def f(xs:List[?3]): ?4 = xs.map(x => x+1)
def g(ys:?2): ?1 = ys.foldLeft(0)(_ + _)
g (f (List (1,2,3,4)))

• In which order should we try to resolve the types?
• What is the type ?1
• What is the type ?2
• What is the type ?3
• What is the type ?4