# Homework 06

**Due Wednesday, 17 November, 10:10am.** Please hand it in to Hossein before the beginning of the exercise session.

## Problem 1

Determine if the following piece of codes type check according to the type rules.

a)
The class Array has a field *length* in which the length of the array is stored.

def swap(lst: Array[Int], a: Int, b: Int): Array[Int] = { if (a >= lst.length || b >= lst.length) lst else { val swap = lst(a) lst(a) = lst(b) lst(b) = swap lst } }

b)

For solving this part you have to write down the type rule corresponding to **new**.

class Shape class Rectangle(width: Int, length: Int) extends Shape { def area : Int = width * length } class Square(length: Int) extends Rectangle(length,length) new Square(5).area

## Problem 2

We can partition the set of positive integers into the set of and the set of numbers.

Assume that we have the types *pos*, *even* and *odd* in a programming language.
Give the type rules for addition, multiplication, division and exponentiation.

For division, consider the following cases.

- Division rounds up the result: x / y is interpreted as .

- Division returns the quotient: x / y is interpreted as .

Specify in which case(s) there may be run-time error. Is there any way to prevent the run-time error at the type checking phase?