# Homework 4

## Due Friday, Nov 16, 2018 at 8pm

$wget https://eecs490.github.io/homework/hw4/starter-files.tar.gz$ tar xzf starter-files.tar.gz

1. Semantic equivalence. Write a transition rule in big-step operational semantics that specifies the evaluation of a let* form in Scheme in terms of let and let*. As an example, the expression

(let* ((v1 e1) (v2 e2) (v3 e3)) body)


is equivalent to

(let ((v1 e1)) (let* ((v2 e2) (v3 e3)) body))


Fill in the recursive rule below:

\begin{equation*} \cfrac{}{\langle(\textbf{let*}~ ((v_1~ e_1)~ \dots~ (v_k~ e_k))~ body), \sigma\rangle \rightarrow~~~~~~~~~~~~~~}~~~ \textrm{if}~ k > 1 \end{equation*}

Also fill in the rule for the base case:

\begin{equation*} \cfrac{}{\langle(\textbf{let*}~ ((v~ e))~ body), \sigma\rangle \rightarrow~~~~~~~~~~~~~~} \end{equation*}

For this question and Q2, you may write subscripts with or without a preceding underscore (e.g. v_1 or v1, and e_k or ek), and you may use the word sigma instead of $$\sigma$$.

2. Scope. Suppose we wanted to add the $$\bf let$$ construct to the simple imperative language defined in lecture, with the following syntax:

\begin{equation*} S ~\rightarrow~ \textbf{let}~ V~ =~ A~ \textbf{in}~ S~ \textbf{end} \end{equation*}

The semantics of this construct are to execute the body $$S$$ of the $$\bf let$$ in the context of a state in which the result of evaluating the given expression $$A$$ is bound to the variable $$V$$. After the $$\bf let$$ is executed, the variable should be restored to its previous value. Fill in the big-step transition rule describing this behavior below:

\begin{equation*} \cfrac{}{\langle\textbf{let}~ v = a~ \textbf{in}~ s, \sigma\rangle \rightarrow~~~~~~~~~~~~~~} \end{equation*}
3. Type systems and recursion. The language that we used to explore type systems does not have a direct mechanism for defining a recursive function. Suppose we wanted to add a $$\bf letrec$$ construct, which is similar in structure to $$\bf let$$:

\begin{equation*} E ~\rightarrow~ (~ \textbf{letrec}~ V~ :~ T~ =~ E~ \textbf{in}~ E~ ) \end{equation*}

A syntactic difference is that the variable must be explicitly typed in a $$\bf letrec$$. Then if the initializer is a function abstraction, it is allowed to refer to itself by name in its body. For example, the following defines a factorial function:

\begin{align*} &(\textbf{letrec}~ fact : Int \rightarrow Int =\\ &~~~~(\textbf{lambda}~ n : Int .~\\ &~~~~~~~~(\textbf{if}~ (n <= 0)~ \textbf{then}~ 1~ \textbf{else}~ (n * (fact~ (n - 1))))\\ &~~~~)\\ &~~~~\textbf{in}~ (fact~ 5)\\ &) \end{align*}

Fill in the following typing rule for the $$\bf letrec$$ construct:

\begin{equation*} \cfrac{ }{ \Gamma ~\vdash~ (\textbf{letrec}~ v : T_1 = t_1~ \textbf{in}~ t_2) ~:~ ~~~~~~~~~~~~~~ } \end{equation*}

For this question, you may write subscripts with or without a preceding underscore (e.g. t_1 or t1), and you may use the word Gamma and the symbols |- instead of $$\Gamma$$ and $$\vdash$$.

4. Vtables. Consider the following C++ code:

struct A {
void foo() {
cout << "A::foo()" << endl;
}
virtual void bar() {
cout << "A::bar()" << endl;
}
};

struct B : A {
virtual void foo() {
cout << "B::foo()" << endl;
}
void bar() {
cout << "B::bar()" << endl;
}
};

int main() {
A *aptr = new B;
aptr->foo();
aptr->bar();
}


This code prints the following when run:

A::foo()
B::bar()

1. Draw the vtables for A and B.
2. Briefly explain how the compiler translates the method calls in main().
5. Dispatch dictionaries and inheritance. In the course notes, we saw a definition of a bank account ADT using functions and dispatch dictionaries. The following is a version of this ADT using built-in Python dictionaries:

def account(initial_balance):
def deposit(amount):
new_balance = dispatch['balance'] + amount
dispatch['balance'] = new_balance
return new_balance

def withdraw(amount):
balance = dispatch['balance']
if amount > balance:
return 'Insufficient funds'
balance -= amount
dispatch['balance'] = balance
return balance

def get_balance():
return dispatch['balance']

dispatch = {}
dispatch['balance'] = initial_balance
dispatch['deposit'] = deposit
dispatch['withdraw'] = withdraw
dispatch['get_balance'] = get_balance

def dispatch_message(message):
return dispatch[message]

return dispatch_message


Implement an ADT for a checking account that is a derived version of a bank account but charges a $1 fee for withdrawal. Fill in the ADT definition for checking_account() in the hw4.py file. Do not repeat code from account(). Instead, implement a scheme for deferring to account() where possible. 6. Java generics. 1. The file Array.java contains the definition of a multidimensional-array class that can hold elements of type String. Read through the provided code. Then make whatever modifications are necessary to make Array a generic class, so that it can hold elements of arbitrary class type. The Array class is built by storing elements in a single-dimensional built-in Java array. Variadic methods are used to allow the Array to have any valid rank (i.e. dimensionality), and the indexOf() method translates a multidimensional index to a single-dimensional location in the underlying Java array. The main() method of ArrayTest provides some tests, and the expected output from running the test is in ArrayTest.correct. In order to install Java on your machine, download the installer for your platform, selecting the Java SE Development Kit. Once it is installed and added to your path, you will be able to invoke javac to compile your code and java to run it. For example, the following compiles ArrayTest.java: $ javac ArrayTest.java


Note: The code will not compile until you make the required changes to Array.java.

Successful compilation produces the Array.class and ArrayTest.class files, and you can then run the main() function of the ArrayTest class with:

$java ArrayTest  Make sure that you are not using OpenJDK or Java EE. 2. Once you have a working generic Array class, implement the indexOfMax1D() method of the Util class in Util.java. This method takes in a generic Array, which must be rank one and have at least one element. It returns the index of the maximum element in the array. In order to be able to compare elements to each other, the element type must implement the appropriate Comparable interface. You will need to modify the header of indexOfMax1D() to enforce that this is the case. The main() method of the UtilTest class contains some tests. Compile and run as follows: $ javac UtilTest.java
\$ java -ea UtilTest


The -ea flag tells Java to enable assertions, which are otherwise disabled by default.

# Submission

Place your solution to question 5 in the provided hw4.py file, question 6a in Array.java, and question 6b in Util.java. Write your answers to questions 1-4 in a PDF file named hw4.pdf. Make sure to list any other students with whom you discussed the homework, as per course policy in the syllabus. Submit hw4.py, Array.java, and Util.java to the autograder before the deadline. Submit hw4.pdf to Gradescope before the deadline.