Assignment 6, CSC430, Winter 2012

1 Guidelines

1.1 Contracts & Test Cases

For this and all remaining assignments, every function you develop must come with the following things:

• A commented header line that expresses the result of the function in terms of its inputs, written in English. Be as precise as you can within the space of a line or two.

• A commented contract line that gives the types of the inputs and the output. The contract should have the form <in-type> ... -> <out-type>. So, for instance, a numerical square function might have the contract line:

  ; number -> number

• Test cases. A function without test cases is incomplete. Write the test cases first, please.

1.2 Handling errors

Your parser and interpreter must detect errors and explicitly signal them by calling (error ...). We will consider an error raised internally by Racket to be a bug in your code.

For example, Racket signals a "divide by zero" error if you attempt to evaluate (/ 1 0). Since we use Racket’s division function to implement division in CFWAE, you may be tempted to leave it to Racket to signal division by zero errors for you. However, you must signal the error yourself by explicitly testing for division by zero before calling Racket’s division procedure.

Note that you may assume that your contracts are respected. That is, if your interp function’s contract indicates that it expects a MOCFAE, you do not need to worry about miscreants that call it with a number or a string or another surprising value.

1.3 Language Level & File Format

Your solution should take the form of a single file. Solve each problem separately, and make sure that each solution appears in a separate part of the file, with comments separating each problem’s solution.

Hand in your solution using the handin server. For help with the handin server, please see the course web page.

2 Pair Programming

This assignment is a pair programming assignment. This means that you may work with at most one other person. You are not required to do so, but I strongly encourage it; if you don’t feel that you need the help, then work with someone who does.

To declare partners, both you and your partner must send me e-mail indicating this fact, at least three days before the assignment is due. In order to submit a pair programming assignment, use the string formed by joining your individual ids with a "+" in between. So, if my login is "clements" and my partner’s is "wilson", then we would submit as "clements+wilson". Don’t submit by yourself if you’re planning to work with a partner; an individual submission will prevent a later pair submission.

3 The Assignment

This assignment requires you to implement mutation of boxes, and expression sequencing.

3.1 Boxes

This assignment adds three new primitive functions: makebox, getbox, and setbox!. It adds one new kind of value, the boxV. These primitive functions behave precisely as in class.

• makebox : a unary operator that accepts a value and returns a boxV containing a newly allocated location, and sets the new location to contain the given value.

• getbox : a unary operator that accepts a boxV and returns the value associated with the box’s location

• changebox : a binary operator that accepts a boxV and an arbitrary new value and sets the location referred to by the box to contain the given new value. The result is the value "false".

3.2 Sequencing

Once we have mutation, we may wish to sequence expressions, as we might do in Racket using begin or as we might do in Java using semicolons. The seq expression must act similarly: it should evaluate all of its subexpressions, and evaluate to the value of the last one. So, for instance, this program:

{with {{b = {makebox 14}}}

{seq {setbox! b 15}

{setbox! b 16}

{getbox b}}}

... would evaluate to 16.

The seq expression must have at least one subexpression.

3.3 No Racket Boxes

Well, to be honest, you are allowed to use boxes in this assignment, just not for mutable structure fields. That is, your prior use of boxes–in the construction of recursive bindings–can continue to use boxes. However, you must implement mutable fields without using a mutable store.

Since you’re not allowed to use Racket’s boxes to represent the target language boxes, you’ll need to use the store monad. This means that functions like interp will have to produce OCFAE-Value computations, rather than plain OCFAE-Values. Your stores will be elements of the Store define-type, containing an immutable hash representing memory and a number representing the next unallocated store location.

You are absolutely not required to reclaim unused memory locations. In fact, don’t even think about it.

3.4 Using Monadic abstractions sanely

To use the monadic abstractions correctly, you have to "play by the rules." In particular:

• Computations aren’t values; you can only put them in bind’s/sdo’s or return them.

• Don’t use (lambda (sto) ...) in your code except in the functions lift and bind.

• Don’t make any reference to a*s in your code, except your monad functions and your test cases.

• Don’t use run in interp or in anything that interp calls. The only use of run should be at the top level. (Brr! Violating this is scary.)

3.5 Order of Evaluation

Order of evaluation is now observable. Order of evaluation should be first-to-last in applications and sequences, and the expressions should be evaluated as needed for switch expressions.

4 Syntax of OCFAE

The syntax of the OCFAE language with these additional features differs from that of the prior assignment only in that it has the new seq form and the three new primitives:

OCFAE		=		num
		|		string
		|		true
		|		false
		|		{operator OCFAE *}
		|		id
		|		{switch CFWAE [CFWAE => CFWAE] * [else CFWAE]}
		|		{with {{Pattern = OCFAE}*} OCFAE}
		|		{withrec {id OCFAE} OCFAE}
		|		{fun {id *} OCFAE}
		|		{OCFAE OCFAE *}
		|		{new {id OCFAE}*}
		|		{ref OCFAE id}
		|		{seq OCFAE OCFAE *}

with

operator		=		+
		|		-
		|		*
		|		/
		|		and
		|		or
		|		equal?
		|		<=
		|		not
		|		number?
		|		string?
		|		makebox
		|		getbox
		|		setbox!

where an id is not true, false, with, switch, else, fun, withrec, new, ref, seq, or one of the operator names

and num is a number.

If a fun or a with expression has duplicate identifiers, we consider it a syntax error. Therefore, such errors must be detected in parse-exp. For example, parsing the following expressions must signal errors:

{with {{x 10} {x 20}} 50}

{fun {x x} 10}

{fun {weight {zebra name weight}} {equal? weight weight}}

...

5 Support Code

The functions you implement must adhere to the following template, without any changes.

; represents an expression

(define-type OCFAE

[num (n number?)]

[bool (b boolean?)]

[str (s string?)]

[unop (op symbol?) (arg OCFAE?)]

[binop (op symbol?) (lhs OCFAE?) (rhs OCFAE?)]

[varref (name symbol?)]

[switch (val OCFAE?) (clauses (listof SClause?)) (elseval OCFAE?)]

[fun (params (listof symbol?)) (body OCFAE?)]

[withrec (name symbol?) (rhs OCFAE?) (body OCFAE?)]

[app (f OCFAE?) (args (listof OCFAE?))]

[newexp (fields (listof (list/c symbol? OCFAE?)))]

[objref (obj OCFAE?) (field symbol?)]

[seq (exps (listof OCFAE?))])

; represents a 'switch' clause

(define-type SClause

[clause (patval OCFAE?) (result OCFAE?)])

; represents a possible result of evaluation

(define-type OCFAE-Value

[numV (n number?)]

[boolV (b boolean?)]

[strV (s string?)]

[closureV (params (listof symbol?))

(body OCFAE?)

(env hash?)]

[objectV (fields (hash/c symbol? OCFAE-Value?))]

[boxV (loc number?)])

; An environment is represented as a hash mapping symbols

;  to (racket) boxes containing OCFAE-Values

; represents a store containing a hash and the next unused address

(define-type Store

[sto (memory (and/c immutable? (hash/c number? OCFAE-Value?)))

(next-loc number?)])

; parse-exp : s-exp -> OCFAE

; Consumes an s-expression and generates the corresponding OCFAE

(define (parse-exp sexp)

...)

;  This procedure interprets the given OCFAE and produces a result

;  in the form of a function that accepts a Store and produces

;  a OCFAE-Value paired with a new store.

;  interp : OCFAE? hash? -> (OCFAE-Value? computation)

(define (interp expr env)

...)