This assignment is all individual work. There is no pair programming.


The purpose of this assignment is to give you additional experience with higher-order, polymorphic functions and to give you practice using continuations for a backtracking search problem. The assignment builds on the previous two assignments, and it adds new ideas and techniques that are described in section 2.10 of Build, Prove, and Compare.


The executable μScheme interpreter is in /comp/105/bin/uscheme; if you are set up with use comp105, you should be able to run uscheme as a command. The interpreter accepts a -q (“quiet”) option, which turns off prompting. Your homework will be graded using uscheme. When using the interpreter interactively, you may find it helpful to use ledit, as in the command

  ledit uscheme

In this assignment, you may find the &trace feature especially useful. It is described in the Scheme homework.

Dire Warnings

The μScheme programs you submit must not use any imperative features. Banish set, while, println, print, printu, and begin from your vocabulary! If you break this rule for any exercise, you get No Credit for that exercise. You may find it useful to use begin and println while debugging, but they must not appear in any code you submit. As a substitute for assignment, use let or let*.

Except as noted below, do not define helper functions at top level. Instead, use let or letrec to define helper functions. When you do use let to define inner helper functions, avoid passing as parameters values that are already available in the environment.

Your solutions must be valid μScheme; in particular, they must pass the following test:

    /comp/105/bin/uscheme -q < myfilename > /dev/null

without any error messages or unit-test failures. If your file produces error messages, we won’t test your solution and you will earn No Credit for functional correctness. (You can still earn credit for structure and organization). If your file includes failing unit tests, you might possibly get some credit for functional correctness, but we cannot guarantee it.

We will evaluate functional correctness by testing your code extensively. Because this testing is automatic, each function must be named be exactly as described in each question. Misnamed functions earn No Credit.

Reading Comprehension (10 percent)

These questions are meant to guide you through the readings that will help you complete the assignment. Keep your answers brief and simple.

As usual, you can download the questions.

  1. Read section 2.12.3, which starts on page 154. Assume y is in the domain of ρ. Recall that evaluating a lambda in some environment creates a closure using that same environment. What is the result of the call to f in the program below? Pick either A or B.

    (val f (lambda () y))
    (val y 10)
    1. It returns 10
    2. An error is raised: Run-time error: name y not found

    Assume y is not in the domain of ρ. What is the result of the call to f in the same program? Pick either A or B.

    1. It returns 10
    2. An error is raised: Run-time error: name y not found

    You are ready to start problem 44.

  2. Set aside an hour to study the conjunctive-normal-form solver in section 2.10.1, which starts on page 140. This will help you a lot in solving problem S.

    1. Look at code chunk 144a on page 144. In English, describe how (one-solution f) produces the answer ((x #t) (y #f)). Walk through each function call, what the input to the function is, how the input is processed, and what the output of the function call is.

      It will help us read your answer and should help you answer this question if each function call is formatted similar to the examples below. But if the formatting gets too cumbersome just make sure you keep the four key parts of each call (function, input, processing, and output) separate. You can use the examples below to start your answer for one-solution.

      function: one-solution

      input: formula is ((x y z) ((not x) (not y) (not z)) (x y (not z)))

      processing: none

      output: what find-cnf-true-assignment returns for the whole formula with no current solutions, a failure continuation that returns no-solution, and a success continuation that returns the first solution

      function: find-cnf-true-assignment

      input: the arguments mentioned in the previous output, where disjunctions is bound to the formula

      processing: disjunctions is not null, so this function calls find-disjunction-true-assignment

      output: what find-disjunction-true-assignment returns for the first disjunction (x y z) with no current solutions, the same failure continuation, and a success continuation that checks the remaining disjunctions (((not x) (not y) (not z)) (x y (not z))). If checking the remaining disjunctions fails we want to try to find another solution to (x y z), so we pass resume as the failure continuation; otherwise if checking suceeds with an answer then that answer is good for the entire search, so we pass suceed as the success continuation

    2. Look at code chunk 144c. As you did with 144a, describe how (one-solution '((x) ((not x)))) produces the answer no-solution. Format this answer like you did for the previous part.

    You are ready to start problem S.

Programming and Language Design (90 percent)

You will explore an alternative semantics for val (44), and you will solve three problems related to Boolean formulas: recognize a formula (F), evaluate a formula (E), and a solve a formula (S). You will also submit test cases for the solver (T). Problems F and E require algebraic laws.

Language-design problem

44. Operational semantics and language design. Do all parts of exercise 44 on page 222 of Build, Prove, and Compare. Be sure your answer to part (b) compiles and runs under uscheme.

Related reading: Rules for evaluating definitions in section 2.12.3, especially the two rules for VAL.

Representing Boolean formulas

This homework involves Boolean formulas. We represent a formula either as a symbol or a record (“struct”), using the following record definitions:

(record not [arg])
(record or  [args])
(record and [args])

In the context of these definitions, a formula is one of the following:

Programming problems

F. Recognizing formulas. Define a function formula?, which when given an arbitrary S-expression, returns #t if the S-expression represents a Boolean formula and #f otherwise. Follow the above definition of formulas exactly. (In case mixing records and S-expressions doesn’t sound proper, recall that record operations in μScheme are syntactic sugar for operations S-expressions. This means passing one of your formula records to formula? is within the function’s contract.)

A straightforward solution that uses if expressions and “how were you formed?” questions takes 10 lines of μScheme. If you want to get clever with short-circuit operators, you can cut that in half.

Your solution must include algebraic laws. If possible, supplement your algebraic laws with a case at the end saying, “When none of the laws above applies, return (you fill in this part).” This supplement will obviate the need to define a law for every possible form of S-expression—you can focus your laws on just the interesting forms.

Related reading: The definition of equal? in section 2.3. The definition of LIST(A) in section 2.6.

E. Evaluating formulas. Define a function eval-formula, which takes two arguments: a formula f and an environment env. If the formula is satisfied in the given environment, (eval-formula f env) returns #t; otherwise it returns #f. Evaluation is defined by induction:

It is part of the contract of eval-formula that (eval-formula f env) may be called only when every variable in formula f is bound in env.

My solution is 10 lines, and it is structurally similar to my straightforward implementation of formula?.

You must document your solution with algebraic laws. As usual, we recommend you write the laws before you write the code.

Related reading: The initial basis of μScheme. And if you are uncertain about structure, the implementation of either the Impcore evaluator or the μScheme evaluator. (You could also look at the implementation of a μScheme evaluator in μScheme, which you would find in section 2.15.4, which starts on page 180, but that section has so much detail that it may be easier just to figure out on your own how to structure an evaluator written in μScheme.)

T. Testing SAT solvers. Create three test cases to test solutions to problem S. Your test cases will be represented by six val bindings, to variables f1, s1, f2, s2, f3, and s3.

As another example, if I wanted to code the test formula

(x ∨ y ∨ z)∧(¬z ∨ ¬y ∨ ¬z)∧(x ∨ y ∨ ¬z),

I might write

(val f1 (make-and
           (list3 (make-or (list3 'x 'y 'z))
                  (make-or (list3 (make-not 'x) (make-not 'y) (make-not 'z)))
                  (make-or (list3 'x 'y (make-not 'z))))))
(val s1 '((x #t) (y #f)))

As a second test case, I might write

(val f2 (make-and (list2 'x (make-not 'x))))  ; x and not x
(val s2 'no-solution)

Put your test cases into the template at

In comments in your test file, explain why these particular test cases are important—your test cases must not be too complicated to be explained. Consider different combinations of the various Boolean operators.

Design test cases that will find bugs in solvers. (We will run every submitted solver on every submitted test case.)

Related reading: The example formulas and satisfying assignments on page 144 (at the very end of section 2.10.1).

S. SAT solving using continuation-passing style. Write a function find-formula-true-asst which, given a satisfiable formula in the form, finds a satisfying assignment—that is, a mapping of variables to Booleans that makes the formula true. Remember De Morgan’s laws, one of which is mentioned on page 131.

Function find-formula-true-asst must take three parameters: a formula, a failure continuation, and a success continuation. A call to

(find-formula-true-asst f fail succ)

searches for an assignment that satisfies formula f. If it finds a satisfying assignment, it calls succ, passing both the satisfying assignment (as an association list) and a resume continuation. If it fails to find a satisfying assignment, it calls fail. Notes:

My solution to this exercise is under 50 lines of μScheme.

You’ll be able to use the ideas in section 2.10.1, but not the code. Instead, try using letrec to define the following mutually recursive functions:

In all the functions above, bool is #t or #f. Before defining each of the functions, we recommend completing the following algebraic laws:

(find-formula-asst x            bool cur fail succeed) == ...,
                                                   where x is a symbol
(find-formula-asst (make-not f)  bool cur fail succeed) == ...
(find-formula-asst (make-or  fs) #t   cur fail succeed) == ...
(find-formula-asst (make-or  fs) #f   cur fail succeed) == ...
(find-formula-asst (make-and fs) #t   cur fail succeed) == ...
(find-formula-asst (make-and fs) #f   cur fail succeed) == ...

(find-all-asst '()         bool cur fail succeed) == ...
(find-all-asst (cons f fs) bool cur fail succeed) == ...

(find-any-asst '()         bool cur fail succeed) == ...
(find-any-asst (cons f fs) bool cur fail succeed) == ...

(find-formula-symbol x bool cur fail succeed) == ..., where x is not bound in cur
(find-formula-symbol x bool cur fail succeed) == ..., where x is bool in cur
(find-formula-symbol x bool cur fail succeed) == ..., where x is (not bool) in cur

Include the completed laws in your solution.

The following unit tests will help make sure your function has the correct interface:

(check-assert (procedure? find-formula-true-asst)) ; correct name
(check-error (find-formula-true-asst))                ; not 0 arguments
(check-error (find-formula-true-asst 'x))             ; not 1 argument
(check-error (find-formula-true-asst 'x (lambda () 'fail)))   ; not 2 args
   (find-formula-true-asst 'x (lambda () 'fail) (lambda (c r) 'succeed) 'z)) ; not 4 args

These additional checks also probe the interface, but they require at least a little bit of a solver—enough so that you call the success or failure continuation with the right number of arguments:

(check-error (find-formula-true-asst 'x (lambda () 'fail) (lambda () 'succeed)))
    ; success continuation expects 2 arguments, not 0
(check-error (find-formula-true-asst 'x (lambda () 'fail) (lambda (_) 'succeed)))
    ; success continuation expects 2 arguments, not 1
(check-error (find-formula-true-asst
                   (make-and (list2 'x (make-not 'x)))
                   (lambda (_) 'fail)
                   (lambda (_) 'succeed)))
    ; failure continuation expects 0 arguments, not 1

And here are some more tests that probe if you can solve a few simple formulas, and if so, if you can call the proper continuation with the proper arguments.

(check-expect   ; x can be solved
   (find-formula-true-asst 'x
                           (lambda () 'fail)
                           (lambda (cur resume) 'succeed))

(check-expect   ; x is solved by '((x #t))
   (find-formula-true-asst 'x
                           (lambda () 'fail)
                           (lambda (cur resume) (find 'x cur)))

(check-expect   ; (make-not 'x) can be solved
   (find-formula-true-asst (make-not 'x)
                           (lambda () 'fail)
                           (lambda (cur resume) 'succeed))

(check-expect   ; (make-not 'x) is solved by '((x #f))
   (find-formula-true-asst (make-not 'x)
                           (lambda () 'fail)
                           (lambda (cur resume) (find 'x cur)))

(check-expect   ; (make-and (list2 'x (make-not 'x))) cannot be solved
   (find-formula-true-asst (make-and (list2 'x (make-not 'x)))
                           (lambda () 'fail)
                           (lambda (cur resume) 'succeed))

You can download all the tests. You can run them at any time with

-> (use solver-interface-tests.scm)

This problem is (forgive me) the most satisfying problem on the assignment.

Related reading: Section 2.10 on continuation passing, especially the CNF solver in section 2.10.1.

What and how to submit

You must submit five files:

As soon as you have the files listed above, run submit105-continuations to submit a preliminary version of your work. Keep submitting until your work is complete; we grade only the last submission.

Avoid common mistakes

The most common mistakes on this assignment have to do with the Boolean-formula solver in problem S. They are