    Next: Logic Programming Up: Variations on a Scheme Previous: Examples of Nondeterministic Programs

## Implementing the Amb Evaluator

The evaluation of an ordinary Scheme expression may return a value, may never terminate, or may signal an error. In nondeterministic Scheme the evaluation of an expression may in addition result in the discovery of a dead end, in which case evaluation must backtrack to a previous choice point. The interpretation of nondeterministic Scheme is complicated by this extra case.

We will construct the amb evaluator for nondeterministic Scheme by modifying the analyzing evaluator of section . As in the analyzing evaluator, evaluation of an expression is accomplished by calling an execution procedure produced by analysis of that expression. The difference between the interpretation of ordinary Scheme and the interpretation of nondeterministic Scheme will be entirely in the execution procedures.

Execution procedures and continuations

Recall that the execution procedures for the ordinary evaluator take one argument: the environment of execution. In contrast, the execution procedures in the amb evaluator take three arguments: the environment, and two procedures called continuation procedures. The evaluation of an expression will finish by calling one of these two continuations: If the evaluation results in a value, the success continuation is called with that value; if the evaluation results in the discovery of a dead end, the failure continuation is called. Constructing and calling appropriate continuations is the mechanism by which the nondeterministic evaluator implements backtracking.

It is the job of the success continuation to receive a value and proceed with the computation. Along with that value, the success continuation is passed another failure continuation, which is to be called subsequently if the use of that value leads to a dead end.

It is the job of the failure continuation to try another branch of the nondeterministic process. The essence of the nondeterministic language is in the fact that expressions may represent choices among alternatives. The evaluation of such an expression must proceed with one of the indicated alternative choices, even though it is not known in advance which choices will lead to acceptable results. To deal with this, the evaluator picks one of the alternatives and passes this value to the success continuation. Together with this value, the evaluator constructs and passes along a failure continuation that can be called later to choose a different alternative.

A failure is triggered during evaluation (that is, a failure continuation is called) when a user program explicitly rejects the current line of attack (for example, a call to require may result in execution of (amb), an expression that always fails--see section ). The failure continuation in hand at that point will cause the most recent choice point to choose another alternative. If there are no more alternatives to be considered at that choice point, a failure at an earlier choice point is triggered, and so on. Failure continuations are also invoked by the driver loop in response to a try-again request, to find another value of the expression.

In addition, if a side-effect operation (such as assignment to a variable) occurs on a branch of the process resulting from a choice, it may be necessary, when the process finds a dead end, to undo the side effect before making a new choice. This is accomplished by having the side-effect operation produce a failure continuation that undoes the side effect and propagates the failure.

In summary, failure continuations are constructed by

• amb expressions--to provide a mechanism to make alternative choices if the current choice made by the amb expression leads to a dead end;

• the top-level driver--to provide a mechanism to report failure when the choices are exhausted;

• assignments--to intercept failures and undo assignments during backtracking.

Failures are initiated only when a dead end is encountered. This occurs

• if the user program executes (amb);

• if the user types try-again at the top-level driver.

Failure continuations are also called during processing of a failure:

• When the failure continuation created by an assignment finishes undoing a side effect, it calls the failure continuation it intercepted, in order to propagate the failure back to the choice point that led to this assignment or to the top level.

• When the failure continuation for an amb runs out of choices, it calls the failure continuation that was originally given to the amb, in order to propagate the failure back to the previous choice point or to the top level.

Structure of the evaluator

The syntax- and data-representation procedures for the amb evaluator, and also the basic analyze procedure, are identical to those in the evaluator of section , except for the fact that we need additional syntax procedures to recognize the amb special form: ```(define (amb? exp) (tagged-list? exp 'amb))

(define (amb-choices exp) (cdr exp))
```
We must also add to the dispatch in analyze a clause that will recognize this special form and generate an appropriate execution procedure:

```((amb? exp) (analyze-amb exp))
```

The top-level procedure ambeval (similar to the version of eval given in section ) analyzes the given expression and applies the resulting execution procedure to the given environment, together with two given continuations:

```(define (ambeval exp env succeed fail)
((analyze exp) env succeed fail))
```

A success continuation is a procedure of two arguments: the value just obtained and another failure continuation to be used if that value leads to a subsequent failure. A failure continuation is a procedure of no arguments. So the general form of an execution procedure is

```(lambda (env succeed fail)
;; succeed is (lambda (value fail) ...)
;; fail is (lambda () ...)
...)
```

For example, executing

```(ambeval exp
the-global-environment
(lambda (value fail) value)
(lambda () 'failed))
```
will attempt to evaluate the given expression and will return either the expression's value (if the evaluation succeeds) or the symbol failed (if the evaluation fails). The call to ambeval in the driver loop shown below uses much more complicated continuation procedures, which continue the loop and support the try-again request.

Most of the complexity of the amb evaluator results from the mechanics of passing the continuations around as the execution procedures call each other. In going through the following code, you should compare each of the execution procedures with the corresponding procedure for the ordinary evaluator given in section .

Simple expressions

The execution procedures for the simplest kinds of expressions are essentially the same as those for the ordinary evaluator, except for the need to manage the continuations. The execution procedures simply succeed with the value of the expression, passing along the failure continuation that was passed to them.

```(define (analyze-self-evaluating exp)
(lambda (env succeed fail)
(succeed exp fail)))

(define (analyze-quoted exp)
(let ((qval (text-of-quotation exp)))
(lambda (env succeed fail)
(succeed qval fail))))

(define (analyze-variable exp)
(lambda (env succeed fail)
(succeed (lookup-variable-value exp env)
fail)))

(define (analyze-lambda exp)
(let ((vars (lambda-parameters exp))
(bproc (analyze-sequence (lambda-body exp))))
(lambda (env succeed fail)
(succeed (make-procedure vars bproc env)
fail))))
```

Notice that looking up a variable always ``succeeds.'' If lookupvariablevalue fails to find the variable, it signals an error, as usual. Such a ``failure'' indicates a program bug--a reference to an unbound variable; it is not an indication that we should try another nondeterministic choice instead of the one that is currently being tried.

Conditionals and sequences

Conditionals are also handled in a similar way as in the ordinary evaluator. The execution procedure generated by analyze-if invokes the predicate execution procedure pproc with a success continuation that checks whether the predicate value is true and goes on to execute either the consequent or the alternative. If the execution of pproc fails, the original failure continuation for the if expression is called.

```(define (analyze-if exp)
(let ((pproc (analyze (if-predicate exp)))
(cproc (analyze (if-consequent exp)))
(aproc (analyze (if-alternative exp))))
(lambda (env succeed fail)
(pproc env
;; success continuation for evaluating the predicate
;; to obtain pred-value
(lambda (pred-value fail2)
(if (true? pred-value)
(cproc env succeed fail2)
(aproc env succeed fail2)))
;; failure continuation for evaluating the predicate
fail))))
```

Sequences are also handled in the same way as in the previous evaluator, except for the machinations in the subprocedure sequentially that are required for passing the continuations. Namely, to sequentially execute a and then b, we call a with a success continuation that calls b.

```(define (analyze-sequence exps)
(define (sequentially a b)
(lambda (env succeed fail)
(a env
;; success continuation for calling a
(lambda (a-value fail2)
(b env succeed fail2))
;; failure continuation for calling a
fail)))
(define (loop first-proc rest-procs)
(if (null? rest-procs)
first-proc
(loop (sequentially first-proc (car rest-procs))
(cdr rest-procs))))
(let ((procs (map analyze exps)))
(if (null? procs)
(error "Empty sequence - ANALYZE"))
(loop (car procs) (cdr procs))))
```

Definitions and assignments

Definitions are another case where we must go to some trouble to manage the continuations, because it is necessary to evaluate the definition-value expression before actually defining the new variable. To accomplish this, the definition-value execution procedure vproc is called with the environment, a success continuation, and the failure continuation. If the execution of vproc succeeds, obtaining a value val for the defined variable, the variable is defined and the success is propagated:

```(define (analyze-definition exp)
(let ((var (definition-variable exp))
(vproc (analyze (definition-value exp))))
(lambda (env succeed fail)
(vproc env
(lambda (val fail2)
(define-variable! var val env)
(succeed 'ok fail2))
fail))))
```

Assignments are more interesting. This is the first place where we really use the continuations, rather than just passing them around. The execution procedure for assignments starts out like the one for definitions. It first attempts to obtain the new value to be assigned to the variable. If this evaluation of vproc fails, the assignment fails.

If vproc succeeds, however, and we go on to make the assignment, we must consider the possibility that this branch of the computation might later fail, which will require us to backtrack out of the assignment. Thus, we must arrange to undo the assignment as part of the backtracking process. This is accomplished by giving vproc a success continuation (marked with the comment ``*1*'' below) that saves the old value of the variable before assigning the new value to the variable and proceeding from the assignment. The failure continuation that is passed along with the value of the assignment (marked with the comment ``*2*'' below) restores the old value of the variable before continuing the failure. That is, a successful assignment provides a failure continuation that will intercept a subsequent failure; whatever failure would otherwise have called fail2 calls this procedure instead, to undo the assignment before actually calling fail2.

```(define (analyze-assignment exp)
(let ((var (assignment-variable exp))
(vproc (analyze (assignment-value exp))))
(lambda (env succeed fail)
(vproc env
(lambda (val fail2)        ; *1*
(let ((old-value
(lookup-variable-value var env)))
(set-variable-value! var val env)
(succeed 'ok
(lambda ()    ; *2*
(set-variable-value! var
old-value
env)
(fail2)))))
fail))))
```

Procedure applications

The execution procedure for applications contains no new ideas except for the technical complexity of managing the continuations. This complexity arises in analyze-application, due to the need to keep track of the success and failure continuations as we evaluate the operands. We use a procedure get-args to evaluate the list of operands, rather than a simple map as in the ordinary evaluator.

```(define (analyze-application exp)
(let ((fproc (analyze (operator exp)))
(aprocs (map analyze (operands exp))))
(lambda (env succeed fail)
(fproc env
(lambda (proc fail2)
(get-args aprocs
env
(lambda (args fail3)
(execute-application
proc args succeed fail3))
fail2))
fail))))
```

In get-args, notice how cdring down the list of aproc execution procedures and consing up the resulting list of args is accomplished by calling each aproc in the list with a success continuation that recursively calls get-args. Each of these recursive calls to get-args has a success continuation whose value is the cons of the newly obtained argument onto the list of accumulated arguments:

```(define (get-args aprocs env succeed fail)
(if (null? aprocs)
(succeed '() fail)
((car aprocs) env
;; success continuation for this aproc
(lambda (arg fail2)
(get-args (cdr aprocs)
env
;; success continuation for recursive
;; call to get-args
(lambda (args fail3)
(succeed (cons arg args)
fail3))
fail2))
fail)))
```

The actual procedure application, which is performed by executeapplication, is accomplished in the same way as for the ordinary evaluator, except for the need to manage the continuations.

```(define (execute-application proc args succeed fail)
(cond ((primitive-procedure? proc)
(succeed (apply-primitive-procedure proc args)
fail))
((compound-procedure? proc)
((procedure-body proc)
(extend-environment (procedure-parameters proc)
args
(procedure-environment proc))
succeed
fail))
(else
(error
"Unknown procedure type - EXECUTE-APPLICATION"
proc))))
```

Evaluating amb expressions

The amb special form is the key element in the nondeterministic language. Here we see the essence of the interpretation process and the reason for keeping track of the continuations. The execution procedure for amb defines a loop try-next that cycles through the execution procedures for all the possible values of the amb expression. Each execution procedure is called with a failure continuation that will try the next one. When there are no more alternatives to try, the entire amb expression fails.

```(define (analyze-amb exp)
(let ((cprocs (map analyze (amb-choices exp))))
(lambda (env succeed fail)
(define (try-next choices)
(if (null? choices)
(fail)
((car choices) env
succeed
(lambda ()
(try-next (cdr choices))))))
(try-next cprocs))))
```

Driver loop

The driver loop for the amb evaluator is complex, due to the mechanism that permits the user to try again in evaluating an expression. The driver uses a procedure called internal-loop, which takes as argument a procedure try-again. The intent is that calling try-again should go on to the next untried alternative in the nondeterministic evaluation. Internal-loop either calls try-again in response to the user typing try-again at the driver loop, or else starts a new evaluation by calling ambeval.

The failure continuation for this call to ambeval informs the user that there are no more values and re-invokes the driver loop.

The success continuation for the call to ambeval is more subtle. We print the obtained value and then invoke the internal loop again with a try-again procedure that will be able to try the next alternative. This next-alternative procedure is the second argument that was passed to the success continuation. Ordinarily, we think of this second argument as a failure continuation to be used if the current evaluation branch later fails. In this case, however, we have completed a successful evaluation, so we can invoke the ``failure'' alternative branch in order to search for additional successful evaluations.

```(define input-prompt ";;; Amb-Eval input:")
(define output-prompt ";;; Amb-Eval value:")

(define (driver-loop)
(define (internal-loop try-again)
(prompt-for-input input-prompt)
(if (eq? input 'try-again)
(try-again)
(begin
(newline)
(display ";;; Starting a new problem ")
(ambeval input
the-global-environment
;; ambeval success
(lambda (val next-alternative)
(announce-output output-prompt)
(user-print val)
(internal-loop next-alternative))
;; ambeval failure
(lambda ()
(announce-output
";;; There are no more values of")
(user-print input)
(driver-loop)))))))
(internal-loop
(lambda ()
(newline)
(display ";;; There is no current problem")
(driver-loop))))
```
The initial call to internal-loop uses a try-again procedure that complains that there is no current problem and restarts the driver loop. This is the behavior that will happen if the user types try-again when there is no evaluation in progress.

Exercise. Implement a new special form ramb that is like amb except that it searches alternatives in a random order, rather than from left to right. Show how this can help with Alyssa's problem in exercise .

Exercise. Implement a new kind of assignment called permanent-set! that is not undone upon failure. For example, we can choose two distinct elements from a list and count the number of trials required to make a successful choice as follows:

```(define count 0)

(let ((x (an-element-of '(a b c)))
(y (an-element-of '(a b c))))
(permanent-set! count (+ count 1))
(require (not (eq? x y)))
(list x y count))
;;; Starting a new problem
;;; Amb-Eval value:
(a b 2)

;;; Amb-Eval input:
try-again
;;; Amb-Eval value:
(a c 3)
```
What values would have been displayed if we had used set! here rather than permanent-set! ?

Exercise. Implement a new construct called if-fail that permits the user to catch the failure of an expression. If-fail takes two expressions. It evaluates the first expression as usual and returns as usual if the evaluation succeeds. If the evaluation fails, however, the value of the second expression is returned, as in the following example:

```;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5))))
(require (even? x))
x)
'all-odd)
;;; Starting a new problem
;;; Amb-Eval value:
all-odd

;;; Amb-Eval input:
(if-fail (let ((x (an-element-of '(1 3 5 8))))
(require (even? x))
x)
'all-odd)
;;; Starting a new problem
;;; Amb-Eval value:
8
```

Exercise. With permanent-set! as described in exercise and if-fail as in exercise , what will be the result of evaluating

```(let ((pairs '()))
(if-fail (let ((p (prime-sum-pair '(1 3 5 8) '(20 35 110))))
(permanent-set! pairs (cons p pairs))
(amb))
pairs))
```

Exercise. If we had not realized that require could be implemented as an ordinary procedure that uses amb, to be defined by the user as part of a nondeterministic program, we would have had to implement it as a special form. This would require syntax procedures

```(define (require? exp) (tagged-list? exp 'require))

```
and a new clause in the dispatch in analyze

```((require? exp) (analyze-require exp))
```
as well the procedure analyze-require that handles require expressions. Complete the following definition of analyze-require.

```(define (analyze-require exp)
(let ((pproc (analyze (require-predicate exp))))
(lambda (env succeed fail)
(pproc env
(lambda (pred-value fail2)
(if ??
??
(succeed 'ok fail2)))
fail))))
```    Next: Logic Programming Up: Variations on a Scheme Previous: Examples of Nondeterministic Programs
Ryan Bender
2000-04-17