SICP Exercise 4.54

Question

I have a question about Exercise 4.54 from Section 4.3.3 of Structure and Interpretation of Computer Programs (http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-28.html#%_sec_4.3.3). This exercise concerns the Amb evaluator.

The exercise is the following:

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))

(define (require-predicate exp) (cadr exp))

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))))

I completed it as follows:

(define (analyze-require exp)
  (let ((pproc (analyze (require-predicate exp))))
    (lambda (env succeed fail)
      (pproc env
             (lambda (pred-value fail2)
               (if (false? pred-value)
                   (fail2) ;; or (fail)
                   (succeed 'ok fail2)))
             fail))))

My doubt is the following:

I know that, during execution, when the predicate value pred-value is false, require should fail; that is, it should call a failure continuation procedure. But I'm a bit confused as to whether it should call (fail) or (fail2). Which one is correct?

Answer 1

(fail2) is the correct one. The procedure conforms to a continuation-passing style, and the correct continuation procedure in this case is fail2.

Answer 2

I think (fail) is correct.

Here we have got pred-value, which means pproc evaluates well for now. But if it is false value, we treat it as bad as pproc evaluates wrong. so the continuation here is fail not fail2.

A test can be found here:

• https://wizardbook.wordpress.com/2011/01/22/exercise-4-54/

Answer 3

(fail2)is correct, well,most of the time it doesn't matter whichever you choose,but for the following conditons:

1.the predicate part contains (amb) such as

(require (amb false true false true true))

2.the predicate part contains (set! <...> <...>)

you can check it out,this is because most of the process's fail-continuation is the same as its caller's except for the above two kinds of processes. Though it sounds some kind of ridiculous.