ch4-ambeval.scm

;;;;AMB EVALUATOR FROM SECTION 4.3 OF
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS

;;;;Matches code in ch4.scm.
;;;; To run the sample programs and exercises, code below also includes
;;;; -- enlarged primitive-procedures list
;;;; -- support for Let (as noted in footnote 56, p.428)

;;;;This file can be loaded into Scheme as a whole.
;;;;**NOTE**This file loads the metacircular evaluator of
;;;;  sections 4.1.1-4.1.4, since it uses the expression representation,
;;;;  environment representation, etc.
;;;;  You may need to change the (load ...) expression to work in your
;;;;  version of Scheme.
;;;;**WARNING: Don't load mceval twice (or you'll lose the primitives
;;;;  interface, due to renamings of apply).

;;;;Then you can initialize and start the evaluator by evaluating
;;;; the two lines at the end of the file ch4-mceval.scm
;;;; (setting up the global environment and starting the driver loop).
;;;;In the driver loop, do
;(define (require p)
;  (if (not p) (amb)))


;;**implementation-dependent loading of evaluator file
;;Note: It is loaded first so that the section 4.2 definition
;; of eval overrides the definition from 4.1.1
(load "ch4-mceval.scm")



;;;Code from SECTION 4.3.3, modified as needed to run it

(define (amb? exp) (tagged-list? exp 'amb))
(define (amb-choices exp) (cdr exp))

;; Define various syntactic predicate procedures to false initially so that eval
;; can run even if all the exercise code hasn't been pasted into the REPL.

;; from exercise 50
(define (ramb? exp) false)

;; from exercise 51
(define (perm-assignment? exp) false)

;; from exercise 52
(define (if-fail? exp) false)

;; from exercise 54
(define (require? exp) false)

;; analyze from 4.1.6, with clause from 4.3.3 added
;; and also support for Let
(define (analyze exp)
  (cond ((self-evaluating? exp)
         (analyze-self-evaluating exp))
        ((quoted? exp) (analyze-quoted exp))
        ((variable? exp) (analyze-variable exp))
        ((assignment? exp) (analyze-assignment exp))
        ((definition? exp) (analyze-definition exp))
        ((if? exp) (analyze-if exp))
        ((lambda? exp) (analyze-lambda exp))
        ((begin? exp) (analyze-sequence (begin-actions exp)))
        ((cond? exp) (analyze (cond->if exp)))
        ((let? exp) (analyze (let->combination exp))) ;**
        ((amb? exp) (analyze-amb exp))                ;**

        ;; from exercise 50
        ((ramb? exp) (analyze-ramb exp))

        ;; from exercise 51
        ((perm-assignment? exp) (analyze-perm-assignment exp))

        ;; from exercise 52
        ((if-fail? exp) (analyze-if-fail exp))

        ;; from exercise 54
        ((require? exp) (analyze-require exp))

        ((application? exp) (analyze-application exp))
        (else
          (error "Unknown expression type -- ANALYZE" exp))))

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

;;;Simple expressions

(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))))

;;;Conditionals and sequences

(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))))

(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

(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))))

(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

(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))))

(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)))

(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))))

;;;amb expressions

(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

(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)
    (let ((input (read)))
      (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))))



;;; Support for Let (as noted in footnote 56, p.428)

(define (let? exp) (tagged-list? exp 'let))
(define (let-bindings exp) (cadr exp))
(define (let-body exp) (cddr exp))

(define (let-var binding) (car binding))
(define (let-val binding) (cadr binding))

(define (make-combination operator operands) (cons operator operands))

(define (let->combination exp)
  ;;make-combination defined in earlier exercise
  (let ((bindings (let-bindings exp)))
    (make-combination (make-lambda (map let-var bindings)
                                   (let-body exp))
                      (map let-val bindings))))



;; A longer list of primitives -- suitable for running everything in 4.3
;; Overrides the list in ch4-mceval.scm
;; Has Not to support Require; various stuff for code in text (including
;;  support for Prime?); integer? and sqrt for exercise code;
;;  eq? for ex. solution

(define primitive-procedures
  (list (list 'car car)
        (list 'cdr cdr)
        (list 'cons cons)
        (list 'null? null?)
        (list 'list list)
        (list 'memq memq)
        (list 'member member)
        (list 'not not)
        (list '+ +)
        (list '- -)
        (list '* *)
        (list '= =)
        (list '> >)
        (list '>= >=)
        (list 'abs abs)
        (list 'remainder remainder)
        (list 'integer? integer?)
        (list 'sqrt sqrt)
        (list 'eq? eq?)
;;      more primitives
        (list 'pair? pair?)
        (list 'append append)
        (list 'even? even?)
        (list 'odd? even?)
        ))


'AMB-EVALUATOR-LOADED