322 lines
11 KiB
Racket
322 lines
11 KiB
Racket
;; Code from the book adapted for racket and my existing support code
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#lang sicp
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(#%require "amb-utilities.rkt"
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"syntax.rkt"
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"environment.rkt"
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"common.rkt"
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"special-forms.rkt"
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"timing.rkt")
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(#%require (only racket current-process-milliseconds))
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(#%require (only racket string-append))
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(#%provide ambeval
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amb-driver-loop
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amb-eval-program)
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(define (amb? exp) (tagged-list? exp 'amb))
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(define (ramb? exp) (tagged-list? exp 'ramb))
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(define (amb-form exp) (car exp)) ; amb or ramb
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(define (amb-choices exp) (cdr exp))
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;; analyze from 4.1.6, with clause from 4.3.3 added
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;; and also support for Let
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(define (analyze exp)
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(cond ((self-evaluating? exp)
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(analyze-self-evaluating exp))
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((quoted? exp) (analyze-quoted exp))
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((variable? exp) (analyze-variable exp))
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((assignment? exp) (analyze-assignment exp))
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((permanent-assignment? exp) (analyze-permanent-assignment exp))
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((definition? exp) (analyze-definition exp))
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((if? exp) (analyze-if exp))
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((if-fail? exp) (analyze-if-fail exp))
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((lambda? exp) (analyze-lambda exp))
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((begin? exp) (analyze-sequence (begin-actions exp)))
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((cond? exp) (analyze (cond->if exp)))
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((let? exp) (analyze (let->combination exp))) ;**
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((amb? exp) (analyze-amb exp)) ;**
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((ramb? exp) (analyze-amb exp))
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((application? exp) (analyze-application exp))
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(else
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(error "Unknown expression type -- ANALYZE" exp))))
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(define (ambeval exp env succeed fail)
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((analyze exp) env succeed fail))
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;;;Simple expressions
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(define (analyze-self-evaluating exp)
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(lambda (env succeed fail)
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(succeed exp fail)))
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(define (analyze-quoted exp)
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(let ((qval (text-of-quotation exp)))
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(lambda (env succeed fail)
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(succeed qval fail))))
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(define (analyze-variable exp)
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(lambda (env succeed fail)
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(succeed (lookup-variable-value exp env)
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fail)))
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(define (analyze-lambda exp)
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(let ((vars (lambda-parameters exp))
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(bproc (analyze-sequence (lambda-body exp))))
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(lambda (env succeed fail)
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(succeed (make-procedure vars bproc env)
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fail))))
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;;;Conditionals and sequences
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(define (analyze-if exp)
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(let ((pproc (analyze (if-predicate exp)))
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(cproc (analyze (if-consequent exp)))
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(aproc (analyze (if-alternative exp))))
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(lambda (env succeed fail)
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(pproc env
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;; success continuation for evaluating the predicate
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;; to obtain pred-value
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(lambda (pred-value fail2)
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(if (true? pred-value)
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(cproc env succeed fail2)
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(aproc env succeed fail2)))
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;; failure continuation for evaluating the predicate
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fail))))
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(define (analyze-sequence exps)
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(define (sequentially a b)
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(lambda (env succeed fail)
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(a env
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;; success continuation for calling a
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(lambda (a-value fail2)
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(b env succeed fail2))
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;; failure continuation for calling a
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fail)))
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(define (loop first-proc rest-procs)
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(if (null? rest-procs)
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first-proc
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(loop (sequentially first-proc (car rest-procs))
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(cdr rest-procs))))
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(let ((procs (map analyze exps)))
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(if (null? procs)
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(error "Empty sequence -- ANALYZE"))
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(loop (car procs) (cdr procs))))
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;;;Definitions and assignments
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(define (analyze-definition exp)
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(let ((var (definition-variable exp))
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(vproc (analyze (definition-value exp))))
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(lambda (env succeed fail)
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(vproc env
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(lambda (val fail2)
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(define-variable! var val env)
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(succeed 'ok fail2))
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fail))))
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(define (analyze-assignment exp)
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(let ((var (assignment-variable exp))
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(vproc (analyze (assignment-value exp))))
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(lambda (env succeed fail)
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(vproc env
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(lambda (val fail2) ; *1*
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(let ((old-value
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(lookup-variable-value var env)))
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(set-variable-value! var val env)
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(succeed 'ok
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(lambda () ; *2*
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(set-variable-value! var
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old-value
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env)
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(fail2)))))
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fail))))
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(define (analyze-permanent-assignment exp)
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(let ((var (assignment-variable exp))
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(vproc (analyze (assignment-value exp))))
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(lambda (env succeed fail)
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(vproc env
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(lambda (val fail2) ; *1*
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(let ((old-value
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(lookup-variable-value var env)))
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(set-variable-value! var val env)
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(succeed 'ok
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fail2)))
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fail))))
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;;;Procedure applications
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(define (analyze-application exp)
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(let ((fproc (analyze (operator exp)))
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(aprocs (map analyze (operands exp))))
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(lambda (env succeed fail)
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(fproc env
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(lambda (proc fail2)
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(get-args aprocs
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env
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(lambda (args fail3)
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(execute-application
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proc args succeed fail3))
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fail2))
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fail))))
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(define (get-args aprocs env succeed fail)
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(if (null? aprocs)
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(succeed '() fail)
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((car aprocs) env
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;; success continuation for this aproc
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(lambda (arg fail2)
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(get-args (cdr aprocs)
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env
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;; success continuation for recursive
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;; call to get-args
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(lambda (args fail3)
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(succeed (cons arg args)
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fail3))
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fail2))
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fail)))
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(define (execute-application proc args succeed fail)
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(cond ((primitive-procedure? proc)
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(succeed (apply-primitive-procedure proc args)
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fail))
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((compound-procedure? proc)
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((procedure-body proc)
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(extend-environment (procedure-parameters proc)
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args
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(procedure-environment proc))
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succeed
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fail))
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(else
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(error
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"Unknown procedure type -- EXECUTE-APPLICATION"
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proc))))
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;;;amb expressions
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;; Make this generic for amb and ramb. For ramb, we pick a random
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;; element rather than car.
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;; If we are in a ramb form, then then next index to select is chosen
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;; randomly. If we are in an amb form, then the index is 0. We
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;; define next-choice and rest-choices in terms of this index.
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(define (remove-element list k)
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(if (= k 0)
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(cdr list)
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(cons (car list)
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(remove-element (cdr list) (- k 1)))))
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(define (analyze-amb exp)
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(let ((form (amb-form exp))
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(cprocs (map analyze (amb-choices exp))))
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(lambda (env succeed fail)
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(define (try-next choices)
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(if (null? choices)
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(fail)
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(let* ((index (if (eq? form 'ramb)
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(random (length choices))
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0))
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(next-choice
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(lambda (l) (list-ref l index)))
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(rest-choices
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(lambda (l) (remove-element l index))))
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((next-choice choices) env
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succeed
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(lambda ()
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(try-next (rest-choices choices)))))))
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(try-next cprocs))))
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;; if-fail is used to catch failure of an expression
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(define (analyze-if-fail exp)
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(let ((tproc (analyze (if-fail-test exp)))
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(cproc (analyze (if-fail-consequent exp))))
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(lambda (env succeed fail)
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;; Use cproc instead of the passed fail procedure
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(tproc env succeed (lambda () (cproc env succeed fail))))))
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;;;Driver loop
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(define input-prompt ";;; Amb-Eval input:")
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(define output-prompt ";;; Amb-Eval value:")
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(define (amb-driver-loop)
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(define env (setup-environment))
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(amb-eval-program-in-env amb-utilities-program env)
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(define start-time 0)
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(define (driver-loop)
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(define (internal-loop try-again)
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(prompt-for-input input-prompt)
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(let ((input (read)))
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(if (eq? input 'try-again)
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(try-again)
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(begin
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(newline)
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(display ";;; Starting a new problem ")
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(newline)
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(set! start-time (current-process-milliseconds))
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(ambeval input
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env
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;; ambeval success
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(lambda (val next-alternative)
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(newline)
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(announce-output output-prompt)
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(display (- (current-process-milliseconds) start-time))
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(newline)
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(user-print val)
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(internal-loop next-alternative))
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;; ambeval failure
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(lambda ()
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(announce-output
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";;; There are no more values of")
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(display (- (current-process-milliseconds) start-time))
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(newline)
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(user-print input)
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(driver-loop)))))))
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(internal-loop
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(lambda ()
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(newline)
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(display ";;; There is no current problem")
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(driver-loop))))
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(driver-loop))
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;;; Support for Let (as noted in footnote 56, p.428)
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(define (let? exp) (tagged-list? exp 'let))
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(define (let-bindings exp) (cadr exp))
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(define (let-body exp) (cddr exp))
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(define (let-var binding) (car binding))
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(define (let-val binding) (cadr binding))
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(define (make-combination operator operands) (cons operator operands))
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(define (let->combination exp)
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;;make-combination defined in earlier exercise
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(let ((bindings (let-bindings exp)))
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(make-combination (make-lambda (map let-var bindings)
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(let-body exp))
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(map let-val bindings))))
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(define (amb-eval-program-in-env program env)
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(eval-program program
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(lambda (exp env)
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(ambeval exp
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env
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(lambda (value fail) value)
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(lambda () 'failed)))
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user-print
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env))
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(define (amb-eval-program program)
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(define env (setup-environment))
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(amb-eval-program-in-env amb-utilities-program env)
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(amb-eval-program-in-env program env))
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'AMB-EVALUATOR-LOADED
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