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- #lang racket
- (require racket/mpair)
- (provide (all-defined-out))
- ;; This is the repl program for exercise 4.31 from section 4.2.2. It is the same as
- ;; the repl program from section 4.1.4, only modified to allow for three different
- ;; evaluation strategies for procedure arguments: eager evaluation by default, lazy
- ;; evaluation without memoization using the lazy keyword, or lazy evaluation with
- ;; memoization using the lazy-memo keyword. There are tests at the end of the file.
- ;; PROGRAM SECTIONS
- ;; 1. my-eval and my-apply
- ;; 2. eval procedures
- ;; 3. self-evaluating expressions, variables, and quotations
- ;; 4. definition and assignment
- ;; 5. thunks, lambdas, procedures and applications
- ;; 6. sequences and begin expressions
- ;; 7. boolean expressions
- ;; 8. if and cond expressions
- ;; 9. let, let*, and named-let
- ;; 10. environment and frames
- ;; 11. primitive procedures and the global environment
- ;; 12. repl operations
- ;; 13. tests
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 1. MY-EVAL AND MY-APPLY ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define (my-eval expr env)
- (cond [(self-evaluating? expr) expr]
- [(variable? expr) (lookup-variable-value expr env)]
- [(quoted? expr) (text-of-quotation expr)]
- [(definition? expr) (eval-definition expr env)]
- [(assignment? expr) (eval-assignment expr env)]
- [(if? expr) (eval-if expr env)]
- [(cond? expr) (my-eval (cond->if expr) env)]
- [(begin? expr) (eval-sequence (begin-actions expr) env)]
- [(and? expr) (eval-and expr env)]
- [(or? expr) (eval-or expr env)]
- [(let? expr) (my-eval (let->combination expr) env)]
- [(let*? expr) (my-eval (let*->nested-lets expr) env)]
- [(named-let? expr) (my-eval (named-let->sequence expr) env)]
- [(lambda? expr)
- (make-procedure (lambda-parameters expr)
- (lambda-body expr)
- env)]
- [(application? expr)
- (my-apply (actual-value (operator expr) env)
- (operands expr)
- env)]
- [else (error "Unknown expression type -- MY-EVAL" expr)]))
- (define (my-apply procedure arguments env)
- (cond [(primitive-procedure? procedure)
- (apply-primitive-procedure
- procedure
- (list-of-arg-values arguments env))]
- [(compound-procedure? procedure)
- (eval-sequence
- (procedure-body procedure)
- (extend-environment
- (procedure-parameters procedure)
- (list-of-args arguments
- (procedure-types procedure)
- env)
- (procedure-environment procedure)))]
- [else (error "Unknown procedure type -- APPLY" procedure)]))
- ;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 2. EVAL PROCEDURES ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;
- (define (eval-definition expr env)
- (define-variable! (definition-variable expr)
- (my-eval (definition-value expr) env)
- env)
- 'ok)
- (define (eval-assignment expr env)
- (set-variable-value! (assignment-variable expr)
- (my-eval (assignment-value expr) env)
- env)
- 'ok)
- (define (list-of-values exprs env)
- (if (no-operands? exprs)
- empty
- (cons (my-eval (first-operand exprs) env)
- (list-of-values (rest-operands exprs) env))))
- (define (eval-sequence exprs env)
- (cond [(last-exp? exprs) (my-eval (first-exp exprs) env)]
- [else (my-eval (first-exp exprs) env)
- (eval-sequence (rest-exps exprs) env)]))
- (define (eval-if expr env)
- (if (true? (actual-value (if-predicate expr) env))
- (my-eval (if-consequent expr) env)
- (my-eval (if-alternative expr) env)))
- (define (eval-and expr env)
- (define ops (and-operands expr))
- (cond [(empty? ops) 'true] ; (and) should be true
- [else
- (define val (my-eval (first ops) env))
- (define rest-ops (rest ops))
- (cond [(empty? rest-ops) val] ; (and x) should be x
- [(false? val) false]
- [else
- (eval-and (make-and rest-ops) env)])]))
- (define (eval-or expr env)
- (define ops (or-operands expr))
- (cond [(empty? ops) 'false] ; (or) should be false
- [else
- (define val (my-eval (first ops) env))
- (define rest-ops (rest ops))
- (cond [(empty? rest-ops) val] ; (or x) should be x
- [(not (false? val)) val]
- [else
- (eval-or (make-or rest-ops) env)])]))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 3. SELF-EVALUATING EXPRESSIONS, VARIABLES, AND QUOTATIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define (tagged-list? expr tag)
- (or (and (pair? expr) (eq? (first expr) tag))
- (and (mpair? expr) (eq? (mcar expr) tag))))
- ;; Only numbers and strings are self-evaluating.
- (define (self-evaluating? expr)
- (cond [(number? expr) true]
- [(string? expr) true]
- [else false]))
- (define (variable? expr)
- (symbol? expr))
- ;; Quotations have the form: (quote <text-of-quotation>)
- (define (quoted? expr)
- (tagged-list? expr 'quote))
- (define (text-of-quotation expr)
- (second expr))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 4. DEFINITION AND ASSIGNMENT ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; Variable definitions have the form: (define <var> <value>)
- ;; Procedure definitions have the form:
- ;; (define (<var> <parameter-1> ... <parameter-n>) <body>)
- ;; which is equivalent to:
- ;; (define <var> (lambda (<parameter-1> ... <parameter-n>) <body>))
- (define (definition? expr)
- (tagged-list? expr 'define))
- (define (procedure-definition? expr)
- (and (definition? expr)
- (pair? (second expr))))
- (define (definition-variable expr)
- (if (procedure-definition? expr)
- (first (second expr))
- (second expr)))
- (define (definition-parameters expr)
- (rest (second expr)))
- (define (definition-value expr)
- (if (procedure-definition? expr)
- (make-lambda (definition-parameters expr)
- (drop expr 2))
- (third expr)))
- ;; All lambda parameters are a two-element list now: the first element is the
- ;; parameter name, and the second element is the parameter type, either 'eager,
- ;; 'lazy, or 'lazy-memo.
- (define (make-all-typed params)
- (map (lambda (p)
- (if (symbol? p)
- (list p 'eager)
- p))
- params))
- ;; Assignments have the form: (set! <var> <value>)
- (define (assignment? expr)
- (tagged-list? expr 'set!))
- (define (assignment-variable expr) (second expr))
- (define (assignment-value expr) (third expr))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 5. THUNKS, LAMBDAS, PROCEDURES AND APPLICATIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define (thunk-memo? obj) (tagged-list? obj 'thunk-memo))
- (define (thunk-no-memo? obj) (tagged-list? obj 'thunk-no-memo))
- (define (thunk-exp my-thunk) (mcar (mcdr my-thunk)))
- (define (thunk-env my-thunk) (mcar (mcdr (mcdr my-thunk))))
- (define (delay-it-memo expr env)
- (mlist 'thunk-memo expr env))
- (define (delay-it-no-memo expr env)
- (mlist 'thunk-no-memo expr env))
- (define (evaluated-thunk? obj) (tagged-list? obj 'evaluated-thunk))
- (define (thunk-value evaluated-thunk) (mcar (mcdr evaluated-thunk)))
- (define (force-it obj) ; changed
- (cond [(thunk-memo? obj)
- (define result (actual-value (thunk-exp obj) (thunk-env obj)))
- (set-mcar! obj 'evaluated-thunk)
- (set-mcar! (mcdr obj) result)
- (set-mcdr! (mcdr obj) empty)
- result]
- [(evaluated-thunk? obj) (thunk-value obj)]
- [(thunk-no-memo? obj)
- (actual-value (thunk-exp obj) (thunk-env obj))]
- [else obj]))
- ;; Lambda expressions have the form:
- ;; (lambda ((<var> <type>) ... ) <body>)
- (define (lambda? expr) (tagged-list? expr 'lambda))
- (define (lambda-parameters expr) (second expr))
- (define (lambda-body expr) (drop expr 2))
- (define (make-lambda parameters body)
- (cons 'lambda (cons (make-all-typed parameters) body))) ; changed
- ;; Procedures:
- (define (make-procedure parameters body env)
- (list 'procedure parameters body env))
- (define (compound-procedure? p)
- (tagged-list? p 'procedure))
- (define (procedure-parameters p) (map first (second p)))
- (define (procedure-types p) (map second (second p)))
- (define (procedure-body p) (third p))
- (define (procedure-environment p) (fourth p))
- ;; Procedure applications have the from:
- ;; (<var> <parameter> ...)
- (define (application? expr) (pair? expr))
- (define (operator expr) (first expr))
- (define (operands expr) (rest expr))
- (define (no-operands? ops) (empty? ops))
- (define (first-operand ops) (first ops))
- (define (rest-operands ops) (rest ops))
- (define (actual-value expr env)
- (force-it (my-eval expr env)))
- (define (list-of-arg-values exprs env)
- (if (no-operands? exprs)
- empty
- (cons (actual-value (first-operand exprs) env)
- (list-of-arg-values (rest-operands exprs)
- env))))
- (define (list-of-args exprs types env)
- (cond [(no-operands? exprs)
- empty]
- [else
- (define e (first-operand exprs))
- (define t (first types))
- (define arg (cond [(eq? t 'eager) (actual-value e env)]
- [(eq? t 'lazy) (delay-it-no-memo e env)]
- [(eq? t 'lazy-memo) (delay-it-memo e env)]
- [else (error "Unknown type -- LIST-OF-ARGS" t)]))
- (cons arg (list-of-args (rest-operands exprs)
- (rest types)
- env))]))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 6. SEQUENCES AND BEGIN EXPRESSIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; Begin has the form: (begin <actions>)
- (define (begin? expr) (tagged-list? expr 'begin))
- (define (begin-actions expr) (rest expr))
- (define (last-exp? seq) (empty? (rest seq)))
- (define (first-exp seq) (first seq))
- (define (rest-exps seq) (rest seq))
- (define (sequence->exp seq)
- (cond [(empty? seq) seq]
- [(last-exp? seq) (first-exp seq)]
- [else (make-begin seq)]))
- ;; begin constructor used by cond->if
- (define (make-begin seq) (cons 'begin seq))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 7. BOOLEAN EXPRESSIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define (true? x) (not (false? x)))
- (define (and? expr) (tagged-list? expr 'and))
- (define (and-operands expr) (rest expr))
- (define (make-and sequence) (cons 'and sequence))
- (define (or? expr) (tagged-list? expr 'or))
- (define (or-operands expr) (rest expr))
- (define (make-or sequence) (cons 'or sequence))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 8. IF AND COND EXPRESSIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; Conditionals have the form: (if <predicate> <consequent> <alternative>)
- ;; If no alternative, use false.
- (define (if? expr) (tagged-list? expr 'if))
- (define (if-predicate expr) (second expr))
- (define (if-consequent expr) (third expr))
- (define (if-alternative expr)
- (if (not (empty? (drop expr 3)))
- (fourth expr)
- 'false))
- (define (make-if predicate consequent alternative)
- (list 'if predicate consequent alternative))
- ;; Cond has the form:
- ;; (cond ((<predicate> <actions>)
- ;; (else <actions>))) ; if no else, assume (else false) clause
- (define (cond? expr) (tagged-list? expr 'cond))
- (define (cond-clauses expr) (rest expr))
- ;; regular cond clause
- (define (cond-predicate clause) (first clause))
- (define (cond-actions clause) (rest clause))
- ;; alternate test clause
- (define (cond-alternate-clause? clause)
- (eq? (second clause) '=>))
- (define (cond-test clause) (first clause))
- (define (cond-recipient clause) (third clause))
- ;; else clause
- (define (cond-else-clause? clause)
- (eq? (cond-predicate clause) 'else))
- ;; derive cond from if
- (define (cond->if expr)
- (expand-clauses (cond-clauses expr)))
- (define (expand-clauses clauses)
- (cond [(empty? clauses) 'false] ; no else clause
- [else
- (define first-clause (first clauses))
- (define rest-clauses (rest clauses))
- (cond [(cond-else-clause? first-clause)
- (if (empty? rest-clauses)
- (sequence->exp (cond-actions first-clause))
- (error "ELSE clause isn't last -- COND->IF" clauses))]
- [(cond-alternate-clause? first-clause)
- (define test (cond-test first-clause)) ; gets evaluated twice
- (make-if test
- (list (cond-recipient first-clause)
- test)
- (expand-clauses rest-clauses))]
- [else
- (make-if (cond-predicate first-clause)
- (sequence->exp (cond-actions first-clause))
- (expand-clauses rest-clauses))])]))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 9. LET, LET*, AND NAMED-LET ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; Let has the form: (let (<bindings>) <body>)
- (define (let? expr)
- (and (tagged-list? expr 'let)
- (list? (second expr))))
- (define (let-bindings expr) (second expr))
- (define (let-parameters expr) (map first (let-bindings expr)))
- (define (let-expressions expr) (map second (let-bindings expr)))
- (define (let-body expr) (drop expr 2))
- (define (let->combination expr)
- (cond [(empty? (let-bindings expr)) ; do not unnecessarily lambda wrap
- (sequence->exp (let-body expr))]
- [else
- (cons (make-lambda (let-parameters expr)
- (let-body expr))
- (let-expressions expr))]))
- (define (make-let bindings body)
- (cons 'let (cons bindings body)))
- ;; Let* has the same form as let, but bindings are sequential.
- (define (let*? expr) (tagged-list? expr 'let*))
- (define (let*-bindings expr) (second expr))
- (define (let*-body expr) (drop expr 2))
- (define (make-let* bindings body)
- (cons 'let* (cons bindings body)))
- (define (let*->nested-lets expr)
- (define bindings (let*-bindings expr))
- (cond [(empty? bindings)
- (sequence->exp (let-body expr))]
- [else
- (list 'let
- (list (first bindings))
- (let*->nested-lets
- (make-let* (rest bindings)
- (let-body expr))))]))
- ;; Named-let has the form: (let <name> (<bindings>) <body>)
- (define (named-let? expr)
- (and (tagged-list? expr 'let)
- (not (list? (second expr)))))
- (define (named-let-name expr) (second expr))
- (define (named-let-bindings expr) (third expr))
- (define (named-let-parameters expr) (map first (named-let-bindings expr)))
- (define (named-let-expressions expr) (map second (named-let-bindings expr)))
- (define (named-let-body expr) (drop expr 3))
- (define (named-let->sequence expr)
- (define bindings (named-let-bindings expr))
- (cond [(empty? bindings)
- (sequence->exp (named-let-body expr))]
- [else
- (list 'begin
- (cons 'define ; first define the named function
- (cons (cons (named-let-name expr)
- (named-let-parameters expr))
- (named-let-body expr)))
- (cons (named-let-name expr) ; then apply it to the expressions
- (named-let-expressions expr)))]))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 10. ENVIRONMENT AND FRAMES ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; An environment is a mutable list of frames. The enclosing environment is the mcdr
- ;; of the list. A frame is a mutable list of bindings with a 'frame header. A
- ;; binding is a var-val pair ie (mcons var val).
- ;; Environments support four procedures:
- ;; lookup-variable-value
- ;; extend-environment
- ;; define-variable
- ;; set-variable-value
- ;; Two helper functions support the environment procedures:
- ;; find-binding-in-frame
- ;; find-binding-in-environment
- (define (enclosing-environment env) (mcdr env))
- (define (first-frame env) (mcar env))
- (define the-empty-environment empty)
- (define the-empty-frame (mlist 'frame))
- (define (empty-frame? frame)
- (empty? (frame-bindings frame)))
- (define (make-frame vars vals)
- (mcons 'frame
- (mmap mcons
- (list->mlist vars)
- (list->mlist vals))))
- (define (frame-bindings frame) (mcdr frame))
- (define (frame-variables frame) (mmap mcar (frame-bindings frame)))
- (define (frame-values frame) (mmap mcdr (frame-bindings frame)))
- (define (binding-variable binding) (mcar binding))
- (define (binding-value binding) (mcdr binding))
- (define (set-value! binding val) (set-mcdr! binding val))
- (define (add-binding-to-frame! var val frame)
- (mappend! frame (mlist (mcons var val))))
- (define (find-binding-in-frame var frame)
- ; Return the var-val pair if present else false.
- (define (loop bindings)
- (cond [(empty? bindings) false]
- [else
- (define b (mcar bindings))
- (if (eq? var (binding-variable b))
- b
- (loop (mcdr bindings)))]))
- (loop (frame-bindings frame)))
- (define (find-binding-in-env var env)
- ; Return the closest binding for var if present else false.
- (cond [(eq? env the-empty-environment) false]
- [else
- (define b (find-binding-in-frame var (first-frame env)))
- (or b (find-binding-in-env var (enclosing-environment env)))]))
- (define (lookup-variable-value var env)
- (define b (find-binding-in-env var env))
- (if b
- (binding-value b)
- (error "Unbound variable" var)))
- (define (extend-environment vars vals base-env)
- (cond [(= (length vars) (length vals))
- (mcons (make-frame vars vals) base-env)]
- [else
- (if (< (length vars) (length vals))
- (error "Too many arguments supplied" vars vals)
- (error "Too few arguments supplied" vars vals))]))
- (define (define-variable! var val env)
- (define frame (first-frame env))
- (define b (find-binding-in-frame var frame))
- (if b
- (set-value! b val)
- (add-binding-to-frame! var val frame)))
- (define (set-variable-value! var val env)
- (define b (find-binding-in-env var env))
- (if b
- (set-value! b val)
- (error "Unbound variable -- SET!" var)))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 11. PRIMITIVE PROCEDURES AND THE GLOBAL ENVIRONMENT ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define primitive-procedures
- (list
- (list 'cons cons)
- (list 'car car)
- (list 'cdr cdr)
- (list '+ +)
- (list '* *)
- (list '- -)
- (list '< <)
- (list '> >)
- (list '<= <=)
- (list '>= >=)
- (list '= =)
- (list 'not not)
- (list 'false? false?)
- (list 'true? (lambda (x) (not (false? x))))
- (list 'empty? empty?)
- (list 'displayln displayln)
- (list 'list list)
- ))
- (define (primitive-procedure? proc)
- (tagged-list? proc 'primitive))
- (define (primitive-implementation proc)
- (second proc))
- (define (primitive-procedure-names)
- (map first primitive-procedures))
- (define (primitive-procedure-objects)
- (map (lambda (proc) (list 'primitive (second proc)))
- primitive-procedures))
- (define apply-in-underlying-scheme apply)
- ;; The metacircular evaluator's apply is my-apply.
- (define (apply-primitive-procedure proc args)
- (apply-in-underlying-scheme
- (primitive-implementation proc) args))
- (define (setup-environment)
- (define initial-env
- (extend-environment (primitive-procedure-names)
- (primitive-procedure-objects)
- the-empty-environment))
- (define-variable! 'true true initial-env)
- (define-variable! 'false false initial-env)
- initial-env)
- (define the-global-environment (setup-environment))
- ;;;;;;;;;;;;;;;;;;;;;;;;;;
- ;; 12. REPL OPERATIONS ;;
- ;;;;;;;;;;;;;;;;;;;;;;;;;;
- (define input-prompt ";;; M-Eval input:")
- (define output-prompt ";;; M-Eval value:")
- (define (driver-loop)
- (prompt-for-input input-prompt)
- (let* ([input (read)]
- [output (actual-value input the-global-environment)])
- (announce-output output-prompt)
- (user-print output))
- (driver-loop))
- (define prompt-for-input displayln)
- (define announce-output displayln)
- (define (user-print object)
- (if (compound-procedure? object)
- (displayln (list 'compound-procedure
- (procedure-parameters object)
- (procedure-body object)
- '<procedure-env>))
- (displayln object)))
- ;;;;;;;;;;;;;;;
- ;; 13. TESTS ;;
- ;;;;;;;;;;;;;;;
- ;; (my-eval '(define (try a (b lazy)) (if (= a 0) 1 b)) the-global-environment)
- ;; ;; 'ok
- ;; (my-eval '(try 0 (/ 1 0)) the-global-environment)
- ;; ;; 1
- ;; ;; the book example with count and (id (id 10)) with lazy-memo:
- ;; (my-eval '(define count 0) the-global-environment)
- ;; ;; 'ok
- ;; (my-eval '(define (id (x lazy-memo)) (set! count (+ count 1)) x)
- ;; the-global-environment)
- ;; ;; 'ok
- ;; (my-eval 'count the-global-environment) ; should be 0
- ;; ;; 0
- ;; (my-eval '(define w (id (id 10))) the-global-environment)
- ;; ;; 'ok
- ;; (my-eval 'count the-global-environment) ; should be 1
- ;; ;; 1
- ;; (actual-value 'w the-global-environment) ; should be 10
- ;; ;; 10
- ;; (my-eval 'count the-global-environment) ; should be 2
- ;; ;; 2
- ;; (actual-value 'w the-global-environment)
- ;; ;; 10
- ;; (my-eval 'count the-global-environment) ; should still be 2
- ;; ;; 2
- ;; ;; the book example with count and (id (id 10)) with lazy, no memo:
- ;; (my-eval '(define count2 0) the-global-environment)
- ;; ;; 'ok
- ;; (my-eval '(define (id2 (x lazy)) (set! count2 (+ count2 1)) x) ; no memo
- ;; the-global-environment)
- ;; ;; 'ok
- ;; (my-eval 'count2 the-global-environment) ; should be 0
- ;; ;; 0
- ;; (my-eval '(define w (id2 (id2 10))) the-global-environment)
- ;; ;; 'ok
- ;; (my-eval 'count2 the-global-environment) ; should be 1
- ;; ;; 1
- ;; (actual-value 'w the-global-environment) ; should be 10
- ;; ;; 10
- ;; (my-eval 'count2 the-global-environment) ; should be 2
- ;; ;; 2
- ;; (actual-value 'w the-global-environment)
- ;; ;; 10
- ;; (my-eval 'count2 the-global-environment) ; should be 3
- ;; ;; 3
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