1 lecture 18 continue evaluator. 2 z9 true#t + twice representing procedures (eval '(define...
Post on 19-Dec-2015
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2
z 9
true #t
+
twice
Representing procedures
(eval '(define twice (lambda (x) (+ x x))) GE)
symbolprimitive
schemeprocedure +
symbolprocedure
symbol+
symbolx
3
Representing the Environment
• (eval ‘(twice 4))• (extend-environment '(x) '(4) GE) GE x: 10
+: (primitive ...)twice: (procedure ..)
E1 x: 4
Abstractly
Concretely
GE
list ofvalues
list ofvariables
frame
x 4
E1
x
10
+ twice primitive
4
Representing the Environment
• (eval ‘(twice 5))• (extend-environment '(x) '(5) GE)
GE
list ofvalues
list ofvariables
frame
x 4
E1
x
10
+ twice primitive
x 5
E2
5
(define (lookup-variable-value var env) (define (env-loop env) (define (scan vars vals) (cond ((null? vars) (env-loop (enclosing-environment env))) ((eq? var (car vars)) (car vals)) (else (scan (cdr vars) (cdr vals))))) (if (eq? env the-empty-environment) (error "Unbound variable" var) (let ((frame (first-frame env))) (scan (frame-variables frame) (frame-values frame))))) (env-loop env))
(define (frame-variables frame) (car frame))(define (frame-values frame) (cdr frame))
(define (enclosing-environment env) (cdr env))(define (first-frame env) (car env))
6
(define (define-variable! var val env) (let ((frame (first-frame env))) (define (scan vars vals) (cond ((null? vars) (add-binding-to-frame! var val frame)) ((eq? var (car vars)) (set-car! vals val)) (else (scan (cdr vars) (cdr vals))))) (scan (frame-variables frame) (frame-values frame))))
(define (eval-definition exp env) (let ((name (cadr exp)) (defined-to-be (eval (caddr exp) env))) (define-variable! name defined-to-be env) ‘undefined))
Assignments
(define (eval-assignment exp env) (set-variable-value! (assignment-variable exp) (eval (assignment-value exp) env) env) 'ok)
(define (assignment? exp) (tagged-list? exp 'set!))
(define (assignment-variable exp) (cadr exp))
(define (assignment-value exp) (caddr exp))
set-variable-value!
(define (set-variable-value! var val env) (define (env-loop env) (define (scan vars vals) (cond ((null? vars) (env-loop (enclosing-environment env))) ((eq? var (car vars)) (set-car! vals val)) (else (scan (cdr vars) (cdr vals)))))
(if (eq? env the-empty-environment) (error "Unbound variable -- SET!" var) (let ((frame (first-frame env))) (scan (frame-variables frame) (frame-values frame))))( (env-loop env))
9
Initialization primitives andinitial env.
(define the-empty-environment '())
(define the-global-environment (setup-environment))
(define (setup-environment) (let ((initial-env (extend-environment (primitive-procedure-names) (primitive-procedure-objects) the-empty-environment))) (define-variable! 'true #t initial-env) (define-variable! 'false #f initial-env) initial-env))
10
(define primitive-procedure (list (list 'car car) (list 'cdr cdr) (list 'cons cons) (list 'null? null?) (list '+ +);; more primitives ))
(define (primitive-procedure-names) (map car primitive-procedures))
(define (primitive-procedure-objects) (map (lambda (proc) (list 'primitive (cadr proc))) primitive-procedures))
11
Read-Eval-Print Loop
(define input-prompt ";;; M-Eval input:")(define output-prompt ";;; M-Eval value:")
(define (prompt-for-input string) (newline) (newline) (display string) (newline))
(define (announce-output string) (newline) (display string) (newline))
(define (driver-loop) (prompt-for-input input-prompt) (let ((input (read))) (let ((output (eval input the-global-env))) (announce-output output-prompt) (user-print output))) (driver-loop))
Apply (reminder)
(define (apply procedure arguments) (cond ((primitive-procedure? procedure) (apply-primitive-procedure procedure arguments)) ((compound-procedure? procedure) (eval-sequence (procedure-body procedure) (extend-environment (procedure-parameters procedure) arguments (procedure-env procedure)))) (else (error "Unknown procedure type -- APPLY" procedure))))
15
Execution Examples
• An iterative algorithm in scheme
(eval '(define odd (lambda (n) (odd (- n 2)))) GE)
• A recursive algorithm in scheme
(eval '(define sum (lambda (n) (+ n (sum (- n 1))))) GE)
• Base case and if check omitted from both algorithmsto simplify the example
18
Summary
• Cycle between eval and apply is the core of the evaluator• eval calls apply with operator and argument values• apply calls eval with expression and environment
• What is still missing from scheme ?• Some special forms• data types other than numbers and booleans
21
Example – Changing Syntax
• Suppose you wanted a "verbose" application syntax:
(CALL <proc> ARGS <arg1> <arg2> ...)
• Changes – only in the syntax routines!
(define (application? exp) (tagged-list? 'CALL))
(define (operator app) (cadr app))
(define (operands app) (cdddr app))
22
Implementing "Syntactic Sugar"
• Idea:• Implement a simple fundamental "core" in the evaluator• Easy way to add alternative/convenient syntax?
• "let" as sugared procedure application:
(let ((<name1> <val1>) (<name2> <val2>)) <body>)
((lambda (<name1> <name2>) <body>) <val1> <val2>)
23
Detect and Transform the Alternative Syntax
(define (eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp) (lookup-variable-value exp env)) ((quoted? exp) (text-of-quotation exp)) . . . ((cond? exp) (eval (cond->if exp) env)) ((let? exp) (eval (let->combination exp) env)) ((application? exp) (apply (eval (operator exp) env)
(list-of-values (operands exp) env))) (else (error "Unknown expression" exp))))
24
Implementing cond: Syntax procedures
(define (cond-clauses exp) (cdr exp))
(define (cond-else-clause? clause) (eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) (car clause))
(define (cond-actions clause) (cdr clause))
(cond ((= x 23) (+ x 1))
(else (- x 1)))
(if (= x 23) (+ x 1) (- x 1))
26
Transforming sequence of expression toan expression
(define (sequence->exp seq) (cond ((null? seq) seq) ((last-exp? seq) (first-exp seq)) (else (make-begin seq))))
(define (make-begin seq) (cons 'begin seq))
2x*
car y
begin
28
Implementing cond
(define (cond->if exp) (expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses) (if (null? clauses) 'false ; no else clause (let ((first (car clauses)) (rest (cdr clauses))) (if (cond-else-clause? first) (if (null? rest) (sequence->exp (cond-actions first)) (error "ELSE clause isn't last -- COND->IF" clauses)) (make-if (cond-predicate first) (sequence->exp (cond-actions first)) (expand-clauses rest))))))
29
Details of cond syntax transformation
(cond ((= x 23) (+ x 1))
(else (- x 1)))
cond
else
23x=
1x+
1x-
35
Named Procedures
Support (define (foo <parm>) <body>)
(define (eval-definition exp env) (define-variable! (definition-variable exp) (eval (definition-value exp) env) env))
(define (definition-variable exp) (if (symbol? (cadr exp)) (cadr exp) (caadr exp)))
(define (definition-value exp) (if (symbol? (cadr exp)) (caddr exp) (make-lambda (cdadr exp) ;formal params
(cddr exp)))) ;body