(define (eval exp env) ; ...... ((let? exp) (eval (let->combination exp) env)) ; ...... ) (define (let? exp) (tagged-list? exp 'let)) (define (let-assignment exp) (cadr exp)) (define (let-body exp) (cddr exp)) (define (let-exp assignment) (if (null? assignment) '() (cons (cadr (car assignment)) (let-exp (cdr assignment))))) (define (let-var assignment) (if (null? assignment) '() (cons (car (car assignment)) (let-var (cdr assignment))))) (define (let->combination exp) (transform-let (let-assignment exp) (let-body exp))) (define (transform-let assignment body) (cons (make-lambda (let-var assignment) body) (let-exp assignment)))
Weiqun Zhang's Blog
Programming and random stuff
-
Recent Comments
zhilongji on SICP Exercise 4.9 hunglun on SICP Solutions Curly Brace on SICP Exercise 4.8 leon on SICP Exercise 4.6 prity on SICP Exercise 2.55
[...] also SICP Exercise 4.6 Subscribe in a [...]
[...] have implemented let in SICP Exercise 4.6. In this exercise, let’s change the syntax of let [...]
[...] SICP Exercise 4.6 for the definitions of let-assignment, let-body, let-var and [...]
[...] SICP Exercise 4.6 for procedures let? and let->combination. Subscribe in a [...]
let-exp y let-var could’ve been implemented in terms of map (using car and cadr as its arguments procedure respectively).
I think
(define (let-body exp) (cddr exp))
should be
(define (let-body exp) (caddr exp))
;if allowing using higher-order procedures, the solution can be more succinct:
(define (let-clauses exp) (cdr exp))
(define (let-parameters exp) (cadr exp))
(define (let-body exp) (cddr exp))
(define (let->combination exp)
(cons
(cons ‘lambda
(cons (map car (let-parameters exp))
(let-body exp)))
(map cadr (let-parameters exp))))