SICP 問題 4.06
letを導入.
lambdaに変形することで定義する.
;; (let ((a 1) (b 2) (c 3)) ;; (+ a b c)) ;; ((lambda (a b c) ;; (+ a b c)) 1 2 3) (define (let? exp) (tagged-list? exp 'let)) (define (let-parameters exp) (cadr exp)) (define (let-variables exp) (map car (let-parameters exp))) (define (let-expressions exp) (map cadr (let-parameters exp))) (define (let-bodys exp) (cddr exp)) (define (let->lambda exp) (cons (make-lambda (let-variables exp) (let-bodys exp)) (let-expressions exp)))
gosh> (let->lambda '(let ((var1 exp1) (var2 exp2) (var3 exp3)) body1 body2)) ((lambda (var1 var2 var3) body1 body2) exp1 exp2 exp3)
(define (eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp) (lookup-valiable-value exp env)) ((quoted? exp) (text-of-quotation exp)) ((assignment? exp) (eval-assignment exp env)) ((definition? exp) (eval-definition exp env)) ((if? exp) (eval-if exp env)) ((lambda? exp) (make-procedure (lambda-parameters exp) (lambda-body exp) env)) ((let? exp) (eval (let->lambda exp) env)) ;;letを追加 ((begin? exp) (eval-sequence (begin-actions exp) env)) ((cond? exp) (eval (cond->if exp) env)) ((and? exp) (eval (and->if exp) env)) ((or? exp) (eval (or->if exp) env)) ((application? exp) (apply (eval (operator exp) env) (list-of-values (operands exp) env))) (else (error "Unknown expression type: EVAL" exp))))