SICP 問題 4.20
;; a ;; letrecをlet式に変換すし,導出された式として実装する. ;; eval (define (eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp) (lookup-variable-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*? exp) (eval (let*->nested-lets exp) env)) ((letrec? exp) (eval (letrec->let exp) env)) ;;letrecを追加 ((begin? exp) (eval-sequence (begin-actions exp) env)) ((cond? exp) (eval (cond->if exp) env)) ((and? exp) (eval-and exp env)) ((or? exp) (eval-or exp env)) ((application? exp) (my-apply (eval (operator exp) env) (list-of-values (operands exp) env))) (else (error "Unknown expression type --EVAL" exp)))) ;; 選択子 (define (letrec? exp) (tagged-list? exp 'letrec)) (define (letrec-parameters exp) (cadr exp)) (define (letrec-variables exp) (map car (letrec-parameters exp))) (define (letrec-expressions exp) (map cadr (letrec-parameters exp))) (define (letrec-body exp) (cddr exp)) (define (letrec->let exp) (make-let (map (lambda (x) (list x ''*unassigned*)) (letrec-variables exp)) (append (map (lambda (x y) (list 'set! x y)) (letrec-variables exp) (letrec-expressions exp)) (letrec-body exp))))
gosh> (driver-loop) ;;; M-Eval input: (letrec ((fact (lambda (n) (if (= n 1) 1 (* n (fact (- n 1))))))) (fact 10)) ;;; M-Eval value: 3628800
;; b ;; 環境図はパス ;; Louiの言うことを素直に書いてみると以下の通りになる. (define (f x) (let ((even? (lambda (n) (if (= n 0) true (odd? (- n 1))))) (odd? (lambda (n) (if (= n 0) false (even? (- n 1)))))) body ...)) ;; これだと相互再帰部分でeven?を評価するときにはまだodd?が評価されていないためエラーになる.