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(wat-aro)

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SICP 問題 4.02

scheme SICP
;; 本文のeval
(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))
        ((begin? exp)
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type: EVAL" exp))))

;; evalのcondの順番を変えてapplyをassignmentより前にしようとしている
;; a
(define x 3)
;; (application? exp)は(pair? exp)なので(define x 3)もapplyされる.
;; applyの節はすべてのリストにマッチするので最後でないといけない.

;; b
;; 手続き適用がcallで始まるように構文を変える.
;; Louisの希望通り,assignmentよりも前にapplyをもってくる.
;; 後はselectorとapplication?を変更すればよい.
(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-valiable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((application? exp)
         (apply (eval (operator exp) env)
                (list-of-values (operands exp) env)))
        ((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))
        ((begin? exp)
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        (else
         (error "Unknown expression type: EVAL" exp))))

(define (application? exp) (tagged-list? exp 'call))
(define (operator exp) (cadr exp))
(define (operands exp) (csdr exp))