SICP 問題 4.22

letを使えるようにする．

(define (analyze exp)
  (cond ((self-evaluating? exp) (analyze-self-evaluating exp))
        ((quoted? exp) (analyze-quoted exp))
        ((variable? exp) (analyze-variable exp))
        ((assignment? exp) (analyze-assignment exp))
        ((definition? exp) (analyze-definition exp))
        ((if? exp) (analyze-if exp))
        ((lambda? exp) (analyze-lambda exp))
        ((let? exp) (analyze (let->combination exp)))
        ((begin? exp) (analyze-sequence (begin-actions exp)))
        ((cond? exp) (analyze (cond->if exp)))
        ((application? exp) (analyze-application exp))
        (else (error "Unknown expression type: ANALYZE" exp))))

(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->combination exp)
  (if (symbol? (cadr exp)) ;; 2番目の要素がシンボルならnamed-let
      (named-let->define (named-let-func-name exp)
                         (named-let-variables exp)
                         (named-let-expressions exp)
                         (named-let-bodys exp))
      (cons (make-lambda (let-variables exp)
                     (let-bodys exp))
        (let-expressions exp))))

;; make-procedureのbodyのところについていたscan-out-definesのせいでエラー．
;; make-proceduren渡される時点でbodyはanalyzeした後のclosureになっているので
;; analyze-lambdaの中で内部定義をletに吐き出す．
(define (make-procedure parameters body env)
  (list 'procedure parameters body env))

(define (analyze-lambda exp)
  (let ((vars (lambda-parameters exp))
        (bproc (analyze-sequence (scan-out-defines (lambda-body exp)))))
    (lambda (env)
      (make-procedure vars bproc env))))