4-8.scm

;; named let
(define (named-let-package-install)
  (define (let-name let)
    (if (symbol? (car let))
        (car let)
        false))
  (define (let-body let)
    (if (symbol? (car let))
        (caddr let)
        (cadr let)))
  (define (let-bingdings let)
    (if (symbol? (car let))
        (cadr let)
        (car let)))
  (define (let-variables bindings)
    (if (null? bindings)
        '()
        (cons (car (car bindings))
              (let-variables (cdr bindings)))))
  (define (let-expressions bindings)
    (if (null? bindings)
        '()
        (cons (cadr (car bindings))
              (let-expressions (cdr bindings)))))
  (define (let->combination let)
    (let ((bindings (let-bindings let))
          (name (let-name let)))
      (if name
          (make-begin
           (list (make-definition name
                                  (make-lambda (let-variables bindings)
                                               (let-body let)))
                 (make-call name (let-expressions bindings))))
          (make-call (make-lambda (let-variables paras)
                                  (let-body let))
                     (let-expressions paras)))))
  (define (eval-self let env)
    (eval (let->combination let) env))

  (define (tag x) (attach-tag 'let x))
  (put 'let 'eval eval-self)
  (put 'let 'make tag))