model.rkt

#lang racket
 
; A blog is a (make-blog home posts)
; where home is a string, posts is a (listof post)
(struct blog (home posts) #:mutable #:prefab)
 
; and post is a (make-post blog title body comments)
; where title is a string, body is a string,
; and comments is a (listof string)
(struct post (title body comments) #:mutable #:prefab)
 
; initialize-blog! : path? -> blog
; Reads a blog from a path, if not present, returns default
(define (initialize-blog! home)
  (local [(define (log-missing-exn-handler exn)
            (blog
             (path->string home)
             (list (post "First Post"
                         "This is my first post"
                         (list "First comment!"))
                   (post "Second Post"
                         "This is another post"
                         (list)))))
          (define the-blog
            (with-handlers ([exn? log-missing-exn-handler])
              (with-input-from-file home read)))]
    (set-blog-home! the-blog (path->string home))
    the-blog))
 
; save-blog! : blog -> void
; Saves the contents of a blog to its home
(define (save-blog! a-blog)
  (local [(define (write-to-blog)
            (write a-blog))]
    (with-output-to-file (blog-home a-blog)
      write-to-blog
      #:exists 'replace)))
 
; blog-insert-post!: blog string string -> void
; Consumes a blog and a post, adds the post at the top of the blog.
(define (blog-insert-post! a-blog title body)
  (set-blog-posts!
   a-blog
   (cons (post title body empty) (blog-posts a-blog)))
  (save-blog! a-blog))
 
; post-insert-comment!: blog post string -> void
; Consumes a blog, a post and a comment string.  As a side-efect,
; adds the comment to the bottom of the post's list of comments.
(define (post-insert-comment! a-blog a-post a-comment)
  (set-post-comments!
   a-post
   (append (post-comments a-post) (list a-comment)))
  (save-blog! a-blog))
 
(provide blog? blog-posts
         post? post-title post-body post-comments
         initialize-blog!
         blog-insert-post! post-insert-comment!)