Continue: Web Applications in Racket

How do we make dynamic web applications? In this tutorial, we show how to use Racket to achieve this goal. We explain how to start up a web server, how to generate dynamic web content, and how to interact with the user. Our working example will be a simple web journal—a “blog.”

This tutorial is intended for students who have read enough of How to Design Programs to know how to use structures, higher-order functions, the local syntax, and a little bit of mutation.

1 Getting Started

Everything you need in this tutorial is provided in Racket; we will be using the DrRacket module language.

Enter the following into DrRacket’s Definitions window, then press the Run button.

#lang web-server/insta

(define (start request)

(response/xexpr

'(html

(head (title "My Blog"))

(body (h1 "Under construction")))))

If a web browser comes up with an “Under Construction” page, then clap your hands with delight, because you’ve built your first web application! We haven’t yet gotten it to do much, but we’ll get there. For now, press the Stop button to shut the server down.

2 The Application

We want to motivate this tutorial by showing you how to develop a blog. Users of the blog should be able to create new posts and add comments to existing posts. We’ll approach the task iteratively, pointing out one or two pitfalls along the way. The game plan will be approximately as follows:

• Show a static list of posts.

• Allow a user to add new posts to the system.

• Extend the model to let a user add comments to a post.

• Allow all users to share the same set of posts.

• Serialize our data structures to disk.

By the end of the tutorial, we’ll have a simple blogging application up and running.

3 Basic Blog

We begin by defining the necessary data structures. A post is:

(struct post (title body))

struct (struct post (title body))

title : string?

body : string?

Exercise. Make a few examples of posts.

Next we define a blog to be simply a list of posts:

value blog : (listof post?)

Here, then, is a very simple example of a blog:

(define BLOG (list (post "First Post!"

"Hey, this is my first post!")))

Now let’s get our web application to show it.

4 Rendering HTML

When a web browser visits our blog’s URL, the browser constructs a request structure and sends it across the network to our application. We need a function, which we’ll call start, to consume such requests and produce responses to them. One basic kind of response is to show an HTML page; this is done by the function response/xexpr, which takes an X-expression representing the desired HTML. An X-expression is defined as

(define xexpr/c

(flat-rec-contract

xexpr

(or/c string?

(cons/c symbol? (listof xexpr))

(cons/c symbol?

(cons/c (listof (list/c symbol? string?))

(listof xexpr))))))

and the following examples illustrate how natural it is to use X-expressions to represent HTML.

The first alternative in xexpr/c is string?. For example, the HTML hello is represented as "hello". To guarantee valid HTML, strings are automatically escaped when output. Thus, the X-expression "<b>Unfinished tag" is rendered as the HTML &lt;b&gt;Unfinished tag, and not as <b>Unfinished tag. Similarly, "<i>Finished\ntag</i>" is rendered as &lt;i&gt;Finished tag&lt;/i&gt;, and not as <i>Finished tag</i>.

The second alternative in xexpr/c is the recursive contract (cons/c symbol? (listof xexpr)). For example, the HTML <p>This is an example</p> is represented by the X-expression

'(p "This is an example").

And finally, the third alternative in xexpr/c allows for parameters in HTML tags. As examples, <a href="link.html">Past</a> is represented by

'(a ((href "link.html")) "Past")

and <p>This is <div class="emph">another</div> example.</p> is represented by

'(p "This is " (div ((class "emph")) "another") " example.").

We could also have produced these X-expressions “manually,” using cons and list, but that can get notationally heavy. For example, the following Racket expressions both evaluate to the same X-expression:

(list 'html (list 'head (list 'title "Some title"))

(list 'body (list 'p "This is a simple static page.")))

'(html (head (title "Some title"))

(body (p "This is a simple static page.")))

But the latter is much easier to read and type, because it uses a leading forward quote mark to express the list structure concisely. This is how to construct static html responses with aplomb! (For more on the extended list abbreviation form, see Section 13 of How to Design Programs.)

It turns out, however, that this simple kind of list abbreviation cannot produce web content that is dynamic. For if we try to inject expressions into an X-expression constructed by simple list abbreviation, those expressions will be treated as part of the list structure, literally! What we need instead is a notation that gives us the convenience of quoted list abbreviations, but that also allows us to treat portions of the list structure as normal expressions. That is, we would like to define a template whose placeholders can be expressed easily and filled in dynamically.

Racket provides this templating functionality, in the form of a notation called quasiquote. In quasiquotation a list is abbreviated not with a leading forward quote but with a leading back quote. If we wish any subexpression of this backquoted list to be evaluated normally (“unquoted”), then all we have to do is place a comma in front that subexpression. For example:

; render-greeting: string -> response

; Consumes a name, and produces a dynamic response.

(define (render-greeting a-name)

(response/xexpr

`(html (head (title "Welcome"))

(body (p ,(string-append "Hello " a-name))))))

Exercise. Write a function that consumes a post and produces an X-expression representing that content.

value render-post : (post? . -> . xexpr/c)

As an example, we want:

(render-post (post "First post!" "This is a first post."))

to produce:

'(div ((class "post")) "First post!" (p "This is a first post."))

————

We will sometimes want to embed a list of X-expressions into another list that acts as a template. For example, given the list of X-expressions '((li "Larry") (li "Curly") (li "Moe")), we may want to create the single X-expression

'(ul (li "Larry")

(li "Curly")

(li "Moe"))

This can’t be done using plain unquoting, because placing a comma in front of '((li "Larry") (li "Curly") (li "Moe")) will unquote the entire list, yielding the malformed expression '(ul ((li "Larry") (li "Curly") (li "Moe"))).

Instead, we must splice the list in, like so: `(ul ,@((li "Larry") (li "Curly") (li "Moe"))). The unquote-splicing form, ,@expression, allows us conveniently to splice a list of X-expression fragments into a larger template list. To generalize the example, here are two helper functions that convert any list of X-expressions into one X-expression representing an unordered, itemized HTML list:

; render-as-itemized-list: (listof xexpr) -> xexpr

; Consumes a list of items, and produces a rendering

; as an unordered list.

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

; Consumes an xexpr, and produces a rendering

; as a list item.

(define (render-as-item a-fragment)

`(li ,a-fragment))

Exercise. Write a function render-posts that consumes a (listof post?) and produces an X-expression for that content.

value render-posts : ((listof post?) . -> . xexpr/c)

As examples,

(render-posts empty)

should produce

'(div ((class "posts")))

and

(render-posts (list (post "Post 1" "Body 1")

(post "Post 2" "Body 2")))

should produce

'(div ((class "posts"))

(div ((class "post")) "Post 1" (p "Body 1"))

(div ((class "post")) "Post 2" (p "Body 2")))

————

Now that we have the render-posts function handy, let’s revisit our web application and change our start function to return an interesting response.

#lang web-server/insta

; A blog is a (listof post)

; and a post is a (post title body)

(struct post (title body))

; BLOG: blog

; The static blog.

(define BLOG

(list (post "Second Post" "This is another post")

(post "First Post" "This is my first post")))

; start: request -> response

; Consumes a request, and produces a page that displays all of the

; web content.

(define (start request)

(render-blog-page BLOG request))

; render-blog-page: blog request -> response

; Consumes a blog and a request, and produces an HTML page

; of the content of the blog.

(define (render-blog-page a-blog request)

(response/xexpr

`(html (head (title "My Blog"))

(body (h1 "My Blog")

,(render-posts a-blog)))))

; render-post: post -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

(define (render-post a-post)

`(div ((class "post"))

,(post-title a-post)

(p ,(post-body a-post))))

; render-posts: blog -> xexpr

; Consumes a blog, produces an xexpr fragment

; of all its posts.

(define (render-posts a-blog)

`(div ((class "posts"))

,@(map render-post a-blog)))

If we press Run, we should see the blog posts in our web browser.

5 Inspecting Requests

Our application is still a bit too static: we build the page dynamically, but we don’t yet provide a way for the user to create new posts. Let’s tackle that now, by providing a form that lets the user add a new blog entry. When the user presses the submit button, we want the new post to appear at the top of the page.

We haven’t yet done anything with the request object that we’ve been passing around. As you may already have guessed, it isn’t really supposed to be ignored so much! When a user fills out a web form and submits it, the user’s browser constructs a new request that contains the form’s values, which we can extract on our end, using the function request-bindings:

value request-bindings : (request? . -> . bindings?)

To extract a single web form value from a set of bindings, Racket provides the function extract-binding/single, which also takes the name of the corresponding field of the web form:

value extract-binding/single : (symbol? bindings? . -> . string?)

To verify that a set of bindings contains a particular field, use exists-binding?:

value exists-binding? : (symbol? bindings? . -> . boolean?)

With these functions, we can design functions that consume requests and respond to them usefully.

Exercise. Write a function can-parse-post? that consumes a set of bindings. It should produce #t if there exist bindings both for the symbols 'title and 'body, and #f otherwise.

value can-parse-post? : (bindings? . -> . boolean?)

Exercise. Write a function parse-post that consumes a set of bindings. Assuming that the bindings structure has values for the symbols 'title and 'body, parse-post should produce a post containing those values.

value parse-post : (bindings? . -> . post?)

Now that we have these helper functions, we can extend our web application to handle form input. We’ll add a small form at the bottom of the web page, and we’ll adjust our program to handle the addition of new posts. So our new start method will check that the request has a parsable post, will then try to extend the set of posts, and will finally display the new set of blog posts:

#lang web-server/insta

; A blog is a (listof post)

; and a post is a (post title body)

(struct post (title body))

; BLOG: blog

; The static blog.

(define BLOG

(list (post "Second Post" "This is another post")

(post "First Post" "This is my first post")))

; start: request -> response

; Consumes a request and produces a page that displays all of the

; web content.

(define (start request)

(local [(define a-blog

(cond [(can-parse-post? (request-bindings request))

(cons (parse-post (request-bindings request))

BLOG)]

[else

BLOG]))]

(render-blog-page a-blog request)))

; can-parse-post?: bindings -> boolean

; Produces true if bindings contains values for 'title and 'body.

(define (can-parse-post? bindings)

(and (exists-binding? 'title bindings)

(exists-binding? 'body bindings)))

; parse-post: bindings -> post

; Consumes a bindings, and produces a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)))

; render-blog-page: blog request -> response

; Consumes a blog and a request, and produces an HTML page

; of the content of the blog.

(define (render-blog-page a-blog request)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts a-blog)

(form

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

; render-post: post -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

(define (render-post a-post)

`(div ((class "post"))

,(post-title a-post)

(p ,(post-body a-post))))

; render-posts: blog -> xexpr

; Consumes a blog, produces an xexpr fragment

; of all its posts.

(define (render-posts a-blog)

`(div ((class "posts"))

,@(map render-post a-blog)))

This solution seems to work, but it has a flaw! Try to add two new posts. What happens?

6 Advanced Control Flow

For the moment, let’s ignore the admittedly huge problem of having a blog that accepts only one new blog entry. Don’t worry, we’ll fix this!

The more pressing problem right now is a higher-level one: although we do have a function, start, that responds to requests directed at our application’s URL, that function has begun to take on too much responsibility. In particular, start now handles two different kinds of requests: those for showing a blog, and those for adding new blog posts. It has become a kind of traffic cop — a dispatcher — for all of our web application’s behaviors, including any new functionality we may want to add later. Life would be easier for start (and for us) if different kinds of requests were instead directed automatically to different functions. Is this possible in Racket?

Yes! The web server library provides a function, send/suspend/dispatch, that allows us to create URLs that direct requests aimed at them to specific functions in our application. We demonstrate with a dizzying example. In a new file, enter the following in DrRacket’s Definitions window.

#lang web-server/insta

; start: request -> response

(define (start request)

(phase-1 request))

; phase-1: request -> response

(define (phase-1 request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html

(body (h1 "Phase 1")

(a ((href ,(embed/url phase-2)))

"click me!")))))]

(send/suspend/dispatch response-generator)))

; phase-2: request -> response

(define (phase-2 request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html

(body (h1 "Phase 2")

(a ((href ,(embed/url phase-1)))

"click me!")))))]

(send/suspend/dispatch response-generator)))

This is a web application that goes round and round. When a user first visits the application, the user starts off in phase-1. The generated page has a hyperlink that, when clicked, continues to phase-2. The user can click back, and falls back to phase-1, and the cycle repeats.

Let’s look more closely at the send/suspend/dispatch mechanism. send/suspend/dispatch consumes a response-generating function and gives it another function, called embed/url, that we’ll use to build special URLs. What makes these URLs special is this: when a web browser visits one of them, our web application restarts, not from start, but from the handler that we associate with the URL. In the handler phase-1, the use of embed/url associates the link with the handler phase-2, and vice versa.

We can be even more sophisticated about the handlers associated with embed/url. Because a handler is just a request-consuming function, it can be defined within a local and so can see all the other variables in the scope of its definition. Here’s another loopy example:

#lang web-server/insta

; start: request -> response

(define (start request)

(show-counter 0 request))

; show-counter: number request -> doesn't return

; Displays a number that's hyperlinked: when the link is pressed,

; returns a new page with the incremented number.

(define (show-counter n request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Counting example"))

(body

(a ((href ,(embed/url next-number-handler)))

,(number->string n))))))

(define (next-number-handler request)

(show-counter (+ n 1) request))]

(send/suspend/dispatch response-generator)))

This example shows that we can accumulate the results of an interaction. Even though the user starts off by visiting and seeing zero, the handlers produced by next-number-handler continue the interaction, accumulating a larger and larger number.

We’re going in circles now, so let’s move forward and return to our blog application. We’ll adjust the form’s action so that it directs a submission request to a URL associated with a separate handler, called insert-post-handler.

#lang web-server/insta

; A blog is a (listof post)

; and a post is a (post title body)

(struct post (title body))

; BLOG: blog

; The static blog.

(define BLOG

(list (post "Second Post" "This is another post")

(post "First Post" "This is my first post")))

; start: request -> doesn't return

; Consumes a request and produces a page that displays all of the

; web content.

(define (start request)

(render-blog-page BLOG request))

; parse-post: bindings -> post

; Extracts a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)))

; render-blog-page: blog request -> doesn't return

; Consumes a blog and a request, and produces an HTML page

; of the content of the blog.

(define (render-blog-page a-blog request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts a-blog)

(form ((action

,(embed/url insert-post-handler)))

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

(define (insert-post-handler request)

(render-blog-page

(cons (parse-post (request-bindings request))

a-blog)

request))]

(send/suspend/dispatch response-generator)))

; render-post: post -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

(define (render-post a-post)

`(div ((class "post"))

,(post-title a-post)

(p ,(post-body a-post))))

; render-posts: blog -> xexpr

; Consumes a blog, produces an xexpr fragment

; of all its posts.

(define (render-posts a-blog)

`(div ((class "posts"))

,@(map render-post a-blog)))

Note that the structure of the render-blog-page function is very similar to that of our last show-counter example. The user can finally add and see multiple posts to the blog.

Unfortunately, our design still suffers from a problem, which can be seen by adding a few posts to the system, and then visiting the web application’s URL in a new browser window. What happens when you try this?

7 Share and Share Alike

The problem with our application is that each browser window keeps track of its own distinct blog. For most people, this defeats the purpose of a blog, which is to share with others! When we insert a new post, rather than creating a new blog value, we’d like to modify the blog. In other words, we’d like to make a structural change. (How to Design Programs, Chapter 41). So let’s switch from just the BLOG binding to a list and instead bind it to a mutable structure. If we were to just use a structure, we’d write the following:

(struct blog (posts))

But, by default, structures in Racket are immutable. To gain access to structure mutators, we’ll need to override this default, by adding the #:mutable keyword to some of our structure definitions. In particular, if we want to allow changes to a blog, we must change our definition of the blog structure to the following:

(struct blog (posts) #:mutable)

struct (struct blog (posts))

posts : (listof post?)

A mutable structure provides functions that change its fields; in this case, we are provided the structure mutator set-blog-posts!, which allows us to change the posts of a blog:

value set-blog-posts! : (blog? (listof post?) . -> . void)

Exercise. Write a function blog-insert-post!

value blog-insert-post! : (blog? post? . -> . void)

whose intended side effect is to extend a blog’s posts.

————

We must now modify the web application to use our new data representation of a blog. Since the blog is now referred to by the global variable BLOG, it no longer needs to be passed as a parameter to handlers like render-blog-page. Here is our updated web application, after adjustments that incorporate insert-blog-post!, and after a bit of variable cleanup:

#lang web-server/insta

; A blog is a (blog posts)

; where posts is a (listof post)

(struct blog (posts) #:mutable)

; and post is a (post title body)

; where title is a string, and body is a string

(struct post (title body))

; BLOG: blog

; The initial BLOG.

(define BLOG

(blog

(list (post "Second Post" "This is another post")

(post "First Post" "This is my first post"))))

; blog-insert-post!: blog post -> void

; Consumes a blog and a post, adds the post at the top of the blog.

(define (blog-insert-post! a-blog a-post)

(set-blog-posts! a-blog

(cons a-post (blog-posts a-blog))))

; start: request -> doesn't return

; Consumes a request and produces a page that displays

; all of the web content.

(define (start request)

(render-blog-page request))

; parse-post: bindings -> post

; Extracts a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)))

; render-blog-page: request -> doesn't return

; Produces an HTML page of the content of the BLOG.

(define (render-blog-page request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts)

(form ((action

,(embed/url insert-post-handler)))

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

(define (insert-post-handler request)

(blog-insert-post!

BLOG (parse-post (request-bindings request)))

(render-blog-page request))]

(send/suspend/dispatch response-generator)))

; render-post: post -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

(define (render-post a-post)

`(div ((class "post"))

,(post-title a-post)

(p ,(post-body a-post))))

; render-posts: -> xexpr

; Consumes a blog, produces an xexpr fragment

; of all its posts.

(define (render-posts)

`(div ((class "posts"))

,@(map render-post (blog-posts BLOG))))

Now visit the blog from two separate browser windows and add posts from each of them. You’ll be glad to see that both windows share the same blog!

8 Extending the Model

Next, let’s extend the application so that a post can include a list of comments. The data definition becomes:

struct (struct   post (title body comments)     #:mutable)

title : string?

body : string?

comments : (listof string?)

Exercise. Write the updated data structure definition for posts. Make sure to make the structure mutable, since we intend to add comments to posts.

Exercise. Make up a few examples of posts.

Exercise. Define a function post-add-comment!

value post-add-comment! : (post? string? . -> . void)

whose intended side effect is to add a new comment to the end of the post’s list of comments.

Exercise. Adjust render-post so that the produced fragment will include the comments in an itemized list.

Exercise. Because we’ve extended post to include comments, you also need to adjust other, post-manipulating parts of the application, such as uses of post.

————

Now that we’ve adjusted our functions to accommodate post’s new structure, our web application should be runnable. The user may even see some of the fruits of our labor: if the initial BLOG has a post with comments, the user should now see them. On the other hand, something is obviously missing: the user is given no interface through which to add comments!

9 Breaking Up the Display

How should we incorporate comments more fully into the user’s web experience? Seeing all the posts and comments on one page may be a bit overwhelming, so maybe we should hold off on showing the comments on the main blog page. Instead, let’s make a secondary “detail” view of a post and present its comments there. Accordingly, the top-level view of a blog will show only the title and body of a post, and the number of its comments.

So now we need a way to visit a post’s detail page. One way to do this is to hyperlink a post’s title: if one wishes to see a post’s detail page, one should only have to click the post’s title. In that post’s detail page, we can even add a form to let the user add new comments. The page flow of this new version of our web application is then depicted simply as:

Each node (bubble) in this diagram corresponds to a request-consuming handler. As you might expect, we’ll be using send/suspend/dispatch some more. Every arrow in the diagram will be realized as a URL that we generate with embed/url.

This approach has a slightly messy consequence. Previously we rendered the list of posts without any hyperlinks. But since any function that generates a special dispatching URL must use embed/url to do so, we’ll need to adjust render-posts and render-post to consume and use embed/url itself when it makes those hyperlinked titles.

We now have a pretty sophisticated web application, one that permits the creation of posts and the addition of comments. Here is what it looks like:

#lang web-server/insta

; A blog is a (blog posts)

; where posts is a (listof post)

(struct blog (posts) #:mutable)

; and post is a (post title body comments)

; where title is a string, body is a string,

; and comments is a (listof string)

(struct post (title body comments) #:mutable)

; BLOG: blog

; The initial BLOG.

(define BLOG

(blog

(list (post "Second Post"

"This is another post"

(list))

(post "First Post"

"This is my first post"

(list "First comment!")))))

; blog-insert-post!: blog post -> void

; Consumes a blog and a post, adds the post at the top of the blog.

(define (blog-insert-post! a-blog a-post)

(set-blog-posts! a-blog

(cons a-post (blog-posts a-blog))))

; post-insert-comment!: post string -> void

; Consumes 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-post a-comment)

(set-post-comments!

a-post

(append (post-comments a-post) (list a-comment))))

; start: request -> doesn't return

; Consumes a request, and produces a page that displays

; all of the web content.

(define (start request)

(render-blog-page request))

; render-blog-page: request -> doesn't return

; Produces an HTML page of the content of the

; BLOG.

(define (render-blog-page request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts embed/url)

(form ((action

,(embed/url insert-post-handler)))

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

; parse-post: bindings -> post

; Extracts a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)

(list)))

(define (insert-post-handler request)

(blog-insert-post!

BLOG (parse-post (request-bindings request)))

(render-blog-page request))]

(send/suspend/dispatch response-generator)))

; render-post-detail-page: post request -> doesn't return

; Consumes a post and request, and produces a detail page

; of the post. The user will be able to insert new comments.

(define (render-post-detail-page a-post request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Post Details"))

(body

(h1 "Post Details")

(h2 ,(post-title a-post))

(p ,(post-body a-post))

,(render-as-itemized-list

(post-comments a-post))

(form ((action

,(embed/url insert-comment-handler)))

(input ((name "comment")))

(input ((type "submit"))))))))

(define (parse-comment bindings)

(extract-binding/single 'comment bindings))

(define (insert-comment-handler a-request)

(post-insert-comment!

a-post (parse-comment (request-bindings a-request)))

(render-post-detail-page a-post a-request))]

(send/suspend/dispatch response-generator)))

; render-post: post (handler -> string) -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

; The fragment contains a link to show a detailed view of the post.

(define (render-post a-post embed/url)

(local [(define (view-post-handler request)

(render-post-detail-page a-post request))]

`(div ((class "post"))

(a ((href ,(embed/url view-post-handler)))

,(post-title a-post))

(p ,(post-body a-post))

(div ,(number->string (length (post-comments a-post)))

" comment(s)"))))

; render-posts: (handler -> string) -> xexpr

; Consumes a embed/url, and produces an xexpr fragment

; of all its posts.

(define (render-posts embed/url)

(local [(define (render-post/embed/url a-post)

(render-post a-post embed/url))]

`(div ((class "posts"))

,@(map render-post/embed/url (blog-posts BLOG)))))

; render-as-itemized-list: (listof xexpr) -> xexpr

; Consumes a list of items, and produces a rendering as

; an unorderered list.

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

; Consumes an xexpr, and produces a rendering

; as a list item.

(define (render-as-item a-fragment)

`(li ,a-fragment))

But it still suffers from a problem: once in a post-detail-page, the only way for the user to return to the blog is to use the Back button! That’s disruptive, and it might allow the user get “stuck” in a dark corner of the web application. To solve this problem, let’s improve the page flow.

10 Adding a Back Button

Perhaps we should simply add a BACK link from the render-post-detail-page, one that returns us to the top-level blog. Here’s the corresponding page flow diagram:

Exercise. Adjust render-post-detail-page to include another link that goes back to render-blog-page.

And since a user may have a change of heart about a comment, let’s enrich the flow to give the user a chance to back out of submitting one.

Note that, although this change may seem complicated, it doesn’t affect the general shape of our handlers:

#lang web-server/insta

; A blog is a (blog posts)

; where posts is a (listof post)

(struct blog (posts) #:mutable)

; and post is a (post title body comments)

; where title is a string, body is a string,

; and comments is a (listof string)

(struct post (title body comments) #:mutable)

; BLOG: blog

; The initial BLOG.

(define BLOG

(blog

(list (post "Second Post"

"This is another post"

(list))

(post "First Post"

"This is my first post"

(list "First comment!")))))

; blog-insert-post!: blog post -> void

; Consumes a blog and a post, adds the post at the top of the blog.

(define (blog-insert-post! a-blog a-post)

(set-blog-posts! a-blog

(cons a-post (blog-posts a-blog))))

; post-insert-comment!: post string -> void

; Consumes 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-post a-comment)

(set-post-comments!

a-post

(append (post-comments a-post) (list a-comment))))

; start: request -> doesn't return

; Consumes a request and produces a page that displays

; all of the web content.

(define (start request)

(render-blog-page request))

; render-blog-page: request -> doesn't return

; Produces an HTML page of the content of the

; BLOG.

(define (render-blog-page request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts embed/url)

(form ((action

,(embed/url insert-post-handler)))

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

; parse-post: bindings -> post

; Extracts a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)

(list)))

(define (insert-post-handler request)

(blog-insert-post!

BLOG (parse-post (request-bindings request)))

(render-blog-page request))]

(send/suspend/dispatch response-generator)))

; render-post-detail-page: post request -> doesn't return

; Consumes a post and produces a detail page of the post.

; The user will be able to either insert new comments

; or go back to render-blog-page.

(define (render-post-detail-page a-post request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Post Details"))

(body

(h1 "Post Details")

(h2 ,(post-title a-post))

(p ,(post-body a-post))

,(render-as-itemized-list

(post-comments a-post))

(form ((action

,(embed/url insert-comment-handler)))

(input ((name "comment")))

(input ((type "submit"))))

(a ((href ,(embed/url back-handler)))

"Back to the blog")))))

(define (parse-comment bindings)

(extract-binding/single 'comment bindings))

(define (insert-comment-handler request)

(render-confirm-add-comment-page

(parse-comment (request-bindings request))

a-post

request))

(define (back-handler request)

(render-blog-page request))]

(send/suspend/dispatch response-generator)))

; render-confirm-add-comment-page :

; comment post request -> doesn't return

; Consumes a comment that we intend to add to a post, as well

; as the request. If the user follows through, adds a comment

; and goes back to the display page. Otherwise, goes back to

; the detail page of the post.

(define (render-confirm-add-comment-page a-comment a-post request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Add a Comment"))

(body

(h1 "Add a Comment")

"The comment: " (div (p ,a-comment))

"will be added to "

(div ,(post-title a-post))

(p (a ((href ,(embed/url yes-handler)))

"Yes, add the comment."))

(p (a ((href ,(embed/url cancel-handler)))

"No, I changed my mind!"))))))

(define (yes-handler request)

(post-insert-comment! a-post a-comment)

(render-post-detail-page a-post request))

(define (cancel-handler request)

(render-post-detail-page a-post request))]

(send/suspend/dispatch response-generator)))

; render-post: post (handler -> string) -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

; The fragment contains a link to show a detailed view of the post.

(define (render-post a-post embed/url)

(local [(define (view-post-handler request)

(render-post-detail-page a-post request))]

`(div ((class "post"))

(a ((href ,(embed/url view-post-handler)))

,(post-title a-post))

(p ,(post-body a-post))

(div ,(number->string (length (post-comments a-post)))

" comment(s)"))))

; render-posts: (handler -> string) -> xexpr

; Consumes a embed/url, produces an xexpr fragment

; of all its posts.

(define (render-posts embed/url)

(local [(define (render-post/embed/url a-post)

(render-post a-post embed/url))]

`(div ((class "posts"))

,@(map render-post/embed/url (blog-posts BLOG)))))

; render-as-itemized-list: (listof xexpr) -> xexpr

; Consumes a list of items, and produces a rendering as

; an unorderered list.

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

; Consumes an xexpr, and produces a rendering

; as a list item.

(define (render-as-item a-fragment)

`(li ,a-fragment))

11 Decorating With Style!

Our web application is now functionally complete. But it’s visually lacking, so let’s try to improve its appearance. One way to add visual panache to our web pages is to use a cascading style sheet. For example, if we’d like to make all of our paragraphs green, we might insert the following style declaration into a response.

'(style ((type "text/css")) "p { color: green }")

It is tempting to embed such declarations directly into our responses. But our source file is already quite busy, and, as a matter of principle, we should separate logical representation from visual presentation. So, rather than embed the .css in the HTML response directly, let’s instead add a link reference to a separate .css file.

Up till now, all the content produced by our web application has come from a response-generating handler. But this dynamic generation of HTML is not necessary for content that doesn’t change. Examples of such static resources include images, documents, and .css files. To serve them alongside our web applications, we inform the web server of a directory that we have created specially for static files. The function static-files-path,

value static-files-path : (path-string? -> void)

tells the web server to look in the given path whenever it receives a URL that looks like a request for a static resource.

Exercise. Create a simple web application called "test-static.rkt" with the following content:

#lang web-server/insta

(define (start request)

(response/xexpr

'(html (head (title "Testing")

(link ((rel "stylesheet")

(href "/test-static.css")

(type "text/css"))))

(body (h1 "Testing")

(h2 "This is a header")

(p "This is " (span ((class "hot")) "hot") ".")))))

(static-files-path "htdocs")

Make a subdirectory called "htdocs", rooted in the same directory as the "test-static.rkt" source. Just to see that we can serve this .css page, create a very simple .css file "test-static.css" in "htdocs/" with the following content:

Now run the application and look at the browser’s output. A Spartan web page should appear, but it should still have some color in its cheeks.

————

Exercise. Improve the presentation of the blog web application by writing an external style sheet that suits your tastes. Adjust all of the HTML response handlers to include a link to the style sheet.

12 The Double Submit Error

Our application has yet another subtle problem. To see it, bring the blog application up again, and add a post. Then reload the page. Reload the page again.

What you are observing is the well known “double-submit” problem. Whenever a user presses Reload, a request is sent to our application, and the problem is that some requests make the application mutate data structures.

A common technique that web developers use to dodge the double-submission problem is to redirect state-mutating requests to a different URL, one that is safe to reload. This trick is implemented in Racket by the function redirect/get:

value redirect/get : (-> request?)

Its immediate side effect is to force the user’s browser to follow a redirection to a safe URL, and it gives us back that fresh new request.

For example, consider a toy application that lets the user add names to a roster:

#lang web-server/insta

; A roster is a (roster names)

; where names is a list of string.

(struct roster (names) #:mutable)

; roster-add-name!: roster string -> void

; Given a roster and a name, adds the name

; to the end of the roster.

(define (roster-add-name! a-roster a-name)

(set-roster-names! a-roster

(append (roster-names a-roster)

(list a-name))))

(define ROSTER (roster '("kathi" "shriram" "dan")))

; start: request -> doesn't return

(define (start request)

(show-roster request))

; show-roster: request -> doesn't return

(define (show-roster request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Roster"))

(body (h1 "Roster")

,(render-as-itemized-list

(roster-names ROSTER))

(form ((action

,(embed/url add-name-handler)))

(input ((name "a-name")))

(input ((type "submit"))))))))

(define (parse-name bindings)

(extract-binding/single 'a-name bindings))

(define (add-name-handler request)

(roster-add-name!

ROSTER (parse-name (request-bindings request)))

(show-roster request))]

(send/suspend/dispatch response-generator)))

; render-as-itemized-list: (listof xexpr) -> xexpr

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

(define (render-as-item a-fragment)

`(li ,a-fragment))

This application suffers from the same problem as our blog does: if the user adds a name, and then presses reload, then the same name will be added twice.

We can fix this by changing a single expression; can you find it below?

#lang web-server/insta

; A roster is a (roster names)

; where names is a list of string.

(struct roster (names) #:mutable)

; roster-add-name!: roster string -> void

; Given a roster and a name, adds the name

; to the end of the roster.

(define (roster-add-name! a-roster a-name)

(set-roster-names! a-roster

(append (roster-names a-roster)

(list a-name))))

(define ROSTER (roster '("kathi" "shriram" "dan")))

; start: request -> doesn't return

(define (start request)

(show-roster request))

; show-roster: request -> doesn't return

(define (show-roster request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Roster"))

(body (h1 "Roster")

,(render-as-itemized-list

(roster-names ROSTER))

(form ((action

,(embed/url add-name-handler)))

(input ((name "a-name")))

(input ((type "submit"))))))))

(define (parse-name bindings)

(extract-binding/single 'a-name bindings))

(define (add-name-handler request)

(roster-add-name!

ROSTER (parse-name (request-bindings request)))

(show-roster (redirect/get)))]

(send/suspend/dispatch response-generator)))

; render-as-itemized-list: (listof xexpr) -> xexpr

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

(define (render-as-item a-fragment)

`(li ,a-fragment))

So the double-submit error is easy to prevent: whenever you have handlers that mutate the state of the system, use redirect/get when sending back your response.

Exercise. Use redirect/get to fix the double-submit error in the blog application.

With these minor fixes, our blog application now looks like this:

#lang web-server/insta

; A blog is a (blog posts)

; where posts is a (listof post)

(struct blog (posts) #:mutable)

; and post is a (post title body comments)

; where title is a string, body is a string,

; and comments is a (listof string)

(struct post (title body comments) #:mutable)

; BLOG: blog

; The initial BLOG.

(define BLOG

(blog

(list (post "Second Post"

"This is another post"

(list))

(post "First Post"

"This is my first post"

(list "First comment!")))))

; blog-insert-post!: blog post -> void

; Consumes a blog and a post, adds the post at the top of the blog.

(define (blog-insert-post! a-blog a-post)

(set-blog-posts! a-blog

(cons a-post (blog-posts a-blog))))

; post-insert-comment!: post string -> void

; Consumes 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-post a-comment)

(set-post-comments!

a-post

(append (post-comments a-post) (list a-comment))))

; start: request -> doesn't return

; Consumes a request and produces a page that displays

; all of the web content.

(define (start request)

(render-blog-page request))

; render-blog-page: request -> doesn't return

; Produces an HTML page of the content of the

; BLOG.

(define (render-blog-page request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "My Blog"))

(body

(h1 "My Blog")

,(render-posts embed/url)

(form ((action

,(embed/url insert-post-handler)))

(input ((name "title")))

(input ((name "body")))

(input ((type "submit"))))))))

; parse-post: bindings -> post

; Extracts a post out of the bindings.

(define (parse-post bindings)

(post (extract-binding/single 'title bindings)

(extract-binding/single 'body bindings)

(list)))

(define (insert-post-handler request)

(blog-insert-post!

BLOG (parse-post (request-bindings request)))

(render-blog-page (redirect/get)))]

(send/suspend/dispatch response-generator)))

; render-post-detail-page: post request -> doesn't return

; Consumes a post and produces a detail page of the post.

; The user will be able to either insert new comments

; or go back to render-blog-page.

(define (render-post-detail-page a-post request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Post Details"))

(body

(h1 "Post Details")

(h2 ,(post-title a-post))

(p ,(post-body a-post))

,(render-as-itemized-list

(post-comments a-post))

(form ((action

,(embed/url insert-comment-handler)))

(input ((name "comment")))

(input ((type "submit"))))

(a ((href ,(embed/url back-handler)))

"Back to the blog")))))

(define (parse-comment bindings)

(extract-binding/single 'comment bindings))

(define (insert-comment-handler request)

(render-confirm-add-comment-page

(parse-comment (request-bindings request))

a-post

request))

(define (back-handler request)

(render-blog-page request))]

(send/suspend/dispatch response-generator)))

; render-confirm-add-comment-page :

; comment post request -> doesn't return

; Consumes a comment that we intend to add to a post, as well

; as the request. If the user follows through, adds a comment

; and goes back to the display page. Otherwise, goes back to

; the detail page of the post.

(define (render-confirm-add-comment-page a-comment a-post request)

(local [(define (response-generator embed/url)

(response/xexpr

`(html (head (title "Add a Comment"))

(body

(h1 "Add a Comment")

"The comment: " (div (p ,a-comment))

"will be added to "

(div ,(post-title a-post))

(p (a ((href ,(embed/url yes-handler)))

"Yes, add the comment."))

(p (a ((href ,(embed/url cancel-handler)))

"No, I changed my mind!"))))))

(define (yes-handler request)

(post-insert-comment! a-post a-comment)

(render-post-detail-page a-post (redirect/get)))

(define (cancel-handler request)

(render-post-detail-page a-post request))]

(send/suspend/dispatch response-generator)))

; render-post: post (handler -> string) -> xexpr

; Consumes a post, produces an xexpr fragment of the post.

; The fragment contains a link to show a detailed view of the post.

(define (render-post a-post embed/url)

(local [(define (view-post-handler request)

(render-post-detail-page a-post request))]

`(div ((class "post"))

(a ((href ,(embed/url view-post-handler)))

,(post-title a-post))

(p ,(post-body a-post))

(div ,(number->string (length (post-comments a-post)))

" comment(s)"))))

; render-posts: (handler -> string) -> xexpr

; Consumes a embed/url, produces an xexpr fragment

; of all its posts.

(define (render-posts embed/url)

(local [(define (render-post/embed/url a-post)

(render-post a-post embed/url))]

`(div ((class "posts"))

,@(map render-post/embed/url (blog-posts BLOG)))))

; render-as-itemized-list: (listof xexpr) -> xexpr

; Consumes a list of items, and produces a rendering as

; an unorderered list.

(define (render-as-itemized-list fragments)

`(ul ,@(map render-as-item fragments)))

; render-as-item: xexpr -> xexpr

; Consumes an xexpr, and produces a rendering

; as a list item.

(define (render-as-item a-fragment)

`(li ,a-fragment))