Tutorial Part 4. Continuation Passing Style (CPS)
web4r provides a way to write web applications in continuation passing style.
Why to write web applications in CPS
- Easy to write control flow
- Enables us to use free variables captured by closure instead of the session object to save state
a/cont
a/cont embeds continuation within a link node.
Examples:
(defpage test ()
(a/cont [p "2nd page!"] "click here"))
form/cont
form/cont embeds continuation within a form node.
- To upload files, use multipart-form/cont instead.
- form/cont and multipart-form/cont inserts a submit button if there isn't one.
Example1:
(defpage test ()
(form/cont [p "2nd page!"]))
Example2:
(defpage test ()
(form/cont (let ((foo (hunchentoot:post-parameter "foo")))
(a/cont [p "You said: " foo] "click here"))
[input :type "text" :name "foo" /]))
page-lambda
page-lambda creates a page precedence which is used to bound paths and get/post parameters.
Examples:
(defpage test ()
(form/cont (page-lambda (:post foo)
(a/cont [p "You said: " foo] "click here"))
[input :type "text" :name "foo" /]))
; => Generated pages are the same with the previous example.
last-post
(last-post NAME) inside form/cont is expanded to bound the last post parameter named NAME at compile time.
Examples:
(defpage test ()
(form/cont (a/cont [p "You said: " (last-post "foo")] "click here")
[input :type "text" :name "foo" /]))
; Expanded code (image):
; (defpage test ()
; (form/cont (let ((#:G1152 (hunchentoot:post-parameter "foo")))
; (a/cont [p "You said: " #:G1152] "click here"))
; [input :type "text" :name "foo" /]))
; => Generated pages are the same with the previous example.