Red Tape

Form Arguments

Sometimes a simple form isn't enough. When you need dynamic functionality in your forms you can use Red Tape's form arguments.

Once you've gone through this document, read the rendering guide.

The Problem

Let's use a simple example to demonstrate why form arguments are necessary and how they work. Suppose you have a site that hosts videos. You'd like to have a "delete video" form to let users delete their videos. A first crack at it might look like this:

(defn video-exists [video-id]
  ; Checks that the given video id exists...

(defform delete-video-form {}
  :video-id [video-exists])

Nothing new here. But this form will let anyone delete any video. In real life you probably only want users to be able to delete their own videos.

Defining Form Arguments

To define form arguments you can use the :arguments entry in the form options map, passing a vector of symbols. Let's change our delete-video-form form:

(defform delete-video-form {:arguments [user]}
  :video-id [video-exists])

The form will now take a user argument. So calling the form without data would be done like this:

(let [current-user ...
      fresh-delete-form (delete-video-form current-user)])

And calling it with data would look like this:

(let [current-user ...
      post-data ...
      form (delete-video-form current-user post-data)])

Using Form Arguments

Using form arguments relies on one key idea: your cleaner definitions are evaluated in a context where the form arguments are bound to the symbols you specified.

Here's how we could use our user argument:

(defn video-owned-by [user video-id]
  ; Check that the given user owns the given video id.

(defform delete-video-form {:arguments [user]}
  :video-id [video-exists
             #(video-owned-by user %)])

(let [current-user ...
      post-data ...
      form (delete-video-form current-user post-data)]

Notice how the second cleaner in the defform is defined as #(video-owned-by user %). user here is the form argument, so the anonymouse function will be created with the appropriate user each time delete-form is called.

Initial Data

The initial data map is also evaluated in a context where the form arguments are bound. You can use this to define default values based on the form arguments.

For example: suppose you have a profile page where users can change their email address and profile. A form for this might look like:

(defform profile-form {:arguments [user]
                       :initial {:email (:email user)
                                 :bio (:bio user)}}
   :email [#(cleaners/matches #"\S+@\S+" %
                              "Please enter a valid email address.")]
   :bio [])

(profile-form some-user)
; =>
 :results nil
 :errors nil
 :data {:email ""
        :bio "Computers are terrible."}}

(profile-form some-user {:email "this is not an email"
                         :bio "I like cats."})
; =>
 :results nil
 :errors {:email "Please enter a valid email address"}
 :data {:email "this is not an email"
        :bio "I like cats."}}

Notice how in the fresh form the :data is prepopulated with the user's information that was pulled from the form argument. Once the user actually enters some data, the initial data is ignored.