24 Days of Hackage: digestive-functors

Form processing is a task I used to dread doing in other programming languages, but not any more! Just about all non-trivial websites have some form of user interaction, and it can be notoriously hard to sanitize all of the data that users submit into something that you can be sure is correct. Not only are there horrible edge cases to deal with, you also need to render the form and present any validation errors back to users. And to complicate things even more, you usually want to validate as much of the form as possible, returning all errors at once - it wouldn’t provide a very good user experience if you only got one error at a time!

It sounds horrible right? Well luckily, it’s mostly a solved problem. In 2008, Cooper, Lindley, Wadler and Yallop wrote the Essence of Form Abstraction which introduced ‘formlets’, with a paper I recommend giving at least a light read at some point to understand how the design came about. Originally there was a fairly literal translation to haskell in the formlets library, and this has now been superceded by Jasper Van der Jeugt’s library: digestive-functors.

Essentially, digestive-functors gives you an Applicative instance to construct a data type by evaluating a form. If you’re not familiar with applicative functors, a layman explanation might be that they allow you to apply arguments to a function, where the arguments themselves come from some sort of side effect - in this case the form evaluation. Let’s have a look at an example:

data Category = Category { catName :: Text }

data BlogPost = BlogPost { postTitle :: Text
                         , postBody :: Text
                         , postCategory :: Category
                         }

postForm :: Monad m => Form Html m BlogPost
postForm = BlogPost
             <$> "title" .: text Nothing
             <*> "body" .: text Nothing
             <*> (Category <$> "category" .: text Nothing)

I’ve defined 2 data types - one for categories of posts, and one for blog posts themselves. We use digestive-functors in postForm to produce a form that runs in any monad, uses Html for errors, and will eventually return a BlogPost. This is a great start, but it’s not really doing much - there’s no validation! We don’t want to accept BlogPosts which have no title or body, for example, so lets start by fixing that:

postForm :: Monad m => Form Html m BlogPost
postForm = BlogPost
             <$> "title" .: nonEmptyText
             <*> "body" .: nonEmptyText
             <*> (Category <$> "category" .: nonEmptyText)
  where
    nonEmptyText =
      check "Cannot be empty" (not . Data.Text.null) $ text Nothing

The check combinator allows us to apply any predicate (that is, a -> Bool) to any form that returns an a. In this case, we use the predicate not . Data.Text.null which will return True if the text form returns a Text value that is not empty. The beauty is that nonEmptyText is itself still a Form - so we could apply further validation on top of this, if we so wanted!

Validation is often not pure though - we might need to check the database to make certain checks (for example, checking for duplicates first). Lets assume in this example we have a BlogWebsite monad, which amongst other things has the operation lookupCategory :: Text -> BlogWebsite (Maybe Category). We can then use this in our postForm to lookup categories, and only allow BlogPosts to be created in a valid category:

postForm :: Form Text BlogWebsite BlogPost
postForm = BlogPost
             <$> "title" .: nonEmptyText
             <*> "body" .: nonEmptyText
             <*> "category" .: category
  where
    nonEmptyText =
      check "Cannot be empty" (not . Data.Text.null) $
        text Nothing
    category =
      let failure = Error "Category does not exist"
      in validateM (maybe failure Success) <$> lookupCategory)
           nonEmptyText

We’ve now built a monadic Form - category - which uses the nonEmptyText Form we already wrote, and if the text is not empty, attempts to use lookupCategory to find the category. We then unwrap the Maybe by converting to the Result type - turning Nothing into an Error and Just into Success.

Alright, this is looking great! But we haven’t actually been able to use it… Afterall, we need a BlogPost not a Form! There are 2 parts to being able to run a Form - the View, an something that can provide an environment to lookup form fields in. Lets use digestive-functors-blaze for the rendering, first:

renderForm :: View Html -> Html
renderForm v = do
  form v "POST" $ do
    H.p $ do
      label "title" v "Post title: "
      inputText "title" v
    H.p $ do
      label "body" v "Post: "
      inputTextArea "body" v
    H.p $ do
      label "category" v "Category: "
      inputText "category" v
    inputSubmit "Submit Post"

Fairly straight forward here - I’m using blaze-html for most of the rendering, and using a few functions from digestive-functors-blaze to provide the form fields and labels. There are other rendering libraries if you don’t like blaze - including digestive-functors-heist for using Heist templates.

Finally, lets put it all together with a Snap action, so we can actually serve stuff:

blogPostHandler :: Handler BlogWebsite BlogWebsite ()
blogPostHandler = do
  (view, result) <- runForm "blog-post" blogPost
  case result of
    Nothing -> blaze $ renderForm view
    Just blogPost -> do
      with blogSite $ addPost blogPost
      redirect (postUlr blogPost)

Voila! So I guess form processing doesn’t have to be so hard after all.

Further Reading

If this post has piqued your interest, I can highly recommend a read through the official tutorial and the various other examples.

It was also mentioned in comments on Reddit that I haven’t quite clarified that digestive-functors is framework agnostic - digestive-functors also works with Happstack and JSON. In fact, here’s a list of all digestive-functor-derived modules on Hackage at the time of writing:


You can contact me via email at ollie@ocharles.org.uk or tweet to me @acid2. I share almost all of my work at GitHub. This post is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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