Web/Libraries/Formlets

Introduction

Formlets are a way of building HTML forms that are type-safe, handle errors, abstract and are easy to combine into bigger forms. Here's an example:

name :: Form String
name = input Nothing

The input function takes a Maybe String, and produces a XHtmlForm String. The Maybe String is used for default values. If you give it a nothing, it won't have a default value. If you pass in a (Just "value"), it will be pre-populated with the value "value".

You can easily combine formlets using the Applicative Functor combinators. Suppose you have a User-datatype:

data User = User {name :: String, age :: Integer, email :: String}

You can then build a form that produces a user:

userForm :: Form User
userForm = User <$> name <*> inputInteger <*> input Nothing

You can also have more advanced widgets, like a radio-choice, that's where you use enumRadio:

enumRadio :: (Monad m, Enum a) => [(a, String)] -> Maybe a -> Form a

So it asks for a list of pairs with a value and the corresponding label, a possible default-value and it will return something of type a.

chooseBool :: Form Bool
chooseBool = enumRadio [(True, "Yes"), (False, "No")] True

Now we have a widget for booleans that we can use everywhere in our forms!

A working example

Below is a self-contained example that uses Formlets and Happstack-Server.

Prepare your system

First install Formlets and Happstack-Server on your system:

$ cabal install formlets happstack-server

The example code

Put the following in a file called Main.hs:

module Main where

import           Control.Applicative
import           Control.Applicative.Error
import           Control.Applicative.State
import           Data.List                  as List
import           Happstack.Server
import           Text.Formlets
import           Text.XHtml.Strict.Formlets
import           Text.XHtml.Strict          ((+++),(<<),(!))
import qualified Text.XHtml.Strict          as X

type MyForm a = XHtmlForm IO a

data Date = Date {month :: Integer, day :: Integer}
  deriving Show

validDate :: Date -> Bool
validDate (Date m d) = m `elem` [1..12] && d `elem` [1..31]

dateComponent :: MyForm Date 
dateComponent = Date <$> inputInteger (Just 1) <*> inputInteger (Just 16)

dateFull :: MyForm Date
dateFull = dateComponent `check` ensure validDate "This is not a valid date"

handleDate :: ServerPartT IO Response
handleDate = withForm "date" dateFull showErrorsInline (\d -> okHtml $ show d)

data User = User {name :: String, pass :: String, birthdate :: Date}
  deriving Show

userFull :: MyForm User
userFull = User <$> input Nothing <*> password Nothing <*> dateFull

handleUser = withForm "user" userFull showErrorsInline (\u -> okHtml $ show u)

withForm
  :: String
  -> MyForm a
  -> (X.Html -> [String] -> ServerPartT IO Response)
  -> (a -> ServerPartT IO Response)
  -> ServerPartT IO Response 
withForm name frm handleErrors handleOk = dir name $ msum
  [ methodSP GET $ createForm [] frm >>= okHtml
  , withDataFn lookPairs $ \d ->
      methodSP POST $ handleOk' $ simple d
  ]
  where
    handleOk' d = do
      let (extractor, html, _) = runFormState d "" frm
      v <- liftIO extractor  
      case v of
        Failure faults -> do 
          f <- createForm d frm
          handleErrors f faults
        Success s      -> handleOk s
    simple d = List.map (\(k,v) -> (k, Left v)) d

showErrorsInline :: X.Html -> [String] -> ServerPartT IO Response
showErrorsInline renderedForm errors =
  okHtml $ X.toHtml (show errors) +++ renderedForm

createForm :: Env -> MyForm a -> ServerPartT IO X.Html
createForm env frm = do
  let (extractor, xml, endState) = runFormState env "" frm
  xml' <- liftIO xml
  return $ X.form ! [X.method "POST"] << (xml' +++ X.submit "submit" "Submit")

okHtml :: (X.HTML a) => a -> ServerPartT IO Response
okHtml content = ok $ toResponse $ htmlPage $ content

htmlPage :: (X.HTML a) => a -> X.Html
htmlPage content = (X.header << (X.thetitle << "Testing forms"))
  +++ (X.body << content)

main = simpleHTTP (nullConf {port = 5000}) (handleDate `mplus` handleUser)

Running the example

Start up the Happstack server with

$ runhaskell Main.hs

and point your web browser at http://localhost:5000/date/ and http://localhost:5000/user/.

Other resources

• Chris Done gives many examples in this blog post

References

• Formlets library on hackage
• Papers on formlets
• Applicative Functors wikibook