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!
The basics
Simple validation
Monadic validation
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/.