Web/Libraries/Formlets
From HaskellWiki
Contents |
1 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!
2 The basics
2.1 Simple validation
2.2 Monadic validation
3 A working example
Below is a self-contained example that uses Formlets and HAppS-Server.
3.1 Prepare your system
First install Formlets and HAppS-Server on your system:
$ cabal install formlets happs-server
3.2 The example code
Put the following in a file calledmodule Main where import HAppS.Server import Control.Applicative import Control.Applicative.Error import Control.Applicative.State import Text.XHtml.Strict.Formlets import Text.XHtml.Strict ((+++), (<<), (!)) import qualified Text.XHtml.Strict as X import qualified Data.Map as M 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 :: [ServerPart 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] -> Web Response) -> (a -> Web Response) -> [ServerPart Response] withForm name frm handleErrors handleOk = [dir name [ method GET $ createForm [] frm >>= okHtml , withDataFn lookPairs $ \d -> [method 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 = map (\(k,v) -> (k, Left v)) d showErrorsInline :: X.Html -> [String] -> Web Response showErrorsInline renderedForm errors = okHtml $ X.toHtml (show errors) +++ renderedForm createForm :: Env -> MyForm a -> Web 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 -> Web 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 ++ handleUser)
3.3 Running the example
Start up the HAppS server with
$ runhaskell Main.hs
and point your web browser at http://localhost:5000/date/ and http://localhost:5000/user/.
