<p>Forms can be a tedious part of web development since they require synchronization of code in many different areas: the HTML form declaration, parsing of the form and reconstructing a datatype from the raw values. The Yesod form library simplifies things greatly. We'll start off with a basic application.</p>
> {-# LANGUAGE TypeFamilies, QuasiQuotes, OverloadedStrings, MultiParamTypeClasses, TemplateHaskell #-}
> import Yesod hiding (Form)
> import Control.Applicative
> import Data.Text (Text)
> data FormExample = FormExample
> mkYesod "FormExample" [parseRoutes|
> / RootR GET
> |]
> type Form a = Html -> MForm FormExample FormExample (FormResult a, Widget)
> type Formlet a = Maybe a -> Form a
> instance Yesod FormExample where approot _ = ""
> instance RenderMessage FormExample FormMessage where
> renderMessage _ _ = defaultFormMessage
Next, we'll declare a Person datatype with a name and age. After that, we'll create a formlet. A formlet is a declarative approach to forms. It takes a Maybe value and constructs either a blank form, a form based on the original value, or a form based on the values submitted by the user. It also attempts to construct a datatype, failing on validation errors.
> data Person = Person { name :: Text, age :: Int }
> deriving Show
> personFormlet :: Formlet Person
> personFormlet p = renderTable $ Person
> <$> areq textField "Name" (fmap name p)
> <*> areq intField "Age" (fmap age p)
We use an applicative approach and stay mostly declarative. The "fmap name p" bit is just a way to get the name from within a value of type "Maybe Person".
> getRootR :: Handler RepHtml
> getRootR = do
> ((res, wform), enctype) <- runFormGet $ personFormlet Nothing
<p>We use runFormGet to bind to GET (query-string) parameters; we could also use runFormPost. The "Nothing" is the initial value of the form. You could also supply a "Just Person" value if you like. There is a three-tuple returned, containing the parsed value, the HTML form as a widget and the encoding type for the form.</p>
<p>We use a widget for the form since it allows embedding CSS and Javascript code in forms directly. This allows unobtrusive adding of rich Javascript controls like date pickers.</p>
> defaultLayout $ do
> setTitle "Form Example"
> form <- extractBody wform
<p>extractBody returns the HTML of a widget and "passes" all of the other declarations (the CSS, Javascript, etc) up to the parent widget. The rest of this is just standard Hamlet code and our main function.</p>
> addHamlet [hamlet|
> <p>Last result: #{show res}
> <form enctype="#{enctype}">
> <table>
> ^{form}
> <tr>
> <td colspan="2">
> <input type="submit">
> |]
>
> main :: IO ()
> main = warpDebug 3000 FormExample