## Haskell literate mode

> {-# LANGUAGE OverloadedStrings #-} > {-# OPTIONS_GHC -fno-warn-unused-do-bind #-} > import Control.Applicative ((<$>), (<*>)) > import Data.Maybe (isJust) > import Data.Text (Text) > import Text.Blaze ((!)) > import qualified Data.Text as T > import qualified Happstack.Server as Happstack > import qualified Text.Blaze.Html5 as H > import qualified Text.Blaze.Html5.Attributes as A > import Text.Digestive > import Text.Digestive.Blaze.Html5 > import Text.Digestive.Happstack > import Text.Digestive.util Simple forms and validation --------------------------- Let's start by creating a very simple datatype to represent a user: > data User = User > { userName :: Text > , userMail :: Text > } deriving (Show) And dive in immediately to create a `Form` for a user. The `Form v m a` type has three parameters: - `v`: the type for messages and errors (usually a `String`-like type, `Text` in this case); - `m`: the monad we are operating in, not specified here; - `a`: the return type of the `Form`, in this case, this is obviously `User`. > userForm :: Monad m => Form Text m User We create forms by using the `Applicative` interface. A few form types are provided in the `Text.Digestive.Form` module, such as `text`, `string`, `bool`... In the `digestive-functors` library, the developer is required to label each field using the `.:` operator. This might look like a bit of a burden, but it allows you to do some really useful stuff, like separating the `Form` from the actual HTML layout. > userForm = User > <$> "name" .: text Nothing > <*> "mail" .: check "Not a valid email address" checkEmail (text Nothing) The `check` function enables you to validate the result of a form. For example, we can validate the email address with a really naive `checkEmail` function. > checkEmail :: Text -> Bool > checkEmail = isJust . T.find (== '@') More validation --------------- For our example, we also want descriptions of Haskell libraries, and in order to do that, we need package versions... > type Version = [Int] We want to let the user input a version number such as `0.1.0.0`. This means we need to validate if the input `Text` is of this form, and then we need to parse it to a `Version` type. Fortunately, we can do this in a single function: `validate` allows conversion between values, which can optionally fail. `readMaybe :: Read a => String -> Maybe a` is a utility function imported from `Text.Digestive.util`. > validateVersion :: Text -> Result Text Version > validateVersion = maybe (Error "Cannot parse version") Success . > mapM (readMaybe . T.unpack) . T.split (== '.') A quick test in GHCi: ghci> validateVersion (T.pack "0.3.2.1") Success [0,3,2,1] ghci> validateVersion (T.pack "0.oops") Error "Cannot parse version" It works! This means we can now easily add a `Package` type and a `Form` for it: > data Category = Web | Text | Math > deriving (Bounded, Enum, Eq, Show) > data Package = Package Text Version Category > deriving (Show) > packageForm :: Monad m => Form Text m Package > packageForm = Package > <$> "name" .: text Nothing > <*> "version" .: validate validateVersion (text (Just "0.0.0.1")) > <*> "category" .: choice categories Nothing > where > categories = [(x, T.pack (show x)) | x <- [minBound .. maxBound]] Composing forms --------------- A release has an author and a package. Let's use this to illustrate the composability of the digestive-functors library: we can reuse the forms we have written earlier on. > data Release = Release User Package > deriving (Show) > releaseForm :: Monad m => Form Text m Release > releaseForm = Release > <$> "author" .: userForm > <*> "package" .: packageForm Views ----- As mentioned before, one of the advantages of using digestive-functors is separation of forms and their actual HTML layout. In order to do this, we have another type, `View`. We can get a `View` from a `Form` by supplying input. A `View` contains more information than a `Form`, it has: - the original form; - the input given by the user; - any errors that have occurred. It is this view that we convert to HTML. For this tutorial, we use the [blaze-html] library, and some helpers from the `digestive-functors-blaze` library. [blaze-html]: http://jaspervdj.be/blaze/ Let's write a view for the `User` form. As you can see, we here refer to the different fields in the `userForm`. The `errorList` will generate a list of errors for the `"mail"` field. > userView :: View H.Html -> H.Html > userView view = do > label "name" view "Name: " > inputText "name" view > H.br > > errorList "mail" view > label "mail" view "Email address: " > inputText "mail" view > H.br Like forms, views are also composable: let's illustrate that by adding a view for the `releaseForm`, in which we reuse `userView`. In order to do this, we take only the parts relevant to the author from the view by using `subView`. We can then pass the resulting view to our own `userView`. We have no special view code for `Package`, so we can just add that to `releaseView` as well. `childErrorList` will generate a list of errors for each child of the specified form. In this case, this means a list of errors from `"package.name"` and `"package.version"`. Note how we use `foo.bar` to refer to nested forms. > releaseView :: View H.Html -> H.Html > releaseView view = do > H.h2 "Author" > userView $ subView "author" view > > H.h2 "Package" > childErrorList "package" view > > label "package.name" view "Name: " > inputText "package.name" view > H.br > > label "package.version" view "Version: " > inputText "package.version" view > H.br > > label "package.category" view "Category: " > inputSelect "package.category" view > H.br The attentive reader might have wondered what the type parameter for `View` is: it is the `String`-like type used for e.g. error messages. But wait! We have releaseForm :: Monad m => Form Text m Release releaseView :: View H.Html -> H.Html ... doesn't this mean that we need a `View Text` rather than a `View Html`? The answer is yes -- but having `View Html` allows us to write these views more easily with the `digestive-functors-blaze` library. Fortunately, we will be able to fix this using the `Functor` instance of `View`. fmap :: Monad m => (v -> w) -> View v -> View w A backend --------- To finish this tutorial, we need to be able to actually run this code. We need an HTTP server for that, and we use [Happstack] for this tutorial. The `digestive-functors-happstack` library gives about everything we need for this. [Happstack]: http://happstack.com/ > site :: Happstack.ServerPart Happstack.Response > site = do > Happstack.decodeBody $ Happstack.defaultBodyPolicy "/tmp" 4096 4096 4096 > r <- runForm "test" releaseForm > case r of > (view, Nothing) -> do > let view' = fmap H.toHtml view > Happstack.ok $ Happstack.toResponse $ > template $ > form view' "/" $ do > releaseView view' > H.br > inputSubmit "Submit" > (_, Just release) -> Happstack.ok $ Happstack.toResponse $ > template $ do > css > H.h1 "Release received" > H.p $ H.toHtml $ show release > > main :: IO () > main = Happstack.simpleHTTP Happstack.nullConf site Utilities --------- > template :: H.Html -> H.Html > template body = H.docTypeHtml $ do > H.head $ do > H.title "digestive-functors tutorial" > css > H.body body > css :: H.Html > css = H.style ! A.type_ "text/css" $ do > "label {width: 130px; float: left; clear: both}" > "ul.digestive-functors-error-list {" > " color: red;" > " list-style-type: none;" > " padding-left: 0px;" > "}"

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