2016-07-23 07:29:20 +02:00
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<div id=nav>
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2019-10-15 10:40:42 +02:00
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<a href="https://codemirror.net"><h1>CodeMirror</h1><img id=logo
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2016-07-23 07:29:20 +02:00
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src="../../doc/logo.png"></a>
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<ul>
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<li><a href="../../index.html">Home</a>
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<li><a href="../../doc/manual.html">Manual</a>
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<li><a href="https://github.com/codemirror/codemirror">Code</a>
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<ul>
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<li><a href="../index.html">Language modes</a>
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<li><a class=active href="#">Haskell-literate</a>
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</div>
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<article>
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<h2>Haskell literate mode</h2>
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<form>
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<textarea id="code" name="code">
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> {-# LANGUAGE OverloadedStrings #-}
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> {-# OPTIONS_GHC -fno-warn-unused-do-bind #-}
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> import Control.Applicative ((<$>), (<*>))
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> import Data.Maybe (isJust)
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> import Data.Text (Text)
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> import Text.Blaze ((!))
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> import qualified Data.Text as T
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> import qualified Happstack.Server as Happstack
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> import qualified Text.Blaze.Html5 as H
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> import qualified Text.Blaze.Html5.Attributes as A
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> import Text.Digestive
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> import Text.Digestive.Blaze.Html5
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> import Text.Digestive.Happstack
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> import Text.Digestive.Util
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Simple forms and validation
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---------------------------
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Let's start by creating a very simple datatype to represent a user:
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> data User = User
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> { userName :: Text
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> , userMail :: Text
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> } deriving (Show)
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And dive in immediately to create a `Form` for a user. The `Form v m a` type
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has three parameters:
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- `v`: the type for messages and errors (usually a `String`-like type, `Text` in
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this case);
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- `m`: the monad we are operating in, not specified here;
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- `a`: the return type of the `Form`, in this case, this is obviously `User`.
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> userForm :: Monad m => Form Text m User
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We create forms by using the `Applicative` interface. A few form types are
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provided in the `Text.Digestive.Form` module, such as `text`, `string`,
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`bool`...
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In the `digestive-functors` library, the developer is required to label each
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field using the `.:` operator. This might look like a bit of a burden, but it
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allows you to do some really useful stuff, like separating the `Form` from the
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actual HTML layout.
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> userForm = User
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> <$> "name" .: text Nothing
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> <*> "mail" .: check "Not a valid email address" checkEmail (text Nothing)
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The `check` function enables you to validate the result of a form. For example,
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we can validate the email address with a really naive `checkEmail` function.
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> checkEmail :: Text -> Bool
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> checkEmail = isJust . T.find (== '@')
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More validation
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---------------
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For our example, we also want descriptions of Haskell libraries, and in order to
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do that, we need package versions...
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> type Version = [Int]
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We want to let the user input a version number such as `0.1.0.0`. This means we
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need to validate if the input `Text` is of this form, and then we need to parse
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it to a `Version` type. Fortunately, we can do this in a single function:
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`validate` allows conversion between values, which can optionally fail.
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`readMaybe :: Read a => String -> Maybe a` is a utility function imported from
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`Text.Digestive.Util`.
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> validateVersion :: Text -> Result Text Version
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> validateVersion = maybe (Error "Cannot parse version") Success .
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> mapM (readMaybe . T.unpack) . T.split (== '.')
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A quick test in GHCi:
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ghci> validateVersion (T.pack "0.3.2.1")
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Success [0,3,2,1]
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ghci> validateVersion (T.pack "0.oops")
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Error "Cannot parse version"
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It works! This means we can now easily add a `Package` type and a `Form` for it:
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> data Category = Web | Text | Math
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> deriving (Bounded, Enum, Eq, Show)
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> data Package = Package Text Version Category
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> deriving (Show)
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> packageForm :: Monad m => Form Text m Package
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> packageForm = Package
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> <$> "name" .: text Nothing
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> <*> "version" .: validate validateVersion (text (Just "0.0.0.1"))
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> <*> "category" .: choice categories Nothing
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> where
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> categories = [(x, T.pack (show x)) | x <- [minBound .. maxBound]]
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Composing forms
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---------------
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A release has an author and a package. Let's use this to illustrate the
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composability of the digestive-functors library: we can reuse the forms we have
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written earlier on.
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> data Release = Release User Package
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> deriving (Show)
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> releaseForm :: Monad m => Form Text m Release
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> releaseForm = Release
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> <$> "author" .: userForm
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> <*> "package" .: packageForm
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Views
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-----
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As mentioned before, one of the advantages of using digestive-functors is
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separation of forms and their actual HTML layout. In order to do this, we have
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another type, `View`.
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We can get a `View` from a `Form` by supplying input. A `View` contains more
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information than a `Form`, it has:
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- the original form;
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- the input given by the user;
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- any errors that have occurred.
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It is this view that we convert to HTML. For this tutorial, we use the
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[blaze-html] library, and some helpers from the `digestive-functors-blaze`
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library.
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[blaze-html]: http://jaspervdj.be/blaze/
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Let's write a view for the `User` form. As you can see, we here refer to the
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different fields in the `userForm`. The `errorList` will generate a list of
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errors for the `"mail"` field.
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> userView :: View H.Html -> H.Html
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> userView view = do
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> label "name" view "Name: "
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> inputText "name" view
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> H.br
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>
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> errorList "mail" view
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> label "mail" view "Email address: "
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> inputText "mail" view
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> H.br
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Like forms, views are also composable: let's illustrate that by adding a view
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for the `releaseForm`, in which we reuse `userView`. In order to do this, we
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take only the parts relevant to the author from the view by using `subView`. We
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can then pass the resulting view to our own `userView`.
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We have no special view code for `Package`, so we can just add that to
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`releaseView` as well. `childErrorList` will generate a list of errors for each
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child of the specified form. In this case, this means a list of errors from
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`"package.name"` and `"package.version"`. Note how we use `foo.bar` to refer to
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nested forms.
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> releaseView :: View H.Html -> H.Html
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> releaseView view = do
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> H.h2 "Author"
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> userView $ subView "author" view
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>
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> H.h2 "Package"
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> childErrorList "package" view
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>
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> label "package.name" view "Name: "
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> inputText "package.name" view
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> H.br
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>
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> label "package.version" view "Version: "
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> inputText "package.version" view
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> H.br
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>
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> label "package.category" view "Category: "
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> inputSelect "package.category" view
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> H.br
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The attentive reader might have wondered what the type parameter for `View` is:
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it is the `String`-like type used for e.g. error messages.
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But wait! We have
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releaseForm :: Monad m => Form Text m Release
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releaseView :: View H.Html -> H.Html
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... doesn't this mean that we need a `View Text` rather than a `View Html`? The
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answer is yes -- but having `View Html` allows us to write these views more
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easily with the `digestive-functors-blaze` library. Fortunately, we will be able
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to fix this using the `Functor` instance of `View`.
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fmap :: Monad m => (v -> w) -> View v -> View w
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A backend
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---------
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To finish this tutorial, we need to be able to actually run this code. We need
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an HTTP server for that, and we use [Happstack] for this tutorial. The
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`digestive-functors-happstack` library gives about everything we need for this.
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[Happstack]: http://happstack.com/
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> site :: Happstack.ServerPart Happstack.Response
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> site = do
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> Happstack.decodeBody $ Happstack.defaultBodyPolicy "/tmp" 4096 4096 4096
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> r <- runForm "test" releaseForm
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> case r of
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> (view, Nothing) -> do
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> let view' = fmap H.toHtml view
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> Happstack.ok $ Happstack.toResponse $
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> template $
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> form view' "/" $ do
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> releaseView view'
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> H.br
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> inputSubmit "Submit"
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> (_, Just release) -> Happstack.ok $ Happstack.toResponse $
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> template $ do
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> css
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> H.h1 "Release received"
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> H.p $ H.toHtml $ show release
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>
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> main :: IO ()
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> main = Happstack.simpleHTTP Happstack.nullConf site
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Utilities
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---------
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> template :: H.Html -> H.Html
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> template body = H.docTypeHtml $ do
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> H.head $ do
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> H.title "digestive-functors tutorial"
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> css
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> H.body body
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> css :: H.Html
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> css = H.style ! A.type_ "text/css" $ do
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> "label {width: 130px; float: left; clear: both}"
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> "ul.digestive-functors-error-list {"
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> " color: red;"
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> " list-style-type: none;"
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> " padding-left: 0px;"
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> "}"
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</textarea>
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</form>
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<p><strong>MIME types
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defined:</strong> <code>text/x-literate-haskell</code>.</p>
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<p>Parser configuration parameters recognized: <code>base</code> to
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set the base mode (defaults to <code>"haskell"</code>).</p>
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<script>
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var editor = CodeMirror.fromTextArea(document.getElementById("code"), {mode: "haskell-literate"});
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</script>
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</article>
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