This module contains the core definitions for the Heist template system.

The Heist template system is based on XML/xhtml. It allows you to build custom XML-based markup languages. With Heist you can define your own domain-specific XML tags implemented with Haskell and use them in your templates.

The most important concept in Heist is the Splice. Splices can be thought of as functions that transform a node into a list of nodes. Heist then substitutes the resulting list of nodes into your template in place of the input node. Splice is implemented as a type synonym type Splice m = TemplateMonad m [Node], and TemplateMonad has a function getParamNode that lets you get the input node.

Suppose you have a place on your page where you want to display a link with the text "Logout username" if the user is currently logged in or a link to the login page if no user is logged in. Assume you have a function getUser :: MyAppMonad (Maybe ByteString) that gets the current user. You can implement this functionality with a Splice as follows:

 import Text.XML.Expat.Tree

 link :: ByteString -> ByteString -> Node
 link target text = X.Element "a" [("href", target)] [X.Text text]
 
 loginLink :: Node
 loginLink = link "/login" "Login"
 
 logoutLink :: ByteString -> Node
 logoutLink user = link "/logout" (B.append "Logout " user)
 
 loginLogoutSplice :: Splice MyAppMonad
 loginLogoutSplice = do
     user <- lift getUser
     return $ [maybe loginLink logoutLink user]

Next, you need to bind that splice to an XML tag. Heist stores information about splices and templates in the TemplateState data structure. The following code demonstrates how this splice would be used.

 mySplices = [ ("loginLogout", loginLogoutSplice) ]
 
 main = do
     ets <- loadTemplates "templates" $
            foldr (uncurry bindSplice) emptyTemplateState mySplices
     let ts = either error id ets
     t <- runMyAppMonad $ renderTemplate ts "index"
     print $ maybe "Page not found" id t

Here we build up our TemplateState by starting with emptyTemplateState and applying bindSplice for all the splices we want to add. Then we pass this to loadTemplates our final TemplateState wrapped in an Either to handle errors. Then we use this TemplateState to render our templates.

Heist hooks allow you to modify templates when they are loaded and before and after they are run. Every time you call one of the addAbcHook functions the hook is added to onto the processing pipeline. The hooks processes the template in the order that they were added to the TemplateState.

The pre-run and post-run hooks are run before and after every template is run/rendered. You should be careful what code you put in these hooks because it can significantly affect the performance of your site.