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|
-- | This module provides means for reading and applying 'Template's.
--
-- Templates are tools to convert items into a string. They are perfectly suited
-- for laying out your site.
--
-- Let's look at an example template:
--
-- > <html>
-- > <head>
-- > <title>My crazy homepage - $title$</title>
-- > </head>
-- > <body>
-- > <div id="header">
-- > <h1>My crazy homepage - $title$</h1>
-- > </div>
-- > <div id="content">
-- > $body$
-- > </div>
-- > <div id="footer">
-- > By reading this you agree that I now own your soul
-- > </div>
-- > </body>
-- > </html>
--
-- As you can see, the format is very simple -- @$key$@ is used to render the
-- @$key$@ field from the page, everything else is literally copied. If you want
-- to literally insert @\"$key$\"@ into your page (for example, when you're
-- writing a Hakyll tutorial) you can use
--
-- > <p>
-- > A literal $$key$$.
-- > </p>
--
-- Because of it's simplicity, these templates can be used for more than HTML:
-- you could make, for example, CSS or JS templates as well.
--
-- Apart from interpolating @$key$@s from the 'Context' you can also
-- use the following macros:
--
-- * @$if(key)$@
--
-- > $if(key)$
-- > <b> Defined </b>
-- > $else$
-- > <b> Non-defined </b>
-- > $endif$
--
-- This example will print @Defined@ if @key@ is defined in the
-- context and @Non-defined@ otherwise. The @$else$@ clause is
-- optional.
--
-- * @$for(key)$@
--
-- The @for@ macro is used for enumerating 'Context' elements that are
-- lists, i.e. constructed using the 'listField' function. Assume that
-- in a context we have an element @listField \"key\" c itms@. Then
-- the snippet
--
-- > $for(key)$
-- > $x$
-- > $sep$,
-- > $endfor$
--
-- would, for each item @i@ in 'itms', lookup @$x$@ in the context @c@
-- with item @i@, interpolate it, and join the resulting list with
-- @,@.
--
-- Another concrete example one may consider is the following. Given the
-- context
--
-- > listField "things" (field "thing" (return . itemBody))
-- > (sequence [makeItem "fruits", makeItem "vegetables"])
--
-- and a template
--
-- > I like
-- > $for(things)$
-- > fresh $thing$$sep$, and
-- > $endfor$
--
-- the resulting page would look like
--
-- > <p>
-- > I like
-- >
-- > fresh fruits, and
-- >
-- > fresh vegetables
-- > </p>
--
-- The @$sep$@ part can be omitted. Usually, you can get by using the
-- 'applyListTemplate' and 'applyJoinListTemplate' functions.
--
-- * @$partial(path)$@
--
-- Loads a template located in a separate file and interpolates it
-- under the current context.
--
-- Assuming that the file @test.html@ contains
--
-- > <b>$key$</b>
--
-- The result of rendering
--
-- > <p>
-- > $partial("test.html")$
-- > </p>
--
-- is the same as the result of rendering
--
-- > <p>
-- > <b>$key$</b>
-- > </p>
--
-- That is, calling @$partial$@ is equivalent to just copying and pasting
-- template code.
--
module Hakyll.Web.Template
( Template
, templateCompiler
, applyTemplate
, loadAndApplyTemplate
, applyAsTemplate
, readTemplate
) where
--------------------------------------------------------------------------------
import Control.Monad (liftM)
import Control.Monad.Error (MonadError (..))
import Data.List (intercalate)
import Data.Monoid (mappend)
import Prelude hiding (id)
--------------------------------------------------------------------------------
import Hakyll.Core.Compiler
import Hakyll.Core.Identifier
import Hakyll.Core.Item
import Hakyll.Web.Template.Context
import Hakyll.Web.Template.Internal
--------------------------------------------------------------------------------
-- | Read a template.
templateCompiler :: Compiler (Item Template)
templateCompiler = cached "Hakyll.Web.Template.templateCompiler" $ do
item <- getResourceString
return $ fmap readTemplate item
--------------------------------------------------------------------------------
applyTemplate :: Template -- ^ Template
-> Context a -- ^ Context
-> Item a -- ^ Page
-> Compiler (Item String) -- ^ Resulting item
applyTemplate tpl context item = do
body <- applyTemplate' tpl context item
return $ itemSetBody body item
--------------------------------------------------------------------------------
applyTemplate' :: Template -- ^ Template
-> Context a -- ^ Context
-> Item a -- ^ Page
-> Compiler String -- ^ Resulting item
applyTemplate' tpl context x = go tpl
where
context' = unContext (context `mappend` missingField)
go = liftM concat . mapM applyElem . unTemplate
applyElem (Chunk c) = return c
applyElem Escaped = return "$"
applyElem (Key k) = context' k x >>= getString k
applyElem (If k t mf) = (context' k x >> go t) `catchError` handler
where
handler _ = case mf of
Nothing -> return ""
Just f -> go f
applyElem (For k b s) = context' k x >>= \cf -> case cf of
StringField _ -> fail $
"Hakyll.Web.Template.applyTemplateWith: expected ListField but " ++
"got StringField for key " ++ show k
ListField c xs -> do
sep <- maybe (return "") go s
bs <- mapM (applyTemplate' b c) xs
return $ intercalate sep bs
applyElem (Partial p) = do
tpl' <- loadBody (fromFilePath p)
applyTemplate' tpl' context x
getString _ (StringField s) = return s
getString k (ListField _ _) = fail $
"Hakyll.Web.Template.applyTemplateWith: expected StringField but " ++
"got ListField for key " ++ show k
--------------------------------------------------------------------------------
-- | The following pattern is so common:
--
-- > tpl <- loadBody "templates/foo.html"
-- > someCompiler
-- > >>= applyTemplate tpl context
--
-- That we have a single function which does this:
--
-- > someCompiler
-- > >>= loadAndApplyTemplate "templates/foo.html" context
loadAndApplyTemplate :: Identifier -- ^ Template identifier
-> Context a -- ^ Context
-> Item a -- ^ Page
-> Compiler (Item String) -- ^ Resulting item
loadAndApplyTemplate identifier context item = do
tpl <- loadBody identifier
applyTemplate tpl context item
--------------------------------------------------------------------------------
-- | It is also possible that you want to substitute @$key$@s within the body of
-- an item. This function does that by interpreting the item body as a template,
-- and then applying it to itself.
applyAsTemplate :: Context String -- ^ Context
-> Item String -- ^ Item and template
-> Compiler (Item String) -- ^ Resulting item
applyAsTemplate context item =
let tpl = readTemplate $ itemBody item
in applyTemplate tpl context item
|
