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+-- | This module provides means for reading and applying 'Template's.
+--
+-- Templates are tools to convert data (pages) 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>
+--
+-- We can use this template to render a 'Page' which has a body and a @$title$@
+-- metadata field.
+--
+-- 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.
+--
+-- In addition to the native format, Hakyll also supports hamlet templates. For
+-- more information on hamlet templates, please refer to:
+-- <http://hackage.haskell.org/package/hamlet>.
+--
+module Hakyll.Web.Template
+    ( Template
+    , applyTemplate
+    , applySelf
+    , templateCompiler
+    , templateCompilerWith
+    , applyTemplateCompiler
+    ) where
+
+import Control.Arrow
+import Data.Maybe (fromMaybe)
+import System.FilePath (takeExtension)
+import qualified Data.Map as M
+
+import Text.Hamlet (HamletSettings, defaultHamletSettings)
+
+import Hakyll.Core.Compiler
+import Hakyll.Core.Identifier
+import Hakyll.Core.ResourceProvider
+import Hakyll.Web.Template.Internal
+import Hakyll.Web.Template.Read
+import Hakyll.Web.Page.Internal
+
+-- | Substitutes @$identifiers@ in the given @Template@ by values from the given
+-- "Page". When a key is not found, it is left as it is. You can specify
+-- the characters used to replace escaped dollars (@$$@) here.
+--
+applyTemplate :: Template -> Page String -> Page String
+applyTemplate template page =
+    fmap (const $ substitute =<< unTemplate template) page
+  where
+    map' = toMap page
+    substitute (Chunk chunk) = chunk
+    substitute (Key key) = fromMaybe ("$" ++ key ++ "$") $ M.lookup key map'
+    substitute (Escaped) = "$"
+
+-- | Apply a page as it's own template. This is often very useful to fill in
+-- certain keys like @$root@ and @$url@.
+--
+applySelf :: Page String -> Page String
+applySelf page = applyTemplate (readTemplate $ pageBody page) page
+
+-- | Read a template. If the extension of the file we're compiling is
+-- @.hml@ or @.hamlet@, it will be considered as a Hamlet template, and parsed
+-- as such.
+--
+templateCompiler :: Compiler Resource Template
+templateCompiler = templateCompilerWith defaultHamletSettings
+
+-- | Version of 'templateCompiler' that enables custom settings.
+--
+templateCompilerWith :: HamletSettings -> Compiler Resource Template
+templateCompilerWith settings =
+    cached "Hakyll.Web.Template.templateCompilerWith" $
+        getIdentifier &&& getResourceString >>^ uncurry read'
+  where
+    read' identifier string =
+        if takeExtension (toFilePath identifier) `elem` [".hml", ".hamlet"]
+            -- Hamlet template
+            then readHamletTemplateWith settings string
+            -- Hakyll template
+            else readTemplate string
+
+applyTemplateCompiler :: Identifier                            -- ^ Template
+                      -> Compiler (Page String) (Page String)  -- ^ Compiler
+applyTemplateCompiler identifier = require identifier (flip applyTemplate)