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+-- | A page is a key-value mapping, representing a page on your site
+--
+-- A page is an important concept in Hakyll. It is a key-value mapping, and has
+-- one field with an arbitrary type. A 'Page' thus consists of
+--
+-- * a key-value mapping (of the type @Map String String@);
+--
+-- * a value (of the type @a@).
+--
+-- Usually, the value will be a 'String' as well, and the value will be the body
+-- of the page.
+--
+-- Pages can be constructed using Haskell, but they are usually parsed from a
+-- file. The file format for pages is pretty straightforward.
+--
+-- > This is a simple page
+-- > consisting of two lines.
+--
+-- This is a valid page with two lines. If we load this in Hakyll, there would
+-- be no metadata, and the body would be the given text. Let's look at a page
+-- with some metadata.
+--
+-- > ---
+-- > title: Alice's Adventures in Wonderland
+-- > author: Lewis Caroll
+-- > year: 1865
+-- > ---
+-- >
+-- > Chapter I
+-- > =========
+-- >
+-- > Down the Rabbit-Hole
+-- > --------------------
+-- >
+-- > Alice was beginning to get very tired of sitting by her sister on the bank,
+-- > and of having nothing to do: once or twice she had peeped into the book her
+-- > sister was reading, but it had no pictures or conversations in it, "and
+-- > what is the use of a book," thought Alice "without pictures or
+-- > conversation?"
+-- >
+-- > ...
+--
+-- As you can see, we construct a metadata header in Hakyll using @---@. Then,
+-- we simply list all @key: value@ pairs, and end with @---@ again. This page
+-- contains three metadata fields and a body. The body is given in markdown
+-- format, which can be easily rendered to HTML by Hakyll, using pandoc.
+--
+{-# LANGUAGE DeriveDataTypeable #-}
+module Hakyll.Web.Page
+    ( Page (..)
+    , fromBody
+    , fromMap
+    , toMap
+    , readPageCompiler
+    , pageCompiler
+    , addDefaultFields
+    , sortByBaseName
+    ) where
+
+import Prelude hiding (id)
+import Control.Category (id)
+import Control.Arrow (arr, (>>^), (&&&), (>>>))
+import System.FilePath (takeBaseName, takeDirectory)
+import qualified Data.Map as M
+import Data.List (sortBy)
+import Data.Ord (comparing)
+
+import Hakyll.Core.Identifier
+import Hakyll.Core.Compiler
+import Hakyll.Core.ResourceProvider
+import Hakyll.Web.Page.Internal
+import Hakyll.Web.Page.Read
+import Hakyll.Web.Page.Metadata
+import Hakyll.Web.Pandoc
+import Hakyll.Web.Template
+import Hakyll.Web.Util.Url
+
+-- | Create a page from a body, without metadata
+--
+fromBody :: a -> Page a
+fromBody = Page M.empty
+
+-- | Read a page (do not render it)
+--
+readPageCompiler :: Compiler Resource (Page String)
+readPageCompiler = getResourceString >>^ readPage
+
+-- | Read a page, add default fields, substitute fields and render using pandoc
+--
+pageCompiler :: Compiler Resource (Page String)
+pageCompiler = cached "Hakyll.Web.Page.pageCompiler" $
+    readPageCompiler >>> addDefaultFields >>> arr applySelf >>> pageRenderPandoc
+
+-- | Add a number of default metadata fields to a page. These fields include:
+--
+-- * @$url@
+--
+-- * @$category@
+--
+-- * @$title@
+--
+-- * @$path@
+--
+addDefaultFields :: Compiler (Page a) (Page a)
+addDefaultFields =   (getRoute &&& id >>^ uncurry addRoute)
+                 >>> (getIdentifier &&& id >>^ uncurry addIdentifier)
+  where
+    -- Add root and url, based on route
+    addRoute Nothing  = id
+    addRoute (Just r) = setField "url" (toUrl r)
+
+    -- Add title and category, based on identifier
+    addIdentifier i = setField "title" (takeBaseName p)
+                    . setField "category" (takeBaseName $ takeDirectory p)
+                    . setField "path" p
+      where
+        p = toFilePath i
+
+-- | Sort posts based on the basename of the post. This is equivalent to a
+-- chronologival sort, because of the @year-month-day-title.extension@ naming
+-- convention in Hakyll.
+--
+sortByBaseName :: [Page a] -> [Page a]
+sortByBaseName = sortBy $ comparing $ takeBaseName . getField "path"