content: --------------------------------------------------------------------------------
  -- | This module provides a declarative Domain Specific Language (DSL) to
  -- generate a static site by specifying transformation 'Rules' (although the
  -- use case is not limited to static sites).
  -- Each rule normally consists of three parts:
  -- 
  -- 1. Source files (like Markdown files) to process (collected with e.g.
  -- 'match' or 'create').
  -- 2. Compilation steps (like Markdown to HTML) to transform files' content
  -- to some output content (steps are collected within 
  -- 'Hakyll.Core.Compiler.Compiler' and executed with 'compile').
  -- 3. Routing to determine if and where an output content will be written out. 
  -- For a static site this determines under which URL the output content will 
  -- be available (configured with 'route' and 'Hakyll.Core.Routes.Routes').
  -- 
  -- A typical usage example looks as follows:
  -- 
  -- > -- write 'match "posts/**.md"' instead of 'match $ fromGlob "posts/**.md"'
  -- > {-# LANGUAGE OverloadedStrings #-}    
  -- > ...
  -- >
  -- > main = hakyll $ do
  -- >
  -- >     -- Rule 1
  -- >     -- Source files: all Markdown files like 'hakyll.md' in the 'posts' directory
  -- >     match "posts/**.md" $ do          
  -- >         -- Routing: Only replace extension, so '<destination-directory>/posts/hakyll.html'.
  -- >         route $ setExtension "html"   
  -- >         -- Compilation step(s): Transform Markdown file content into HTML output.
  -- >         compile pandocCompiler        
  -- >
  -- >     -- Rule 2
  -- >     match "css/*" $ do
  -- >         route idRoute
  -- >         compile compressCssCompiler
  -- >     ...
  -- __The order of rules doesn't matter.__ 
  -- 
  -- See official [Hakyll Rules tutorial](https://jaspervdj.be/hakyll/tutorials/03-rules-routes-compilers.html)
  -- for other examples.
  {-# LANGUAGE GeneralizedNewtypeDeriving #-}
  {-# LANGUAGE OverloadedStrings          #-}
  module Hakyll.Core.Rules
      ( Rules
      , match
      , matchMetadata
      , create
      , version
      , compile
      , route
  
        -- * Advanced usage
      , preprocess
      , Dependency (..)
      , rulesExtraDependencies
      ) where
  
  
  --------------------------------------------------------------------------------
  import           Control.Monad.Reader           (ask, local)
  import           Control.Monad.State            (get, modify, put)
  import           Control.Monad.Trans            (liftIO)
  import           Control.Monad.Writer           (censor, tell)
  import           Data.Maybe                     (fromMaybe)
  import qualified Data.Set                       as S
  
  
  --------------------------------------------------------------------------------
  import           Data.Binary                    (Binary)
  import           Data.Typeable                  (Typeable)
  
  
  --------------------------------------------------------------------------------
  import           Hakyll.Core.Compiler.Internal
  import           Hakyll.Core.Dependencies
  import           Hakyll.Core.Identifier
  import           Hakyll.Core.Identifier.Pattern
  import           Hakyll.Core.Item
  import           Hakyll.Core.Item.SomeItem
  import           Hakyll.Core.Metadata
  import           Hakyll.Core.Routes
  import           Hakyll.Core.Rules.Internal
  import           Hakyll.Core.Writable
  
  
  --------------------------------------------------------------------------------
  -- | Add a route
  tellRoute :: Routes -> Rules ()
  tellRoute route' = Rules $ tell $ RuleSet route' mempty mempty mempty
  
  
  --------------------------------------------------------------------------------
  -- | Add a number of compilers
  tellCompilers :: [(Identifier, Compiler SomeItem)] -> Rules ()
  tellCompilers compilers = Rules $ tell $ RuleSet mempty compilers mempty mempty
  
  
  --------------------------------------------------------------------------------
  -- | Add resources
  tellResources :: [Identifier] -> Rules ()
  tellResources resources' = Rules $ tell $
      RuleSet mempty mempty (S.fromList resources') mempty
  
  
  --------------------------------------------------------------------------------
  -- | Add a pattern
  tellPattern :: Pattern -> Rules ()
  tellPattern pattern = Rules $ tell $ RuleSet mempty mempty mempty pattern
  
  
  --------------------------------------------------------------------------------
  flush :: Rules ()
  flush = Rules $ do
      mcompiler <- rulesCompiler <$> get
      case mcompiler of
          Nothing       -> return ()
          Just compiler -> do
              matches' <- rulesMatches                  <$> ask
              version' <- rulesVersion                  <$> ask
              route'   <- fromMaybe mempty . rulesRoute <$> get
  
              -- The version is possibly not set correctly at this point (yet)
              let ids = map (setVersion version') matches'
  
              {-
              ids      <- case fromLiteral pattern of
                  Just id' -> return [setVersion version' id']
                  Nothing  -> do
                      ids <- unRules $ getMatches pattern
                      unRules $ tellResources ids
                      return $ map (setVersion version') ids
              -}
  
              -- Create a fast pattern for routing that matches exactly the
              -- compilers created in the block given to match
              let fastPattern = fromList ids
  
              -- Write out the compilers and routes
              unRules $ tellRoute $ matchRoute fastPattern route'
              unRules $ tellCompilers $ [(id', compiler) | id' <- ids]
  
      put $ emptyRulesState
  
  
  --------------------------------------------------------------------------------
  matchInternal :: Pattern -> Rules [Identifier] -> Rules () -> Rules ()
  matchInternal pattern getIDs rules = do
      tellPattern pattern
      flush
      ids <- getIDs
      tellResources ids
      Rules $ local (setMatches ids) $ unRules $ rules >> flush
    where
      setMatches ids env = env {rulesMatches = ids}
  
  --------------------------------------------------------------------------------
  {- | Add a selection of which source files to process (using the
  given [glob pattern](https://en.wikipedia.org/wiki/Glob_(programming\))) to the
  given remaining 'Rules' value.
  
  The expanded, relative path of the matched source file on disk (relative to the
  project directory configured with 'Hakyll.Core.Configuration.providerDirectory')
  becomes the identifier under which the compilation result is saved to the
  'Hakyll.Core.Store.Store' (in case you want to 'Hakyll.Core.Compiler.load' it
  within another rule).
  See 'Hakyll.Core.Identifier.Identifier' for details.
  
  === __Examples__
  
  __Select all markdown files within a directory (but without subdirectories)__
  
  > -- Match all Markdown files in the immediate 'posts' directory
  > -- e.g. '<project-directory>/posts/hakyll.md'
  > -- but NOT  '<project-directory>/posts/haskell/monad.md'
  > match "posts/*.md" $ do
  >     route $ setExtension "html"
  >     compile pandocCompiler
  
  __Select all markdown files within a directory (including subdirectories recursively)__
  
  > -- Match all Markdown files in the 'posts' directory and any subdirectory
  > -- e.g. '<project-directory>/posts/hakyll.md'
  > -- and  '<project-directory>/posts/haskell/monad.md'
  > match "posts/**.md" $ do
  >     route $ setExtension "html"
  >     compile pandocCompiler
  See 'Hakyll.Core.Identifier.Pattern.Pattern' or search "glob patterns" online
  for more details. To control where the compilation result will be written out,
  use routing functions like 'Hakyll.Core.Routes.setExtension'.
  -}
  match :: Pattern  -- ^ Glob pattern
        -> Rules () -- ^ Remaining processing parts
        -> Rules () -- ^ Result
  match pattern = matchInternal pattern $ getMatches pattern
  
  
  --------------------------------------------------------------------------------
  {- | Add a selection of which source files to process (using the
  given [glob pattern](https://en.wikipedia.org/wiki/Glob_(programming\)) and
  metadata predicate) to the given remaining 'Rules' values. Same as 'match' but
  allows to filter files further based on their (metadata) content (a file is
  added only when the metadata predicate returns @True@).
  
  The expanded, relative path of the matched source file on disk (relative to the
  project directory configured with 'Hakyll.Core.Configuration.providerDirectory')
  becomes the identifier under which the compilation result is saved to
  the 'Hakyll.Core.Store.Store' (in case you want to 'Hakyll.Core.Compiler.load'
  it within another rule).
  See 'Hakyll.Core.Identifier.Identifier' for details.
  
  === __Examples__
  __Select all markdown files with enabled draft flag within a directory__
  
  > matchMetadata "posts/*.md" (\meta -> maybe False (=="true") $ lookupString "draft" meta) $ do
  >     route $ setExtension "html"
  >     compile pandocCompiler
  
  For example, the following 'posts/hakyll.md' file with @draft: true@ metadata
  would match:
  
  > ---
  > draft: true
  > title: Hakyll Post
  > ...
  > ---
  > In this blog post we learn about Hakyll ...
  Note that files that have @draft: false@ or no such draft field at all, would
  not match. You can use helper functions like 'Hakyll.Core.Metadata.lookupString'
  to access a specific metadata field, and 'Data.Maybe.maybe' to work with
  'Data.Maybe.Maybe'. To control where the compilation result will be written out,
  use routing functions like 'Hakyll.Core.Routes.setExtension'.
  -}
  matchMetadata :: Pattern             -- ^ Glob pattern
                -> (Metadata -> Bool)  -- ^ Metadata predicate
                -> Rules ()            -- ^ Remaining processing parts
                -> Rules ()            -- ^ Result
  matchMetadata pattern metadataPred = matchInternal pattern $
      map fst . filter (metadataPred . snd) <$> getAllMetadata pattern
  
  
  --------------------------------------------------------------------------------
  {- | Assign (and thereby create) the given identifier(s) to content that has no
  underlying source file on disk. That content must be created within the
  'compile' part of the given remaining 'Rules' value. The given identifier is the
  id under which the compilation is saved to the 'Hakyll.Core.Store.Store' (in
  case you want to 'Hakyll.Core.Compiler.load' it within another rule).
  See 'Hakyll.Core.Identifier.Identifier' for details.
  
  Use this function for example to create an overview page that doesn't have or
  need its content prepared in a file (unlike blog posts which normally have a
  corresponding Markdown source file on disk).
  
  === __Examples__
  __Create a webpage without an underlying source file__
  
  > -- saved with implicit identifier 'index.html' to Store
  > create ["index.html"] $ do
  >
  >     -- compilation result is written to '<destination-directory>/index.html'
  >     route idRoute
  >
  >     -- create content without a source file from disk
  >     compile $ makeItem ("<h1>Hello World</h1>" :: String)
  Note how you can use 'Hakyll.Core.Compiler.makeItem' to create content inline
  (to be processed as a 'Hakyll.Core.Compiler.Compiler' value) as if that content
  was loaded from a file (as it's the case when using 'match').
  To control where the compilation result will be written out, use routing
  functions like 'Hakyll.Core.Routes.idRoute'.
  -}
  create :: [Identifier] -- ^ Identifiers to assign to created content in next argument
         -> Rules ()     -- ^ Remaining processing parts that must create content
         -> Rules ()     -- ^ Resulting rule
  create ids rules = do
      flush
      -- TODO Maybe check if the resources exist and call tellResources on that
      Rules $ local setMatches $ unRules $ rules >> flush
    where
      setMatches env = env {rulesMatches = ids}
  
  
  --------------------------------------------------------------------------------
  {- | Add the given version name to the implicit identifier(s) under which the
  compilation result of the given remaining 'Rules' value is saved to the
  'Hakyll.Core.Store.Store'.
  See 'Hakyll.Core.Identifier.Identifier' for details.
  
  Use this wrapper function for example when you need to compile the same source
  file into two or more different results, each with a different version name.
  The version is needed to distinguish between these different compilation results
  in the store, otherwise they would get the same conflicting identifier in the
  store.
  
  Warning:
  __If you add a version name with this function, you need to supply the same name__
  when you 'Hakyll.Core.Compiler.load' the content from the store from
  within another rule.
  
  === __Examples__
  __Compile source file into differently versioned outputs and load both__
  
  > -- e.g. file on disk: 'posts/hakyll.md'
  >
  > -- saved with implicit identifier ('posts/hakyll.md', no-version)
  > match "posts/*" $ do
  >     route $ setExtension "html"
  >     compile pandocCompiler
  >
  > -- saved with implicit identifier ('posts/hakyll.md', version 'raw')
  > match "posts/*" $ version "raw" $ do
  >     route idRoute
  >     compile getResourceBody
  >
  > -- use compilation results from rules above
  > create ["index.html"] $ do
  >     route idRoute
  >     compile $ do
  >         -- load no-version version
  >         compiledPost <- load (fromFilePath "posts/hakyll.md")
  >         -- load version 'raw'
  >         rawPost <- load . setVersion (Just "raw") $ fromFilePath "posts/hakyll.md"
  >         ...
  Note how a version name is needed to distinguish the unversioned and the "raw"
  version when loading the Hakyll post for the @index.html@ page.
  To control where the compilation result will be written out, use routing
  functions like 'Hakyll.Core.Routes.idRoute' and 'Hakyll.Core.Routes.setExtension'.
  -}
  version :: String   -- ^ Version name to add
          -> Rules () -- ^ Remaining processing parts
          -> Rules () -- ^ Result
  version v rules = do
      flush
      Rules $ local setVersion' $ unRules $ rules >> flush
    where
      setVersion' env = env {rulesVersion = Just v}
  
  
  --------------------------------------------------------------------------------
  {- | Add (or replace) the given compilation steps within the given
  'Hakyll.Core.Compiler.Compiler' value to the current 'Rules' value.
  __This functions controls HOW the content within a rule is processed__ (use one
  of the 'match' functions to control WHAT content is processed).
  
  The compilation result is saved to the 'Hakyll.Core.Store.Store' under an
  implicit identifier.
  See 'Hakyll.Core.Identifier.Identifier' for details.
  
  If there's routing attached to the rule where this function is used, the
  compilation result is also written out to a file according to that route.
  See 'route' and 'Hakyll.Core.Routes.Routes' for details.
  
  === __Examples__
  __Compile Markdown to HTML__
  
  > -- Select all Markdown files in 'posts' directory
  > match "posts/**.md" $ do
  >
  >     route $ setExtension "html"
  >
  >     -- use pandoc to transform Markdown to HTML in a single step
  >     compile pandocCompiler
  Note how we set the content to be processed with
  'Hakyll.Web.Pandoc.pandocCompiler'. The content comes implicitly from the
  matched Markdown files on disk. We don't have to pass that content around
  manually. Every file is processed the same way within this one rule.
  
  To control where the compilation result will be written out, use routing
  functions like 'Hakyll.Core.Routes.setExtension'.
  Here the compilation result of a file like @posts\/hakyll.md@ is written out
  to @posts\/hakyll.html@.
  
  __Compile Markdown to HTML and embed it in a template__
  
  > -- Select all Markdown files in 'posts' directory
  > match "posts/**.md" $ do
  >     route $ setExtension "html"
  >     compile $
  >         pandocCompiler >>=
  >           loadAndApplyTemplate "templates/post.html" defaultContext
  >
  > -- To Hakyll templates are just plain files that have to be processed
  > -- and placed into the store like any other file (but without routing).
  > -- e.g. file on disk: 'templates/post.html'
  > match "templates/*" $ compile templateBodyCompiler
  Note how a Markdown post that is compiled to HTML using
  'Hakyll.Web.Pandoc.pandocCompiler' in a first step and then embedded into
  a HTMl 'Hakyll.Web.Template.Template' in a second step by using
  'Hakyll.Web.Template.loadAndApplyTemplate'.
  We can use templates to control the design and layout of a webpage.
  A template may look as follows:
  
  > <h1>$title$</h1>
  > $body$
  See "Hakyll.Web.Template" to see examples of the templating syntax.
  -}
  compile :: (Binary a, Typeable a, Writable a) => Compiler (Item a) -- ^ How to transform content
                                                -> Rules ()          -- ^ Result
  compile compiler = Rules $ modify $ \s ->
      s {rulesCompiler = Just (fmap SomeItem compiler)}
  
  
  --------------------------------------------------------------------------------
  {- | Add (or replace) routing in the current 'Rules' value.
  __This functions controls IF and WHERE the compiled results are written out__
  (use one of the 'match' functions to control WHAT content is processed and
  'compile' to control HOW).
  See 'Hakyll.Core.Routes.Routes' and 'Hakyll.Core.Identifier.Identifier' for
  details on how output filepaths are computed.
  
  Hint:
  __If there's no route attached to a rule, the compilation result is not written out__.
  However, the compilation result is saved to the 'Hakyll.Core.Store.Store'
  and can be loaded and used within another rule. This behavior is needed,
  for example, for templates.
  
  === __Examples__
  __Rules with and without routing__
  
  > -- e.g. file on disk: 'templates/post.html'
  >
  > -- Rule 1 (without routing)
  > match "templates/*" $ do
  >     -- compilation result saved to store with implicit identifier, e.g. 'templates/post.html'
  >     compile templateCompiler
  >
  > -- Rule 2 (with routing)
  > match "posts/**.md" $ do
  >     route $ setExtension "html"
  >     compile $ do
  >        -- load compiled result of other rule with explicit identifier.
  >        postTemplate <- loadBody "templates/post.html"
  >        pandocCompiler >>= applyTemplate postTemplate defaultContext
  Note that we don't set a route in the first rule to avoid writing out our
  compiled templates.
  However, we can still 'Hakyll.Core.Compiler.load' (or
  'Hakyll.Core.Compiler.loadBody') the compiled templates to apply them in a
  second rule.
  The content for 'Hakyll.Web.Template.templateCompiler' comes implicitly from the
  matched template files on disk. We don't have to pass that content around
  manually. See 'match' and 'compile' for details.
  
  To control where a compilation result will be written out (as done in the second
  rule), use routing functions like 'Hakyll.Core.Routes.setExtension'.
  
  See "Hakyll.Web.Template" for examples of templates and the templating syntax.
  -}
  route :: Routes   -- ^ Where to output compilation results
        -> Rules () -- ^ Result
  route route' = Rules $ modify $ \s -> s {rulesRoute = Just route'}
  
  
  --------------------------------------------------------------------------------
  -- | Execute an 'IO' action immediately while the rules are being evaluated.
  -- This should be avoided if possible, but occasionally comes in useful.
  preprocess :: IO a -> Rules a
  preprocess = Rules . liftIO
  
  
  --------------------------------------------------------------------------------
  -- | Advanced usage: add extra dependencies to compilers. Basically this is
  -- needed when you're doing unsafe tricky stuff in the rules monad, but you
  -- still want correct builds.
  --
  -- A useful utility for this purpose is 'makePatternDependency'.
  rulesExtraDependencies :: [Dependency] -> Rules a -> Rules a
  rulesExtraDependencies deps rules =
      -- Note that we add the dependencies seemingly twice here. However, this is
      -- done so that 'rulesExtraDependencies' works both if we have something
      -- like:
      --
      -- > match "*.css" $ rulesExtraDependencies [foo] $ ...
      --
      -- and something like:
      --
      -- > rulesExtraDependencies [foo] $ match "*.css" $ ...
      --
      -- (1) takes care of the latter and (2) of the former.
      Rules $ censor fixRuleSet $ do
          x <- unRules rules
          fixCompiler
          return x
    where
      -- (1) Adds the dependencies to the compilers we are yet to create
      fixCompiler = modify $ \s -> case rulesCompiler s of
          Nothing -> s
          Just c  -> s
              { rulesCompiler = Just $ compilerTellDependencies deps >> c
              }
  
      -- (2) Adds the dependencies to the compilers that are already in the ruleset
      fixRuleSet ruleSet = ruleSet
          { rulesCompilers =
              [ (i, compilerTellDependencies deps >> c)
              | (i, c) <- rulesCompilers ruleSet
              ]
          }
  