{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes       #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE CPP               #-}
module Yesod.Core.Class.Yesod where

import           Control.Monad.Logger               (logErrorS)
import           Yesod.Core.Content
import           Yesod.Core.Handler

import           Yesod.Routes.Class

import           Blaze.ByteString.Builder           (Builder)
import           Blaze.ByteString.Builder.Char.Utf8 (fromText)
import           Control.Arrow                      ((***), second)
import           Control.Monad                      (forM, when, void)
import           Control.Monad.IO.Class             (MonadIO (liftIO))
import           Control.Monad.Logger               (LogLevel (LevelInfo, LevelOther),
                                                     LogSource)
import qualified Data.ByteString.Char8              as S8
import qualified Data.ByteString.Lazy               as L
import Data.Aeson (object, (.=))
import           Data.List                          (foldl')
import           Data.List                          (nub)
import qualified Data.Map                           as Map
import           Data.Maybe                         (fromMaybe)
import           Data.Monoid
import           Data.Text                          (Text)
import qualified Data.Text                          as T
import qualified Data.Text.Encoding                 as TE
import qualified Data.Text.Encoding.Error           as TEE
import           Data.Text.Lazy.Builder             (toLazyText)
import           Data.Text.Lazy.Encoding            (encodeUtf8)
import           Data.Word                          (Word64)
import           Language.Haskell.TH.Syntax         (Loc (..))
import           Network.HTTP.Types                 (encodePath)
import qualified Network.Wai                        as W
import           Data.Default                       (def)
import           Network.Wai.Parse                  (lbsBackEnd,
                                                     tempFileBackEnd)
import           System.IO                          (stdout)
#if MIN_VERSION_fast_logger(2, 0, 0)
import           Network.Wai.Logger                 (ZonedDate, clockDateCacher)
import           System.Log.FastLogger
import qualified GHC.IO.FD
#else
import           System.Log.FastLogger.Date         (ZonedDate)
import           System.Log.FastLogger              (LogStr (..), Logger,
                                                     loggerDate, loggerPutStr,
                                                     mkLogger)
#endif
import           Text.Blaze                         (customAttribute, textTag,
                                                     toValue, (!))
import           Text.Blaze                         (preEscapedToMarkup)
import qualified Text.Blaze.Html5                   as TBH
import           Text.Hamlet
import           Text.Julius
import qualified Web.ClientSession                  as CS
import           Web.Cookie                         (parseCookies)
import           Web.Cookie                         (SetCookie (..))
import           Yesod.Core.Types
import           Yesod.Core.Internal.Session
import           Yesod.Core.Widget
import Control.Monad.Trans.Class (lift)

-- | Define settings for a Yesod applications. All methods have intelligent
-- defaults, and therefore no implementation is required.
class RenderRoute site => Yesod site where
    -- | An absolute URL to the root of the application. Do not include
    -- trailing slash.
    --
    -- Default value: 'ApprootRelative'. This is valid under the following
    -- conditions:
    --
    -- * Your application is served from the root of the domain.
    --
    -- * You do not use any features that require absolute URLs, such as Atom
    -- feeds and XML sitemaps.
    --
    -- If this is not true, you should override with a different
    -- implementation.
    approot :: Approot site
    approot = ApprootRelative

    -- | Output error response pages.
    --
    -- Default value: 'defaultErrorHandler'.
    errorHandler :: ErrorResponse -> HandlerT site IO TypedContent
    errorHandler = defaultErrorHandler

    -- | Applies some form of layout to the contents of a page.
    defaultLayout :: WidgetT site IO () -> HandlerT site IO Html
    defaultLayout w = do
        p <- widgetToPageContent w
        mmsg <- getMessage
        giveUrlRenderer [hamlet|
            $newline never
            $doctype 5
            <html>
                <head>
                    <title>#{pageTitle p}
                    ^{pageHead p}
                <body>
                    $maybe msg <- mmsg
                        <p .message>#{msg}
                    ^{pageBody p}
            |]

    -- | Override the rendering function for a particular URL. One use case for
    -- this is to offload static hosting to a different domain name to avoid
    -- sending cookies.
    urlRenderOverride :: site -> Route site -> Maybe Builder
    urlRenderOverride _ _ = Nothing

    -- | Determine if a request is authorized or not.
    --
    -- Return 'Authorized' if the request is authorized,
    -- 'Unauthorized' a message if unauthorized.
    -- If authentication is required, return 'AuthenticationRequired'.
    isAuthorized :: Route site
                 -> Bool -- ^ is this a write request?
                 -> HandlerT site IO AuthResult
    isAuthorized _ _ = return Authorized

    -- | Determines whether the current request is a write request. By default,
    -- this assumes you are following RESTful principles, and determines this
    -- from request method. In particular, all except the following request
    -- methods are considered write: GET HEAD OPTIONS TRACE.
    --
    -- This function is used to determine if a request is authorized; see
    -- 'isAuthorized'.
    isWriteRequest :: Route site -> HandlerT site IO Bool
    isWriteRequest _ = do
        wai <- waiRequest
        return $ W.requestMethod wai `notElem`
            ["GET", "HEAD", "OPTIONS", "TRACE"]

    -- | The default route for authentication.
    --
    -- Used in particular by 'isAuthorized', but library users can do whatever
    -- they want with it.
    authRoute :: site -> Maybe (Route site)
    authRoute _ = Nothing

    -- | A function used to clean up path segments. It returns 'Right' with a
    -- clean path or 'Left' with a new set of pieces the user should be
    -- redirected to. The default implementation enforces:
    --
    -- * No double slashes
    --
    -- * There is no trailing slash.
    --
    -- Note that versions of Yesod prior to 0.7 used a different set of rules
    -- involing trailing slashes.
    cleanPath :: site -> [Text] -> Either [Text] [Text]
    cleanPath _ s =
        if corrected == s
            then Right $ map dropDash s
            else Left corrected
      where
        corrected = filter (not . T.null) s
        dropDash t
            | T.all (== '-') t = T.drop 1 t
            | otherwise = t

    -- | Builds an absolute URL by concatenating the application root with the
    -- pieces of a path and a query string, if any.
    -- Note that the pieces of the path have been previously cleaned up by 'cleanPath'.
    joinPath :: site
             -> T.Text -- ^ application root
             -> [T.Text] -- ^ path pieces
             -> [(T.Text, T.Text)] -- ^ query string
             -> Builder
    joinPath _ ar pieces' qs' =
        fromText ar `mappend` encodePath pieces qs
      where
        pieces = if null pieces' then [""] else map addDash pieces'
        qs = map (TE.encodeUtf8 *** go) qs'
        go "" = Nothing
        go x = Just $ TE.encodeUtf8 x
        addDash t
            | T.all (== '-') t = T.cons '-' t
            | otherwise = t

    -- | This function is used to store some static content to be served as an
    -- external file. The most common case of this is stashing CSS and
    -- JavaScript content in an external file; the "Yesod.Widget" module uses
    -- this feature.
    --
    -- The return value is 'Nothing' if no storing was performed; this is the
    -- default implementation. A 'Just' 'Left' gives the absolute URL of the
    -- file, whereas a 'Just' 'Right' gives the type-safe URL. The former is
    -- necessary when you are serving the content outside the context of a
    -- Yesod application, such as via memcached.
    addStaticContent :: Text -- ^ filename extension
                     -> Text -- ^ mime-type
                     -> L.ByteString -- ^ content
                     -> HandlerT site IO (Maybe (Either Text (Route site, [(Text, Text)])))
    addStaticContent _ _ _ = return Nothing

    -- | Maximum allowed length of the request body, in bytes.
    --
    -- If @Nothing@, no maximum is applied.
    --
    -- Default: 2 megabytes.
    maximumContentLength :: site -> Maybe (Route site) -> Maybe Word64
    maximumContentLength _ _ = Just $ 2 * 1024 * 1024 -- 2 megabytes

    -- | Creates a @Logger@ to use for log messages.
    --
    -- Note that a common technique (endorsed by the scaffolding) is to create
    -- a @Logger@ value and place it in your foundation datatype, and have this
    -- method return that already created value. That way, you can use that
    -- same @Logger@ for printing messages during app initialization.
    --
    -- Default: Sends to stdout and automatically flushes on each write.
    makeLogger :: site -> IO Logger
#if MIN_VERSION_fast_logger(2, 0, 0)
    makeLogger _ = do
#if MIN_VERSION_fast_logger(2, 1, 0)
        loggerSet <- newLoggerSet defaultBufSize Nothing
#else
        loggerSet <- newLoggerSet defaultBufSize GHC.IO.FD.stdout
#endif
        (getter, _) <- clockDateCacher
        return $! Logger loggerSet getter
#else
    makeLogger _ = mkLogger True stdout
#endif

    -- | Send a message to the @Logger@ provided by @getLogger@.
    --
    -- Default implementation: checks if the message should be logged using
    -- 'shouldLog' and, if so, formats using 'formatLogMessage'.
    messageLoggerSource :: site
                        -> Logger
                        -> Loc -- ^ position in source code
                        -> LogSource
                        -> LogLevel
                        -> LogStr -- ^ message
                        -> IO ()
    messageLoggerSource a logger loc source level msg = do
        sl <- shouldLogIO a source level
        when sl $
            formatLogMessage (loggerDate logger) loc source level msg >>= loggerPutStr logger

    -- | Where to Load sripts from. We recommend the default value,
    -- 'BottomOfBody'.  Alternatively use the built in async yepnope loader:
    --
    -- > BottomOfHeadAsync $ loadJsYepnope $ Right $ StaticR js_modernizr_js
    --
    -- Or write your own async js loader.
    jsLoader :: site -> ScriptLoadPosition site
    jsLoader _ = BottomOfBody

    -- | Create a session backend. Returning 'Nothing' disables
    -- sessions. If you'd like to change the way that the session
    -- cookies are created, take a look at
    -- 'customizeSessionCookies'.
    --
    -- Default: Uses clientsession with a 2 hour timeout.
    makeSessionBackend :: site -> IO (Maybe SessionBackend)
    makeSessionBackend _ = fmap Just $ defaultClientSessionBackend 120 CS.defaultKeyFile

    -- | How to store uploaded files.
    --
    -- Default: When the request body is greater than 50kb, store in a temp
    -- file. For chunked request bodies, store in a temp file. Otherwise, store
    -- in memory.
    fileUpload :: site -> W.RequestBodyLength -> FileUpload
    fileUpload _ (W.KnownLength size)
        | size <= 50000 = FileUploadMemory lbsBackEnd
    fileUpload _ _ = FileUploadDisk tempFileBackEnd

    -- | Should we log the given log source/level combination.
    --
    -- Default: Logs everything at or above 'logLevel'
    shouldLog :: site -> LogSource -> LogLevel -> Bool
    shouldLog _ _ level = level >= LevelInfo

    -- | Should we log the given log source/level combination.
    --
    -- Note that this is almost identical to @shouldLog@, except the result
    -- lives in @IO@. This allows you to dynamically alter the logging level of
    -- your application by having this result depend on, e.g., an @IORef@.
    --
    -- The default implementation simply uses @shouldLog@. Future versions of
    -- Yesod will remove @shouldLog@ and use this method exclusively.
    --
    -- Since 1.2.4
    shouldLogIO :: site -> LogSource -> LogLevel -> IO Bool
    shouldLogIO a b c = return (shouldLog a b c)

    -- | A Yesod middleware, which will wrap every handler function. This
    -- allows you to run code before and after a normal handler.
    --
    -- Default: the 'defaultYesodMiddleware' function.
    --
    -- Since: 1.1.6
    yesodMiddleware :: ToTypedContent res => HandlerT site IO res -> HandlerT site IO res
    yesodMiddleware = defaultYesodMiddleware

-- | Default implementation of 'yesodMiddleware'. Adds the response header
-- \"Vary: Accept, Accept-Language\" and performs authorization checks.
--
-- Since 1.2.0
defaultYesodMiddleware :: Yesod site => HandlerT site IO res -> HandlerT site IO res
defaultYesodMiddleware handler = do
    addHeader "Vary" "Accept, Accept-Language"
    authorizationCheck
    handler

-- | Check if a given request is authorized via 'isAuthorized' and
-- 'isWriteRequest'.
--
-- Since 1.2.0
authorizationCheck :: Yesod site => HandlerT site IO ()
authorizationCheck = do
    getCurrentRoute >>= maybe (return ()) checkUrl
  where
    checkUrl url = do
        isWrite <- isWriteRequest url
        ar <- isAuthorized url isWrite
        case ar of
            Authorized -> return ()
            AuthenticationRequired -> do
                master <- getYesod
                case authRoute master of
                    Nothing -> void $ notAuthenticated
                    Just url' -> do
                      void $ selectRep $ do
                          provideRepType typeHtml $ do
                              setUltDestCurrent
                              void $ redirect url'
                          provideRepType typeJson $
                              void $ notAuthenticated
            Unauthorized s' -> permissionDenied s'

-- | Convert a widget to a 'PageContent'.
widgetToPageContent :: (Eq (Route site), Yesod site)
                    => WidgetT site IO ()
                    -> HandlerT site IO (PageContent (Route site))
widgetToPageContent w = do
    master <- getYesod
    hd <- HandlerT return
    ((), GWData (Body body) (Last mTitle) scripts' stylesheets' style jscript (Head head')) <- lift $ unWidgetT w hd
    let title = maybe mempty unTitle mTitle
        scripts = runUniqueList scripts'
        stylesheets = runUniqueList stylesheets'

    render <- getUrlRenderParams
    let renderLoc x =
            case x of
                Nothing -> Nothing
                Just (Left s) -> Just s
                Just (Right (u, p)) -> Just $ render u p
    css <- forM (Map.toList style) $ \(mmedia, content) -> do
        let rendered = toLazyText $ content render
        x <- addStaticContent "css" "text/css; charset=utf-8"
           $ encodeUtf8 rendered
        return (mmedia,
            case x of
                Nothing -> Left $ preEscapedToMarkup rendered
                Just y -> Right $ either id (uncurry render) y)
    jsLoc <-
        case jscript of
            Nothing -> return Nothing
            Just s -> do
                x <- addStaticContent "js" "text/javascript; charset=utf-8"
                   $ encodeUtf8 $ renderJavascriptUrl render s
                return $ renderLoc x

    -- modernizr should be at the end of the <head> http://www.modernizr.com/docs/#installing
    -- the asynchronous loader means your page doesn't have to wait for all the js to load
    let (mcomplete, asyncScripts) = asyncHelper render scripts jscript jsLoc
        regularScriptLoad = [hamlet|
            $newline never
            $forall s <- scripts
                ^{mkScriptTag s}
            $maybe j <- jscript
                $maybe s <- jsLoc
                    <script src="#{s}">
                $nothing
                    <script>^{jelper j}
        |]

        headAll = [hamlet|
            $newline never
            \^{head'}
            $forall s <- stylesheets
                ^{mkLinkTag s}
            $forall s <- css
                $maybe t <- right $ snd s
                    $maybe media <- fst s
                        <link rel=stylesheet media=#{media} href=#{t}>
                    $nothing
                        <link rel=stylesheet href=#{t}>
                $maybe content <- left $ snd s
                    $maybe media <- fst s
                        <style media=#{media}>#{content}
                    $nothing
                        <style>#{content}
            $case jsLoader master
              $of BottomOfBody
              $of BottomOfHeadAsync asyncJsLoader
                  ^{asyncJsLoader asyncScripts mcomplete}
              $of BottomOfHeadBlocking
                  ^{regularScriptLoad}
        |]
    let bodyScript = [hamlet|
            $newline never
            ^{body}
            ^{regularScriptLoad}
        |]

    return $ PageContent title headAll $
        case jsLoader master of
            BottomOfBody -> bodyScript
            _ -> body
  where
    renderLoc' render' (Local url) = render' url []
    renderLoc' _ (Remote s) = s

    addAttr x (y, z) = x ! customAttribute (textTag y) (toValue z)
    mkScriptTag (Script loc attrs) render' =
        foldl' addAttr TBH.script (("src", renderLoc' render' loc) : attrs) $ return ()
    mkLinkTag (Stylesheet loc attrs) render' =
        foldl' addAttr TBH.link
            ( ("rel", "stylesheet")
            : ("href", renderLoc' render' loc)
            : attrs
            )

    runUniqueList :: Eq x => UniqueList x -> [x]
    runUniqueList (UniqueList x) = nub $ x []

-- | The default error handler for 'errorHandler'.
defaultErrorHandler :: Yesod site => ErrorResponse -> HandlerT site IO TypedContent
defaultErrorHandler NotFound = selectRep $ do
    provideRep $ defaultLayout $ do
        r <- waiRequest
        let path' = TE.decodeUtf8With TEE.lenientDecode $ W.rawPathInfo r
        setTitle "Not Found"
        toWidget [hamlet|
            <h1>Not Found
            <p>#{path'}
        |]
    provideRep $ return $ object ["message" .= ("Not Found" :: Text)]

-- For API requests.
-- For a user with a browser,
-- if you specify an authRoute the user will be redirected there and
-- this page will not be shown.
defaultErrorHandler NotAuthenticated = selectRep $ do
    provideRep $ defaultLayout $ do
        setTitle "Not logged in"
        toWidget [hamlet|
            <h1>Not logged in
            <p style="display:none;">Set the authRoute and the user will be redirected there.
        |]

    provideRep $ do
        -- 401 *MUST* include a WWW-Authenticate header
        -- however, there is no standard to indicate a redirection
        --
        -- change this to Basic or Digest if you allow those forms of authentications
        addHeader "WWW-Authenticate" "RedirectJSON realm=\"application\", param=\"authentication_url\""

        -- The client will just use the authentication_url in the JSON
        site <- getYesod
        rend <- getUrlRender
        return $ object $ [
          "message" .= ("Not logged in"::Text)
          ] ++
          case authRoute site of
              Nothing -> []
              Just url -> ["authentication_url" .= rend url]

defaultErrorHandler (PermissionDenied msg) = selectRep $ do
    provideRep $ defaultLayout $ do
        setTitle "Permission Denied"
        toWidget [hamlet|
            <h1>Permission denied
            <p>#{msg}
        |]
    provideRep $
        return $ object $ [
          "message" .= ("Permission Denied. " <> msg)
          ]

defaultErrorHandler (InvalidArgs ia) = selectRep $ do
    provideRep $ defaultLayout $ do
        setTitle "Invalid Arguments"
        toWidget [hamlet|
            <h1>Invalid Arguments
            <ul>
                $forall msg <- ia
                    <li>#{msg}
        |]
    provideRep $ return $ object ["message" .= ("Invalid Arguments" :: Text), "errors" .= ia]
defaultErrorHandler (InternalError e) = do
    $logErrorS "yesod-core" e
    selectRep $ do
        provideRep $ defaultLayout $ do
            setTitle "Internal Server Error"
            toWidget [hamlet|
                <h1>Internal Server Error
                <pre>#{e}
            |]
        provideRep $ return $ object ["message" .= ("Internal Server Error" :: Text), "error" .= e]
defaultErrorHandler (BadMethod m) = selectRep $ do
    provideRep $ defaultLayout $ do
        setTitle"Bad Method"
        toWidget [hamlet|
            <h1>Method Not Supported
            <p>Method <code>#{S8.unpack m}</code> not supported
        |]
    provideRep $ return $ object ["message" .= ("Bad method" :: Text), "method" .= TE.decodeUtf8With TEE.lenientDecode m]

asyncHelper :: (url -> [x] -> Text)
         -> [Script (url)]
         -> Maybe (JavascriptUrl (url))
         -> Maybe Text
         -> (Maybe (HtmlUrl url), [Text])
asyncHelper render scripts jscript jsLoc =
    (mcomplete, scripts'')
  where
    scripts' = map goScript scripts
    scripts'' =
        case jsLoc of
            Just s -> scripts' ++ [s]
            Nothing -> scripts'
    goScript (Script (Local url) _) = render url []
    goScript (Script (Remote s) _) = s
    mcomplete =
        case jsLoc of
            Just{} -> Nothing
            Nothing ->
                case jscript of
                    Nothing -> Nothing
                    Just j -> Just $ jelper j

#if MIN_VERSION_fast_logger(2, 0, 0)
formatLogMessage :: IO ZonedDate
                 -> Loc
                 -> LogSource
                 -> LogLevel
                 -> LogStr -- ^ message
                 -> IO LogStr
formatLogMessage getdate loc src level msg = do
    now <- getdate
    return $
        toLogStr now `mappend`
        " [" `mappend`
        (case level of
            LevelOther t -> toLogStr t
            _ -> toLogStr $ drop 5 $ show level) `mappend`
        (if T.null src
            then mempty
            else "#" `mappend` toLogStr src) `mappend`
        "] " `mappend`
        msg `mappend`
        " @(" `mappend`
        toLogStr (fileLocationToString loc) `mappend`
        ")\n"
#else
formatLogMessage :: IO ZonedDate
                 -> Loc
                 -> LogSource
                 -> LogLevel
                 -> LogStr -- ^ message
                 -> IO [LogStr]
formatLogMessage getdate loc src level msg = do
    now <- getdate
    return
        [ LB now
        , LB " ["
        , LS $
            case level of
                LevelOther t -> T.unpack t
                _ -> drop 5 $ show level
        , LS $
            if T.null src
                then ""
                else "#" ++ T.unpack src
        , LB "] "
        , msg
        , LB " @("
        , LS $ fileLocationToString loc
        , LB ")\n"
        ]
#endif

-- | Customize the cookies used by the session backend.  You may
-- use this function on your definition of 'makeSessionBackend'.
--
-- For example, you could set the cookie domain so that it
-- would work across many subdomains:
--
-- @
-- makeSessionBackend = fmap (customizeSessionCookie addDomain) ...
--   where
--     addDomain cookie = cookie { 'setCookieDomain' = Just \".example.com\" }
-- @
--
-- Default: Do not customize anything ('id').
customizeSessionCookies :: (SetCookie -> SetCookie) -> (SessionBackend -> SessionBackend)
customizeSessionCookies customizeCookie backend = backend'
  where
    customizeHeader (AddCookie cookie) = AddCookie (customizeCookie cookie)
    customizeHeader other              = other
    customizeSaveSession = (fmap . fmap . fmap) customizeHeader
    backend' =
      backend {
        sbLoadSession = \req ->
          second customizeSaveSession `fmap` sbLoadSession backend req
      }


defaultClientSessionBackend :: Int -- ^ minutes
                            -> FilePath -- ^ key file
                            -> IO SessionBackend
defaultClientSessionBackend minutes fp = do
  key <- CS.getKey fp
  let timeout = fromIntegral (minutes * 60)
  (getCachedDate, _closeDateCacher) <- clientSessionDateCacher timeout
  return $ clientSessionBackend key getCachedDate

jsToHtml :: Javascript -> Html
jsToHtml (Javascript b) = preEscapedToMarkup $ toLazyText b

jelper :: JavascriptUrl url -> HtmlUrl url
jelper = fmap jsToHtml

left :: Either a b -> Maybe a
left (Left x) = Just x
left _ = Nothing

right :: Either a b -> Maybe b
right (Right x) = Just x
right _ = Nothing

clientSessionBackend :: CS.Key  -- ^ The encryption key
                     -> IO ClientSessionDateCache -- ^ See 'clientSessionDateCacher'
                     -> SessionBackend
clientSessionBackend key getCachedDate =
  SessionBackend {
    sbLoadSession = loadClientSession key getCachedDate "_SESSION"
  }

loadClientSession :: CS.Key
                  -> IO ClientSessionDateCache -- ^ See 'clientSessionDateCacher'
                  -> S8.ByteString -- ^ session name
                  -> W.Request
                  -> IO (SessionMap, SaveSession)
loadClientSession key getCachedDate sessionName req = load
  where
    load = do
      date <- getCachedDate
      return (sess date, save date)
    sess date = fromMaybe Map.empty $ do
      raw <- lookup "Cookie" $ W.requestHeaders req
      val <- lookup sessionName $ parseCookies raw
      let host = "" -- fixme, properly lock sessions to client address
      decodeClientSession key date host val
    save date sess' = do
      -- We should never cache the IV!  Be careful!
      iv <- liftIO CS.randomIV
      return [AddCookie def
          { setCookieName = sessionName
          , setCookieValue = encodeClientSession key iv date host sess'
          , setCookiePath = Just "/"
          , setCookieExpires = Just (csdcExpires date)
          , setCookieDomain = Nothing
          , setCookieHttpOnly = True
          }]
        where
          host = "" -- fixme, properly lock sessions to client address

-- taken from file-location package
-- turn the TH Loc loaction information into a human readable string
-- leaving out the loc_end parameter
fileLocationToString :: Loc -> String
fileLocationToString loc = (loc_package loc) ++ ':' : (loc_module loc) ++
  ' ' : (loc_filename loc) ++ ':' : (line loc) ++ ':' : (char loc)
  where
    line = show . fst . loc_start
    char = show . snd . loc_start