    {-# LANGUAGE EmptyDataDecls #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE GADTs #-}
    {-# LANGUAGE TypeSynonymInstances #-}
    -- | Module for parsing and rendering Hamlet templates at runtime, not compile
    -- time. This uses the same Hamlet parsing as compile-time Hamlet, but has some
    -- limitations, such as:
    --
    -- * No compile-time checking of validity
    --
    -- * Can't apply functions at runtime
    --
    -- * No URL rendering
    --
    -- > {-# LANGUAGE OverloadedStrings #-}
    -- > import Text.Hamlet.Runtime
    -- > import qualified Data.Map as Map
    -- > import Text.Blaze.Html.Renderer.String (renderHtml)
    -- >
    -- > main :: IO ()
    -- > main = do
    -- >     template <- parseHamletTemplate defaultHamletSettings $ unlines
    -- >         [ "<p>Hello, #{name}"
    -- >         , "$if hungry"
    -- >         , "  <p>Available food:"
    -- >         , "  <ul>"
    -- >         , "    $forall food <- foods"
    -- >         , "      <li>#{food}"
    -- >         ]
    -- >     let hamletDataMap = Map.fromList
    -- >             [ ("name", "Michael")
    -- >             , ("hungry", toHamletData True) -- always True
    -- >             , ("foods", toHamletData
    -- >                 [ "Apples"
    -- >                 , "Bananas"
    -- >                 , "Carrots"
    -- >                 ])
    -- >             ]
    -- >     html <- renderHamletTemplate template hamletDataMap
    -- >     putStrLn $ renderHtml html
    --
    -- @since 2.0.6
    module Text.Hamlet.Runtime
        ( HamletTemplate
        , HamletSettings
        , defaultHamletSettings
        , HamletData
        , ToHamletData (..)
        , parseHamletTemplate
        , readHamletTemplateFile
        , renderHamletTemplate
        ) where

    import Control.Arrow ((***))
    import Control.Monad.Catch (MonadThrow)
    import Text.Hamlet (HamletSettings, defaultHamletSettings)
    import qualified Text.Hamlet.RT as RT
    import Data.Map (Map)
    import qualified Data.Map as Map
    import Data.Text (Text)
    import Data.String
    import Text.Blaze.Html (Html, toHtml)
    import Control.Monad (liftM)
    import Control.Monad.IO.Class
    import Data.Text.Encoding (decodeUtf8With)
    import Data.Text.Encoding.Error (lenientDecode)
    import qualified Data.ByteString as S
    import qualified Data.Text as T

    -- Just to skip a dependency for GHC < 7.10
    data Void
    absurd :: Void -> a
    absurd _ = error "absurd"

    -- | A parsed Hamlet template. See 'parseHamletTemplate' and
    -- 'readHamletTemplateFile'.
    --
    -- @since 2.0.6
    newtype HamletTemplate = HamletTemplate RT.HamletRT

    -- | A piece of data that can be embedded and passed to a Hamlet template (via
    -- 'renderHamletTemplate').
    --
    -- This supplies an 'IsString' instance, so with @OverloadedStrings@ it will
    -- support literal strings, which are converted to HTML via 'toHtml'. For other
    -- datatypes, use 'toHamletData'.
    --
    -- @since 2.0.6
    newtype HamletData = HamletData { unHamletData :: RT.HamletData Void }
    instance IsString HamletData where
        fromString = HamletData . RT.HDHtml . fromString

    -- | Data which can be passed to a Hamlet template.
    --
    -- @since 2.0.6
    class ToHamletData a where
        toHamletData :: a -> HamletData
    instance ToHamletData HamletData where
        toHamletData = id
    instance a ~ HamletData => ToHamletData [a] where
        toHamletData = HamletData . RT.HDList . map (\x -> [([], unHamletData x)])
    instance a ~ HamletData => ToHamletData (Maybe a) where
        toHamletData = HamletData . RT.HDMaybe . fmap (\x -> [([], unHamletData x)])
    instance ToHamletData Text where
        toHamletData = toHamletData . toHtml
    instance ToHamletData Html where
        toHamletData = HamletData . RT.HDHtml
    instance ToHamletData Bool where
        toHamletData = HamletData . RT.HDBool

    -- | Parse an in-memory Hamlet template. This operation may fail if the
    -- template is not parsable.
    --
    -- @since 2.0.6
    parseHamletTemplate :: MonadThrow m => HamletSettings -> String -> m HamletTemplate
    parseHamletTemplate set str = HamletTemplate `liftM` RT.parseHamletRT set str

    -- | Same as 'parseHamletTemplate', but reads from a file. The file is assumed
    -- to be UTF-8 encoded (same assumption as compile-time Hamlet).
    --
    -- @since 2.0.6
    readHamletTemplateFile :: (MonadThrow m, MonadIO m) => HamletSettings -> FilePath -> m HamletTemplate
    readHamletTemplateFile set fp = do
        bs <- liftIO $ S.readFile fp
        parseHamletTemplate set $ T.unpack $ decodeUtf8With lenientDecode bs

    -- | Render a runtime Hamlet template, together with a 'Map' of variables to
    -- pass in, into an 'Html' value. This can fail if the template references a
    -- variable that is not present in the @Map@.
    --
    -- @since 2.0.6
    renderHamletTemplate :: MonadThrow m => HamletTemplate -> Map Text HamletData -> m Html
    renderHamletTemplate (HamletTemplate rt) m =
        RT.renderHamletRT' True rt m' renderUrl
      where
        m' = map (return . T.unpack *** unHamletData) $ Map.toList m
        renderUrl url _ = absurd url