hakyll/src/Hakyll/Core/Identifier.hs

--------------------------------------------------------------------------------
-- | An identifier is a type used to uniquely identify a resource, target...
--
-- One can think of an identifier as something similar to a file path. An
-- identifier is a path as well, with the different elements in the path
-- separated by @/@ characters. Examples of identifiers are:
--
-- * @posts/foo.markdown@
--
-- * @index@
--
-- * @error/404@
--
-- The most important difference between an 'Identifier' and a file path is that
-- the identifier for an item is not necesserily the file path.
--
-- For example, we could have an @index@ identifier, generated by Hakyll. The
-- actual file path would be @index.html@, but we identify it using @index@.
--
-- @posts/foo.markdown@ could be an identifier of an item that is rendered to
-- @posts/foo.html@. In this case, the identifier is the name of the source
-- file of the page.
--
-- An `Identifier` carries the type of the value it identifies. This basically
-- means that an @Identifier (Page String)@ refers to a page.
--
-- It is a phantom type parameter, meaning you can safely change this if you
-- know what you are doing. You can change the type using the 'castIdentifier'
-- function.
--
-- If the @a@ type is not known, Hakyll traditionally uses @Identifier ()@.
{-# LANGUAGE DeriveDataTypeable         #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Hakyll.Core.Identifier
    ( Identifier
    , fromFilePath
    , toFilePath
    , castIdentifier
    ) where


--------------------------------------------------------------------------------
import           Control.DeepSeq (NFData)
import           Data.List       (intercalate)
import           System.FilePath (dropTrailingPathSeparator, splitPath)


--------------------------------------------------------------------------------
import           Data.Binary     (Binary)
import           Data.Typeable   (Typeable)
import           GHC.Exts        (IsString, fromString)


--------------------------------------------------------------------------------
-- | An identifier used to uniquely identify a value
newtype Identifier a = Identifier {unIdentifier :: String}
    deriving (Binary, Eq, NFData, Ord, Typeable)


--------------------------------------------------------------------------------
instance Show (Identifier a) where
    show = toFilePath


--------------------------------------------------------------------------------
instance IsString (Identifier a) where
    fromString = fromFilePath


--------------------------------------------------------------------------------
-- | Parse an identifier from a string
fromFilePath :: String -> Identifier a
fromFilePath = Identifier .  intercalate "/" . filter (not . null) . split'
  where
    split' = map dropTrailingPathSeparator . splitPath


--------------------------------------------------------------------------------
-- | Convert an identifier to a relative 'FilePath'
toFilePath :: Identifier a -> FilePath
toFilePath = unIdentifier


--------------------------------------------------------------------------------
-- | Discard the phantom type parameter of an identifier
castIdentifier :: Identifier a -> Identifier b
castIdentifier (Identifier x) = Identifier x
{-# INLINE castIdentifier #-}