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|
-- | This module provides an API for resource providers. Resource providers
-- allow Hakyll to get content from resources; the type of resource depends on
-- the concrete instance.
--
-- A resource is represented by the 'Resource' type. This is basically just a
-- newtype wrapper around 'Identifier' -- but it has an important effect: it
-- guarantees that a resource with this identifier can be provided by one or
-- more resource providers.
--
-- Therefore, it is not recommended to read files directly -- you should use the
-- provided 'Resource' methods.
--
module Hakyll.Core.ResourceProvider
( Resource (..)
, ResourceProvider (..)
, resourceExists
, resourceDigest
, resourceModified
) where
import Control.Monad ((<=<))
import Data.Word (Word8)
import qualified Data.ByteString.Lazy as LB
import OpenSSL.Digest.ByteString.Lazy (digest)
import OpenSSL.Digest (MessageDigest (MD5))
import Hakyll.Core.Identifier
import Hakyll.Core.Store
-- | A resource
--
-- Invariant: the resource specified by the given identifier must exist
--
newtype Resource = Resource {unResource :: Identifier}
deriving (Eq, Show, Ord)
-- | A value responsible for retrieving and listing resources
--
data ResourceProvider = ResourceProvider
{ -- | A list of all resources this provider is able to provide
resourceList :: [Resource]
, -- | Retrieve a certain resource as string
resourceString :: Resource -> IO String
, -- | Retrieve a certain resource as lazy bytestring
resourceLazyByteString :: Resource -> IO LB.ByteString
}
-- | Check if a given identifier has a resource
--
resourceExists :: ResourceProvider -> Identifier -> Bool
resourceExists provider = flip elem $ map unResource $ resourceList provider
-- | Retrieve a digest for a given resource
--
resourceDigest :: ResourceProvider -> Resource -> IO [Word8]
resourceDigest provider = digest MD5 <=< resourceLazyByteString provider
-- | Check if a resource was modified
--
resourceModified :: ResourceProvider -> Resource -> Store -> IO Bool
resourceModified provider resource store = do
-- Get the latest seen digest from the store
lastDigest <- storeGet store itemName $ unResource resource
-- Calculate the digest for the resource
newDigest <- resourceDigest provider resource
-- Check digests
if Just newDigest == lastDigest
-- All is fine, not modified
then return False
-- Resource modified; store new digest
else do storeSet store itemName (unResource resource) newDigest
return True
where
itemName = "Hakyll.Core.ResourceProvider.resourceModified"
|
