Add target module

1 files changed, 46 insertions, 0 deletions

diff --git a/src/Hakyll/Core/Target/Internal.hs b/src/Hakyll/Core/Target/Internal.hs
new file mode 100644
index 0000000..a58f736
--- /dev/null
+++ b/src/Hakyll/Core/Target/Internal.hs
@@ -0,0 +1,46 @@
+-- | Internal structure of a Target, not exported outside of the library
+--
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+module Hakyll.Core.Target.Internal
+    ( DependencyLookup
+    , TargetM (..)
+    , Target
+    , runTarget
+    ) where
+
+import Control.Monad.Trans (MonadIO)
+import Control.Monad.Reader (ReaderT, runReaderT)
+
+import Hakyll.Core.Identifier
+
+-- | A lookup with which we can get dependencies
+--
+type DependencyLookup a = Identifier -> a
+
+-- | Environment for the target monad
+--
+data TargetEnvironment a = TargetEnvironment
+    { targetIdentifier       :: Identifier
+    , targetDependencyLookup :: DependencyLookup a  -- ^ Dependency lookup
+    }
+
+-- | Monad for targets. In this monad, the user can compose targets and describe
+-- how they should be created.
+--
+newtype TargetM a b = TargetM {unTargetM :: ReaderT (TargetEnvironment a) IO b}
+                    deriving (Monad, Functor, MonadIO)
+
+-- | Simplification of the 'TargetM' type for concrete cases: the type of the
+-- returned item should equal the type of the dependencies.
+--
+type Target a = TargetM a a
+
+-- | Run a target, yielding an actual result.
+--
+runTarget :: Target a -> Identifier -> DependencyLookup a -> IO a
+runTarget target id' lookup' = runReaderT (unTargetM target) env
+  where
+    env = TargetEnvironment
+        { targetIdentifier       = id'
+        , targetDependencyLookup = lookup'
+        }