Add target module

3 files changed, 59 insertions, 1 deletions

diff --git a/src/Hakyll/Core/DirectedGraph/Internal.hs b/src/Hakyll/Core/DirectedGraph/Internal.hs
index 9890fc0..52a712d 100644
--- a/src/Hakyll/Core/DirectedGraph/Internal.hs
+++ b/src/Hakyll/Core/DirectedGraph/Internal.hs
@@ -1,4 +1,5 @@
--- | Internal structure of the DirectedGraph type. Not exported in the library.
+-- | Internal structure of the DirectedGraph type. Not exported outside of the
+-- library.
 --
 module Hakyll.Core.DirectedGraph.Internal
     ( Node (..)
diff --git a/src/Hakyll/Core/Target.hs b/src/Hakyll/Core/Target.hs
new file mode 100644
index 0000000..1f783df
--- /dev/null
+++ b/src/Hakyll/Core/Target.hs
@@ -0,0 +1,11 @@
+-- | A target represents one compilation unit, e.g. a blog post, a CSS file...
+--
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+module Hakyll.Core.Target
+    ( DependencyLookup
+    , TargetM
+    , Target
+    , runTarget
+    ) where
+
+import Hakyll.Core.Target.Internal
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'
+        }