Add utility getMetadataField

1 files changed, 22 insertions, 0 deletions

diff --git a/src/Hakyll/Core/Metadata.hs b/src/Hakyll/Core/Metadata.hs
index 9fc2d3e..7902b94 100644
--- a/src/Hakyll/Core/Metadata.hs
+++ b/src/Hakyll/Core/Metadata.hs
@@ -2,6 +2,8 @@
 module Hakyll.Core.Metadata
     ( Metadata
     , MonadMetadata (..)
+    , getMetadataField
+    , getMetadataField'
     , makePatternDependency
     ) where
 
@@ -9,6 +11,7 @@ module Hakyll.Core.Metadata
 --------------------------------------------------------------------------------
 import           Control.Monad                  (forM)
 import           Data.Map                       (Map)
+import qualified Data.Map                       as M
 
 
 --------------------------------------------------------------------------------
@@ -35,6 +38,25 @@ class Monad m => MonadMetadata m where
 
 
 --------------------------------------------------------------------------------
+getMetadataField :: MonadMetadata m => Identifier -> String -> m (Maybe String)
+getMetadataField identifier key = do
+    metadata <- getMetadata identifier
+    return $ M.lookup key metadata
+
+
+--------------------------------------------------------------------------------
+-- | Version of 'getMetadataField' which throws an error if the field does not
+-- exist.
+getMetadataField' :: MonadMetadata m => Identifier -> String -> m String
+getMetadataField' identifier key = do
+    field <- getMetadataField identifier key
+    case field of
+        Just v  -> return v
+        Nothing -> fail $ "Hakyll.Core.Metadata.getMetadataField': " ++
+            "Item " ++ show identifier ++ " has no metadata field " ++ show key
+
+
+--------------------------------------------------------------------------------
 makePatternDependency :: MonadMetadata m => Pattern -> m Dependency
 makePatternDependency pattern = do
     matches' <- getMatches pattern