Throw an error in case of duplicate compilers

src/Hakyll/Core/Rules/Internal.hs

@@ -16,6 +16,7 @@ import           Control.Applicative            (Applicative, (<$>))
 import           Control.Monad.Reader           (ask)
 import           Control.Monad.RWS              (RWST, runRWST)
 import           Control.Monad.Trans            (liftIO)
+import           Data.Map                       (Map)
 import qualified Data.Map                       as M
 import           Data.Monoid                    (Monoid, mappend, mempty)
 import           Data.Set                       (Set)
@@ -97,15 +98,26 @@ runRules rules provider = do
     (_, _, ruleSet) <- runRWST (unRules rules) env emptyRulesState
 
     -- Ensure compiler uniqueness
-    let ruleSet' = ruleSet
-            { rulesCompilers = M.toList $
-                M.fromListWith (flip const) (rulesCompilers ruleSet)
-            }
+    uniqueCompilers <- case fromListUnique (rulesCompilers ruleSet) of
+        Right m  -> return m
+        Left id' -> error $
+            "Hakyll.Core.Rules.Internal.runRules: duplicate compiler for " ++
+            show id'
 
-    return ruleSet'
+    return ruleSet {rulesCompilers = M.toList uniqueCompilers}
   where
     env = RulesRead
         { rulesProvider = provider
         , rulesMatches  = []
         , rulesVersion  = Nothing
         }
+
+
+--------------------------------------------------------------------------------
+fromListUnique :: Ord k => [(k, v)] -> Either k (Map k v)
+fromListUnique = go M.empty
+  where
+    go m []            = Right m
+    go m ((k, v) : zs) = case M.lookup k m of
+        Nothing -> go (M.insert k v m) zs
+        Just _  -> Left k

src/Hakyll/Core/Runtime.hs

@@ -53,7 +53,8 @@ run config verbosity rules = do
     provider <- newProvider store (shouldIgnoreFile config) $
         providerDirectory config
     Logger.message logger "Running rules..."
-    ruleSet  <- runRules rules provider
+    Logger.flush logger
+    ruleSet <- runRules rules provider
 
     -- Get old facts
     mOldFacts <- Store.get store factsKey