1 files changed, 26 insertions, 7 deletions
diff --git a/lib/Language/Elna/SymbolTable.hs b/lib/Language/Elna/SymbolTable.hs
index b8892c9..eb82073 100644
--- a/lib/Language/Elna/SymbolTable.hs
+++ b/lib/Language/Elna/SymbolTable.hs
@@ -2,15 +2,19 @@ module Language.Elna.SymbolTable
     ( Info(..)
     , ParameterInfo(..)
     , SymbolTable
+    , builtInSymbolTable
     , empty
     , enter
     , fromList
     , lookup
-    , builtInSymbolTable
+    , member
     ) where
 
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.List (sort)
+import Data.List.NonEmpty (NonEmpty)
+import qualified Data.List.NonEmpty as NonEmpty
 import Data.Vector (Vector)
 import Language.Elna.Location (Identifier(..))
 import Language.Elna.Types (Type(..), intType, booleanType)
@@ -25,7 +29,7 @@ instance Semigroup SymbolTable
 
 instance Monoid SymbolTable
   where
-    mempty = SymbolTable HashMap.empty
+    mempty = empty
 
 builtInSymbolTable :: SymbolTable
 builtInSymbolTable = SymbolTable $ HashMap.fromList
@@ -36,15 +40,30 @@ builtInSymbolTable = SymbolTable $ HashMap.fromList
 empty :: SymbolTable
 empty = SymbolTable HashMap.empty
 
-enter :: Identifier -> Info -> SymbolTable -> SymbolTable
-enter identifier info (SymbolTable table) = SymbolTable
-    $ HashMap.insert identifier info table
+enter :: Identifier -> Info -> SymbolTable -> Maybe SymbolTable
+enter identifier info table@(SymbolTable hashTable)
+    | member identifier table = Nothing
+    | otherwise = Just
+        $ SymbolTable
+        $ HashMap.insert identifier info hashTable
 
 lookup :: Identifier -> SymbolTable -> Maybe Info
 lookup identifier (SymbolTable table) = HashMap.lookup identifier table
 
-fromList :: [(Identifier, Info)] -> SymbolTable
-fromList = SymbolTable . HashMap.fromList
+member :: Identifier -> SymbolTable -> Bool
+member identifier (SymbolTable table) = HashMap.member identifier table
+
+fromList :: [(Identifier, Info)] -> Either (NonEmpty Identifier) SymbolTable
+fromList elements
+    | Just identifierDuplicates' <- identifierDuplicates =
+        Left identifierDuplicates'
+    | otherwise = Right $ SymbolTable $ HashMap.fromList elements
+  where
+    identifierDuplicates = NonEmpty.nonEmpty
+        $ fmap NonEmpty.head
+        $ filter ((> 1) . NonEmpty.length)
+        $ NonEmpty.group . sort
+        $ fst <$> elements
 
 data ParameterInfo = ParameterInfo
     { name :: Identifier