diff --git a/src/Eval.hs b/src/Eval.hs
index bdb022a..6b5baa2 100644
--- a/src/Eval.hs
+++ b/src/Eval.hs
@@ -10,26 +10,26 @@ import qualified Data.Set as Set
 
 evalSingle :: Env -> TricuAST -> Env
 evalSingle env term
-  | SDef name [] body <- term =
-      if
-        | Map.member name env ->
-            errorWithoutStackTrace $
-              "Error: Identifier '" ++ name ++ "' is already defined."
-        | otherwise ->
-            let res = evalAST env body
-            in Map.insert "!result" res (Map.insert name res env)
-  | SApp func arg <- term =
-      let res = apply (evalAST env func) (evalAST env arg)
+  | SDef name [] body <- term
+  = case Map.lookup name env of
+      Just existingValue
+        | existingValue == evalAST env body -> env
+        | otherwise -> errorWithoutStackTrace $
+            "Unable to rebind immutable identifier: '" ++ name
+      Nothing ->
+        let res = evalAST env body
+        in Map.insert "!result" res (Map.insert name res env)
+  | SApp func arg <- term 
+  = let res = apply (evalAST env func) (evalAST env arg)
       in Map.insert "!result" res env
-  | SVar name <- term =
-      case Map.lookup name env of
-        Just v  ->
-          Map.insert "!result" v env
+  | SVar name <- term 
+  = case Map.lookup name env of
+        Just v  -> Map.insert "!result" v env
         Nothing ->
           errorWithoutStackTrace $ "Variable `" ++ name ++ "` not defined\n\
           \This error should never occur here. Please report this as an issue."
-  | otherwise =
-      Map.insert "!result" (evalAST env term) env
+  | otherwise 
+  = Map.insert "!result" (evalAST env term) env
 
 evalTricu :: Env -> [TricuAST] -> Env
 evalTricu env x = go env (reorderDefs env x)