1 files changed, 18 insertions, 2 deletions
diff --git a/app/Code.hs b/app/Code.hs
index 8bea782..0556415 100644
--- a/app/Code.hs
+++ b/app/Code.hs
@@ -65,14 +65,30 @@ data Interp =
 
 type PrlgEnv a = StateT Interp (InputT IO) a
 
-type BuiltinFn = PrlgEnv (Maybe (Either String Bool))
+type InterpFn = PrlgEnv (Maybe (Either String Bool))
 
 data Builtin =
-  Builtin BuiltinFn
+  Builtin InterpFn
 
 instance Show Builtin where
   show _ = "Builtin _"
 
+data Dereferenced
+  = FreeRef Int
+  | BoundRef Int Datum
+  | NoRef
+
+-- TRICKY: HeapRefs alone must not form a cycle other than the FreeRef case.
+derefHeap :: Heap -> Int -> Dereferenced
+derefHeap h@(Heap _ hmap) x =
+  case hmap M.!? x of
+    Just (HeapRef x') ->
+      if x == x'
+        then FreeRef x'
+        else derefHeap h x'
+    Just x' -> BoundRef x x'
+    _ -> NoRef
+
 -- TODO are we actually going to use this?
 codeStruct ::
      Monad m