make struct&atom useful

app/Builtins.hs

@@ -167,8 +167,10 @@ struct = do
   heap <- use (cur . heap)
   scope <- use (cur . hvar)
   case derefHeap heap <$> scope M.!? 0 of
-    Just (BoundRef addr (Struct Id {arity = arity, str = str})) ->
+    Just (BoundRef _ (Struct Id {arity = arity, str = str})) ->
       structUnify arity str
+    Just (BoundRef _ _) ->
+      backtrack
     _ -> structAssemble
 
 heapListLength listAtom heap ref = go 0 ref ref (id, fromJust . step)
@@ -217,6 +219,7 @@ structUnify arity str = do
   zoom cur $ do
     gol %= (gcode ++)
     hed %= (hcode ++)
+    unis += 3
   continue
 
 {- terms -}
@@ -229,6 +232,14 @@ var = do
     Just (FreeRef _) -> continue
     _ -> backtrack
 
+atom :: InterpFn
+atom = do
+  heap <- use (cur . heap)
+  scope <- use (cur . hvar)
+  case derefHeap heap <$> scope M.!? 0 of
+    Just (BoundRef _ (C (Atom _))) -> continue
+    _ -> backtrack
+
 number :: InterpFn
 number = do
   heap <- use (cur . heap)
@@ -468,6 +479,7 @@ addPrelude = do
   {- terms -}
   addBi struct "struct" 3
   addBi var "var" 1
+  addBi atom "atom" 1
   addBi number "number" 1
   addBi string "string" 1
   addBi sameTerm "same_term" 2