fix bug with mark_non_var

README.md

@@ -35,7 +35,7 @@ Extend rusty-wam to include the following, among other features:
     - A representation of 'partial strings' as difference lists
       of characters (_done_).
 * `term_expansion/2` and `goal_expansion/2` (_in progress_).
-* Definite Clause Grammars.
+* Definite Clause Grammars (_in progress_).
 * Attributed variables using the SICStus Prolog interface and
   semantics. Adding coroutines like `dif/2`, `freeze/2`, etc.
   is straightforward with attributed variables.

src/prolog/debray_allocator.rs

@@ -232,23 +232,22 @@ impl<'a> Allocator<'a> for DebrayAllocator
     {
         let r = cell.get();
         
-        if r.reg_num() == 0 {
-            let r = match lvl {
-                Level::Shallow => {
-                    let k = self.arg_c;
+        let r = match lvl {
+            Level::Shallow => {
+                let k = self.arg_c;
                 
-                    if let GenContext::Last(chunk_num) = term_loc {
-                        self.evacuate_arg(chunk_num, target);
-                    }
+                if let GenContext::Last(chunk_num) = term_loc {
+                    self.evacuate_arg(chunk_num, target);
+                }
                 
-                    self.arg_c += 1;
-                    RegType::Temp(k)
-                },
-                _ => RegType::Temp(self.alloc_reg_to_non_var())
-            };
+                self.arg_c += 1;
+                RegType::Temp(k)
+            },
+            _ if r.reg_num() == 0 => RegType::Temp(self.alloc_reg_to_non_var()),
+            _ => r
+        };
 
-            cell.set(r);
-        }
+        cell.set(r);        
     }
 
     fn mark_var<Target>(&mut self, var: Rc<Var>, lvl: Level, cell: &'a Cell<VarReg>,

src/prolog/lib/dcgs.pl

@@ -25,7 +25,7 @@ term_expansion(Term0, (ModHead :- ModBody)) :-
     !,
     nonvar(Head),
     Head =.. [RuleName | Args],
-    append(Args, ['$VAR'(0), '$VAR'(N)], ModArgs),
+    append(Args, ['$VAR'(0), '$VAR'(N)], ModArgs), %% problematic line.
     ModHead =.. [RuleName | ModArgs],
     nonvar(Body),
     expand_body(Body, ModBody1, 0, N1),

src/prolog/machine/mod.rs

@@ -347,9 +347,9 @@ impl Machine {
             self.machine_st.copy_and_align_ball_to_heap();
 
             let error_str = self.machine_st.print_exception(Addr::HeapCell(h),
-                                                    &heap_locs,
-                                                    TermFormatter {},
-                                                    PrinterOutputter::new())
+                                                            &heap_locs,
+                                                            TermFormatter {},
+                                                            PrinterOutputter::new())
                                 .result();
 
             EvalSession::from(SessionError::QueryFailureWithException(error_str))