hacky workaround

src/prolog/heap_print.rs

@@ -229,6 +229,27 @@ impl<'a, Formatter: HCValueFormatter, Outputter: HCValueOutputter>
         })
     }
 
+    fn print_constant(&mut self, c: Constant) {
+        match c {
+            Constant::Char(c) if c == '\n' =>
+                self.outputter.append("'\\n'"),
+//            Constant::Char(c) if c == '\f' =>
+//                self.outputter.append("\\f"),
+            Constant::Char(c) if c == '\r' =>
+                self.outputter.append("'\\r'"),
+            Constant::Char(c) if c == '\t' =>
+                self.outputter.append("'\\t'"),
+//            Constant::Char(c) if c == '\b' =>
+//                self.outputter.append("\\b"),
+//            Constant::Char(c) if c == '\\a' =>
+//                self.outputter.append("\a"),
+//            Constant::Char(c) if c == '\\v' =>
+//                self.outputter.append("\\v"),
+            _ =>
+                self.outputter.append(format!("{}", c).as_str())
+        }
+    }
+    
     fn handle_heap_term(&mut self, iter: &mut HCPreOrderIterator)
     {
         let heap_val = match self.check_for_seen(iter) {
@@ -246,7 +267,7 @@ impl<'a, Formatter: HCValueFormatter, Outputter: HCValueOutputter>
                     self.outputter.append("[]");
                 },
             HeapCellValue::Addr(Addr::Con(c)) =>
-                self.outputter.append(format!("{}", c).as_str()),
+                self.print_constant(c),
             HeapCellValue::Addr(Addr::Lis(_)) => {
                 let cell = Rc::new(Cell::new(true));