deallocate old stack frames (#201), start using tags, fix a panic! associated with partial strings

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "scryer-prolog"
-version = "0.8.109"
+version = "0.8.110"
 authors = ["Mark Thom <markjordanthom@gmail.com>"]
 build = "build.rs"
 repository = "https://github.com/mthom/scryer-prolog"

src/prolog/machine/and_stack.rs

@@ -66,6 +66,11 @@ impl AndStack {
             }
         }
     }
+
+    #[inline]
+    pub fn truncate(&mut self, len: usize) {
+        self.0.truncate(len);
+    }
 }
 
 impl Index<usize> for AndStack {

src/prolog/machine/machine_state.rs

@@ -419,11 +419,12 @@ pub(crate) trait CallPolicy: Any {
             machine_st.registers[i] = machine_st.or_stack[b][i].clone();
         }
 
-        // necessary because of restore_snapshot. similarly for other choice instructions.
         machine_st.num_of_args = n;
         machine_st.e = machine_st.or_stack[b].e;
         machine_st.cp = machine_st.or_stack[b].cp.clone();
 
+        machine_st.pop_stack_frames();
+        
         machine_st.or_stack[b].bp = machine_st.p.clone() + offset;
 
         let old_tr = machine_st.or_stack[b].tr;
@@ -468,6 +469,8 @@ pub(crate) trait CallPolicy: Any {
         machine_st.e = machine_st.or_stack[b].e;
         machine_st.cp = machine_st.or_stack[b].cp.clone();
 
+        machine_st.pop_stack_frames();
+        
         machine_st.or_stack[b].bp = machine_st.p.clone() + 1;
 
         let old_tr = machine_st.or_stack[b].tr;
@@ -539,6 +542,8 @@ pub(crate) trait CallPolicy: Any {
         machine_st.b = machine_st.or_stack[b].b;
         machine_st.or_stack.truncate(machine_st.b);
 
+        machine_st.pop_stack_frames();
+
         machine_st.hb = machine_st.heap.h;
         machine_st.p += offset;
 
@@ -584,6 +589,8 @@ pub(crate) trait CallPolicy: Any {
         machine_st.b = machine_st.or_stack[b].b;
         machine_st.or_stack.truncate(machine_st.b);
 
+        machine_st.pop_stack_frames();
+
         machine_st.hb = machine_st.heap.h;
         machine_st.p += 1;
 

src/prolog/machine/machine_state_impl.rs

@@ -1860,8 +1860,12 @@ impl MachineState {
                 let offset = match addr {
                     Addr::HeapCell(_) | Addr::StackCell(..) | Addr::AttrVar(..) => v,
                     Addr::Con(Constant::String(ref s)) if !self.flags.double_quotes.is_atom() => {
-                        if s.is_empty() && !s.is_expandable() {
+                        if s.is_empty() {
+                            if s.is_expandable() {
+                                v
+                            } else {
                                 c
+                            }
                         } else {
                             l
                         }
@@ -3147,6 +3151,21 @@ impl MachineState {
         self.p += 1;
     }
 
+    pub(super) fn pop_stack_frames(&mut self) {
+	if self.and_stack.len() > self.e {
+            let and_gi = self.and_stack[self.e].global_index;
+            let or_gi = self
+                .or_stack
+                .top()
+                .map(|or_fr| or_fr.global_index)
+                .unwrap_or(0);
+
+            if and_gi > or_gi {
+                self.and_stack.truncate(self.e + 1);
+            }
+        }
+    }
+
     fn handle_call_clause(
         &mut self,
         indices: &mut IndexStore,