add order preserving tidy_trail, fix random_labeling/2

weighted_maximum/3 now works

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "scryer-prolog"
-version = "0.8.112"
+version = "0.8.114"
 authors = ["Mark Thom <markjordanthom@gmail.com>"]
 build = "build.rs"
 repository = "https://github.com/mthom/scryer-prolog"

src/prolog/clause_types.rs

@@ -2,6 +2,7 @@ use prolog_parser::ast::*;
 
 use crate::prolog::forms::Number;
 use crate::prolog::machine::machine_indices::*;
+use crate::prolog::rug::rand::RandState;
 
 use ref_thread_local::RefThreadLocal;
 
@@ -81,6 +82,10 @@ pub enum InlinedClauseType {
     IsVar(RegType),
 }
 
+ref_thread_local! {
+    pub static managed RANDOM_STATE: RandState<'static> = RandState::new();
+}
+
 ref_thread_local! {
     pub static managed CLAUSE_TYPE_FORMS: BTreeMap<(&'static str, usize), ClauseType> = {
         let mut m = BTreeMap::new();
@@ -223,12 +228,14 @@ pub enum SystemClauseType {
     GetCutPoint,
     GetDoubleQuotes,
     InstallNewBlock,
+    Maybe,
     ResetBlock,
     ReturnFromAttributeGoals,
     ReturnFromVerifyAttr,
     SetBall,
     SetCutPointByDefault(RegType),
     SetDoubleQuotes,
+    SetSeed,
     SkipMaxList,
     Succeed,
     TermVariables,
@@ -319,6 +326,7 @@ impl SystemClauseType {
                 clause_name!("$install_inference_counter")
             }
             &SystemClauseType::LiftedHeapLength => clause_name!("$lh_length"),
+            &SystemClauseType::Maybe => clause_name!("maybe"),
             &SystemClauseType::ModuleHeadIsDynamic => clause_name!("$module_head_is_dynamic"),
             &SystemClauseType::ModuleOf => clause_name!("$module_of"),
             &SystemClauseType::NoSuchPredicate => clause_name!("$no_such_predicate"),
@@ -329,6 +337,7 @@ impl SystemClauseType {
             &SystemClauseType::RemoveInferenceCounter => clause_name!("$remove_inference_counter"),
             &SystemClauseType::RestoreCutPolicy => clause_name!("$restore_cut_policy"),
             &SystemClauseType::SetCutPoint(_) => clause_name!("$set_cp"),
+            &SystemClauseType::SetSeed => clause_name!("$set_seed"),
             &SystemClauseType::StoreGlobalVar => clause_name!("$store_global_var"),
             &SystemClauseType::StoreGlobalVarWithOffset => {
                 clause_name!("$store_global_var_with_offset")
@@ -417,6 +426,7 @@ impl SystemClauseType {
             ("$install_scc_cleaner", 2) => Some(SystemClauseType::InstallSCCCleaner),
             ("$install_inference_counter", 3) => Some(SystemClauseType::InstallInferenceCounter),
             ("$lh_length", 1) => Some(SystemClauseType::LiftedHeapLength),
+            ("$maybe", 0) => Some(SystemClauseType::Maybe),
             ("$module_of", 2) => Some(SystemClauseType::ModuleOf),
             ("$module_retract_clause", 5) => Some(SystemClauseType::ModuleRetractClause),
             ("$module_head_is_dynamic", 2) => Some(SystemClauseType::ModuleHeadIsDynamic),
@@ -450,6 +460,7 @@ impl SystemClauseType {
             ("$set_ball", 1) => Some(SystemClauseType::SetBall),
             ("$set_cp_by_default", 1) => Some(SystemClauseType::SetCutPointByDefault(temp_v!(1))),
             ("$set_double_quotes", 1) => Some(SystemClauseType::SetDoubleQuotes),
+            ("$set_seed", 1) => Some(SystemClauseType::SetSeed),
             ("$skip_max_list", 4) => Some(SystemClauseType::SkipMaxList),
             ("$store_global_var", 2) => Some(SystemClauseType::StoreGlobalVar),
             ("$store_global_var_with_offset", 2) => Some(SystemClauseType::StoreGlobalVarWithOffset),

src/prolog/lib/atts.pl

@@ -20,7 +20,7 @@
 
 '$default_attr_list'([PG | PGs], Module, AttrVar) -->
     (  { '$module_of'(Module, PG) } -> [Module:put_atts(AttrVar, PG)]
-    ;  true
+    ;  { true }
     ),
     '$default_attr_list'(PGs, Module, AttrVar).
 '$default_attr_list'([], _, _) --> [].

src/prolog/lib/clpb.pl

@@ -34,7 +34,7 @@
 :- use_module(library(lists)).
 :- use_module(library(non_iso)).
 :- use_module(library(dcgs)).
-%:- use_module(library(types)).
+:- use_module(library(error), []).
 
 :- attribute
         clpb/1,
@@ -77,8 +77,8 @@ must_be(list(What), Where, Term) :- !,
 must_be(ground, _, Term) :- !,
         functor(Term, _, _).
 
-must_be(Type, Goal-Arg, Term) :-
-        must_be(Term, Type, Goal, Arg).
+must_be(Type, _, Term) :-
+        error:must_be(Type, Term).
 
 clpz_list(Nil, _) :- Nil == [].
 clpz_list(Ls, Where) :-
@@ -87,7 +87,6 @@ clpz_list(Ls, Where) :-
     ;   Ls = [_|Rest],
         clpz_list(Rest, Where)
     ).
-    
 
 
 instantiation_error(Term) :- instantiation_error(Term, unknown(Term)-1).
@@ -139,7 +138,10 @@ partition(Pred, Ls0, As, Bs) :-
         include(Pred, Ls0, As),
         exclude(Pred, Ls0, Bs).
 
-sum_list(Ls, S) :- sumlist(Ls, S).
+sum_list(Ls, S) :-
+        foldl(sum_, Ls, 0, S).
+
+sum_(L, S0, S) :- S is S0 + L.
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   Pairs.

src/prolog/lib/error.pl

@@ -33,6 +33,10 @@ must_be(Type, Term) :-
 must_be_(Type, _) :-
         var(Type),
         instantiation_error(Type).
+must_be_(var, Term) :-
+        (   var(Term) -> true
+        ;   throw(error(uninstantiation_error, _))
+        ).
 must_be_(integer, Term) :- check_(integer, integer, Term).
 must_be_(atom, Term)    :- check_(atom, atom, Term).
 must_be_(list, Term)    :- check_(ilist, list, Term).
@@ -52,6 +56,7 @@ type(type).
 type(integer).
 type(atom).
 type(list).
+type(var).
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    can_be(Type, Term)

src/prolog/lib/non_iso.pl

@@ -4,8 +4,8 @@
 %% ?- use_module(library(non_iso)).
 
 :- module(non_iso, [bb_b_put/2, bb_get/2, bb_put/2, call_cleanup/2,
-		    call_with_inference_limit/3, forall/2,
-		    setup_call_cleanup/3, variant/2]).
+		    call_with_inference_limit/3, forall/2, maybe/0,
+		    set_random/1, setup_call_cleanup/3, variant/2]).
 
 forall(Generate, Test) :-
     \+ (Generate, \+ Test).
@@ -125,3 +125,17 @@ call_with_inference_limit(_, _, R, Bb, B) :-
     '$call_with_default_policy'(handle_ile(B, Ball, R)).
 
 variant(X, Y) :- '$variant'(X, Y).
+
+% succeeds with probability 0.5.
+maybe :- '$maybe'.
+
+set_random(Seed) :-
+    (  nonvar(Seed) ->
+       (  Seed = seed(S) ->
+	  (  var(S) -> throw(error(instantiation_error, set_random/1))
+	  ;  integer(S) -> '$set_seed'(S)
+	  ;  throw(error(type_error(integer(S), set_random/1)))
+	  )
+       )
+    ;  throw(error(instantiation_error, set_random/1))
+    ).

src/prolog/machine/attributed_variables.pl

@@ -25,6 +25,7 @@ verify_attrs([], _, _, []).
 
 call_verify_attributes(Attrs, _, _, []) :-
     var(Attrs), !.
+call_verify_attributes([], _, _, []).
 call_verify_attributes([Attr|Attrs], Var, Value, ListOfGoalLists) :-
     gather_modules([Attr|Attrs], Modules0),
     sort(Modules0, Modules),

src/prolog/machine/attributed_variables.rs

@@ -14,6 +14,7 @@ pub(super) struct AttrVarInitializer {
     pub(super) attr_var_queue: Vec<usize>,
     pub(super) bindings: Bindings,
     pub(super) cp: LocalCodePtr,
+    pub(super) instigating_p: LocalCodePtr,
     pub(super) verify_attrs_loc: usize,
     pub(super) project_attrs_loc: usize,
 }
@@ -24,6 +25,7 @@ impl AttrVarInitializer {
             attribute_goals: vec![],
             attr_var_queue: vec![],
             bindings: vec![],
+            instigating_p: LocalCodePtr::default(),
             cp: LocalCodePtr::default(),
             verify_attrs_loc,
             project_attrs_loc,
@@ -41,10 +43,12 @@ impl AttrVarInitializer {
 impl MachineState {
     pub(super) fn push_attr_var_binding(&mut self, h: usize, addr: Addr) {
         if self.attr_var_init.bindings.is_empty() {
+            self.attr_var_init.instigating_p = self.p.local();
+            
             if self.last_call {
                 self.attr_var_init.cp = self.cp;
             } else {
-                self.attr_var_init.cp = self.p.local();
+                self.attr_var_init.cp = self.p.local() + 1;
             }
             
             self.p = CodePtr::VerifyAttrInterrupt(self.attr_var_init.verify_attrs_loc);
@@ -201,7 +205,7 @@ impl Machine {
             &mut self.code_repo,
             &mut readline::input_stream(),
         );
-
+               
         self.machine_st
             .print_attribute_goals_string(&self.indices.op_dir)
     }

src/prolog/machine/machine_indices.rs

@@ -178,10 +178,11 @@ impl From<Ref> for Addr {
     }
 }
 
-#[derive(Clone)]
+#[derive(Clone, Copy)]
 pub enum TrailRef {
     Ref(Ref),
-    AttrVarLink(usize, Addr),
+    AttrVarHeapLink(usize),
+    AttrVarListLink(usize, usize),
 }
 
 impl From<Ref> for TrailRef {
@@ -468,6 +469,12 @@ pub struct IndexStore {
 }
 
 impl IndexStore {
+    pub fn reset_global_variable_offsets(&mut self) {
+        for (_, ref mut offset) in self.global_variables.values_mut() {
+            *offset = None;
+        }
+    }
+    
     pub fn predicate_exists(
         &self,
         name: ClauseName,

src/prolog/machine/machine_state_impl.rs

@@ -648,9 +648,15 @@ impl MachineState {
                     self.tr += 1;
                 }
             }
-            TrailRef::AttrVarLink(h, prev_addr) => {
+            TrailRef::AttrVarHeapLink(h) => {
                 if h < self.hb {
-                    self.trail.push(TrailRef::AttrVarLink(h, prev_addr));
+                    self.trail.push(TrailRef::AttrVarHeapLink(h));
+                    self.tr += 1;
+                }
+            }
+            TrailRef::AttrVarListLink(h, l) => {
+                if h < self.hb {
+                    self.trail.push(TrailRef::AttrVarListLink(h, l));
                     self.tr += 1;
                 }
             }
@@ -680,7 +686,7 @@ impl MachineState {
         // additions, now that deleted attributes can be undeleted by
         // backtracking.
         for i in (a1..a2).rev() {
-            match self.trail[i].clone() {
+            match self.trail[i] {
                 TrailRef::Ref(Ref::HeapCell(h)) => {
                     self.heap[h] = HeapCellValue::Addr(Addr::HeapCell(h))
                 }
@@ -690,8 +696,11 @@ impl MachineState {
                 TrailRef::Ref(Ref::StackCell(fr, sc)) => {
                     self.and_stack[fr][sc] = Addr::StackCell(fr, sc)
                 }
-                TrailRef::AttrVarLink(h, prev_addr) => {
-                    self.heap[h] = HeapCellValue::Addr(prev_addr)
+                TrailRef::AttrVarHeapLink(h) => {
+                    self.heap[h] = HeapCellValue::Addr(Addr::HeapCell(h));
+                }
+                TrailRef::AttrVarListLink(h, l) => {
+                    self.heap[h] = HeapCellValue::Addr(Addr::Lis(l));
                 }
             }
         }
@@ -732,28 +741,21 @@ impl MachineState {
         }
 
         let b = self.b - 1;
-        let mut i = self.or_stack[b].tr;
+        let hb = self.hb;
+        let mut offset = 0;
 
-        while i < self.tr {
-            let tr_i = self.trail[i].clone();
-            let hb = self.hb;
-
-            match tr_i {
+        for i in self.or_stack[b].tr .. self.tr {
+            match self.trail[i] {
                 TrailRef::Ref(Ref::AttrVar(tr_i))
-                | TrailRef::Ref(Ref::HeapCell(tr_i))
-                | TrailRef::AttrVarLink(tr_i, _) => {
-                    if tr_i < hb {
-                        i += 1;
-                    } else {
-                        let tr = self.tr;
-                        let val = self.trail[tr - 1].clone();
-                        self.trail[i] = val;
-                        self.trail.pop();
-                        self.tr -= 1;
-                    }
-                }
+              | TrailRef::Ref(Ref::HeapCell(tr_i))
+              | TrailRef::AttrVarHeapLink(tr_i)
+              | TrailRef::AttrVarListLink(tr_i, _) =>
+                  if tr_i >= hb {
+                      offset += 1;
+                  } else {
+                      self.trail[i - offset] = self.trail[i];
+                  },     
                 TrailRef::Ref(Ref::StackCell(fr, _)) => {
-                    let b = self.b - 1;
                     let fr_gi = self.and_stack[fr].global_index;
                     let b_gi = if !self.or_stack.is_empty() {
                         self.or_stack[b].global_index
@@ -761,20 +763,19 @@ impl MachineState {
                         0
                     };
 
-                    if fr_gi < b_gi {
-                        i += 1;
+                    if fr_gi >= b_gi {
+                        offset += 1;
                     } else {
-                        let tr = self.tr;
-                        let val = self.trail[tr - 1].clone();
-                        self.trail[i] = val;
-                        self.trail.pop();
-                        self.tr -= 1;
+                        self.trail[i - offset] = self.trail[i];
                     }
                 }
-            };
+            }
         }
-    }
 
+        self.tr -= offset;
+        self.trail.truncate(self.tr);
+    }
+        
     #[inline]
     fn write_char_to_string(&mut self, s: &mut StringList, c: char) -> bool {
         self.pstr_trail(s.clone());
@@ -1665,7 +1666,7 @@ impl MachineState {
         } else if s.is_expandable() {
             self.heap
                 .push(HeapCellValue::Addr(Addr::Con(Constant::String(s.clone()))));
-            
+
             self.s = h;
             self.mode = MachineMode::Read;
         } else {
@@ -3424,56 +3425,4 @@ impl MachineState {
         self.heap_locs.clear();
         self.lifted_heap.clear();
     }
-
-    pub(super) fn sink_to_snapshot(&mut self) -> MachineState {
-        let mut snapshot = MachineState::with_capacity(0);
-
-        snapshot.hb = self.hb;
-        snapshot.e = self.e;
-        snapshot.b = self.b;
-        snapshot.b0 = self.b0;
-        snapshot.s = self.s;
-        snapshot.tr = self.tr;
-        snapshot.pstr_tr = self.pstr_tr;
-        snapshot.num_of_args = self.num_of_args;
-
-        snapshot.fail = self.fail;
-        snapshot.trail = mem::replace(&mut self.trail, vec![]);
-        snapshot.pstr_trail = mem::replace(&mut self.pstr_trail, vec![]);
-        snapshot.heap = self.heap.take();
-        snapshot.mode = self.mode;
-        snapshot.and_stack = self.and_stack.take();
-        snapshot.or_stack = self.or_stack.take();
-        snapshot.registers = mem::replace(&mut self.registers, vec![]);
-        snapshot.block = self.block;
-
-        snapshot.ball = self.ball.take();
-        snapshot.lifted_heap = mem::replace(&mut self.lifted_heap, vec![]);
-
-        snapshot
-    }
-
-    pub(super) fn absorb_snapshot(&mut self, mut snapshot: MachineState) {
-        self.hb = snapshot.hb;
-        self.e = snapshot.e;
-        self.b = snapshot.b;
-        self.b0 = snapshot.b0;
-        self.s = snapshot.s;
-        self.tr = snapshot.tr;
-        self.pstr_tr = snapshot.pstr_tr;
-        self.num_of_args = snapshot.num_of_args;
-
-        self.fail = snapshot.fail;
-        self.trail = mem::replace(&mut snapshot.trail, vec![]);
-        self.pstr_trail = mem::replace(&mut snapshot.pstr_trail, vec![]);
-        self.heap = snapshot.heap.take();
-        self.mode = snapshot.mode;
-        self.and_stack = snapshot.and_stack.take();
-        self.or_stack = snapshot.or_stack.take();
-        self.registers = mem::replace(&mut snapshot.registers, vec![]);
-        self.block = snapshot.block;
-
-        self.ball = snapshot.ball.take();
-        self.lifted_heap = mem::replace(&mut snapshot.lifted_heap, vec![]);
-    }
 }

src/prolog/machine/mod.rs

@@ -6,6 +6,7 @@ use crate::prolog::fixtures::*;
 use crate::prolog::forms::*;
 use crate::prolog::heap_print::*;
 use crate::prolog::instructions::*;
+use crate::prolog::machine::heap::Heap;
 use crate::prolog::read::*;
 use crate::prolog::write::{next_keypress, ContinueResult};
 
@@ -71,6 +72,7 @@ impl MachinePolicies {
 
 pub struct Machine {
     pub(super) machine_st: MachineState,
+    pub(super) inner_heap: Heap,
     pub(super) policies: MachinePolicies,
     pub(super) indices: IndexStore,
     pub(super) code_repo: CodeRepo,
@@ -227,13 +229,13 @@ impl Machine {
     }
 
     pub fn run_init_code(&mut self, code: Code) {
-	let old_machine_st = self.machine_st.sink_to_snapshot();
+	let old_machine_st = self.sink_to_snapshot();
 	self.machine_st.reset();
 
 	self.code_repo.cached_query = code;
 	self.run_query(&AllocVarDict::new());
 
-	self.machine_st.absorb_snapshot(old_machine_st);
+	self.absorb_snapshot(old_machine_st);
     }
 
     pub fn run_top_level(&mut self) {
@@ -260,6 +262,7 @@ impl Machine {
     pub fn new(prolog_stream: PrologStream) -> Self {
         let mut wam = Machine {
             machine_st: MachineState::new(),
+            inner_heap: Heap::with_capacity(256 * 256),
             policies: MachinePolicies::new(),
             indices: IndexStore::new(),
             code_repo: CodeRepo::new(),
@@ -570,9 +573,14 @@ impl Machine {
                 };
 
                 let stream = parsing_stream(s.as_bytes());
+                let snapshot = self.sink_to_snapshot();
+                let policies = mem::replace(&mut self.policies, MachinePolicies::new());
 
-                let snapshot = self.machine_st.sink_to_snapshot();
                 self.machine_st.reset();
+                self.machine_st.heap = mem::replace(
+                    &mut self.inner_heap,
+                    Heap::with_capacity(0),
+                );
 
                 let result = match stream_to_toplevel(stream, self) {
                     Ok(packet) => compile_term(self, packet),
@@ -580,6 +588,8 @@ impl Machine {
                 };
 
                 self.handle_eval_session(result, snapshot);
+                self.indices.reset_global_variable_offsets();
+                self.policies = policies;
             }
 	    REPLCodePtr::UseModule =>
 		self.use_module(ModuleSource::Library),
@@ -594,10 +604,66 @@ impl Machine {
         self.machine_st.p = CodePtr::Local(p);
     }
 
+    fn sink_to_snapshot(&mut self) -> MachineState {
+        let mut snapshot = MachineState::with_capacity(0);
+
+        snapshot.hb = self.machine_st.hb;
+        snapshot.e = self.machine_st.e;
+        snapshot.b = self.machine_st.b;
+        snapshot.b0 = self.machine_st.b0;
+        snapshot.s = self.machine_st.s;
+        snapshot.tr = self.machine_st.tr;
+        snapshot.pstr_tr = self.machine_st.pstr_tr;
+        snapshot.num_of_args = self.machine_st.num_of_args;
+
+        snapshot.fail = self.machine_st.fail;
+        snapshot.trail = mem::replace(&mut self.machine_st.trail, vec![]);
+        snapshot.pstr_trail = mem::replace(&mut self.machine_st.pstr_trail, vec![]);
+        snapshot.heap = self.machine_st.heap.take();
+        snapshot.mode = self.machine_st.mode;
+        snapshot.and_stack = self.machine_st.and_stack.take();
+        snapshot.or_stack = self.machine_st.or_stack.take();
+        snapshot.registers = mem::replace(&mut self.machine_st.registers, vec![]);
+        snapshot.block = self.machine_st.block;
+
+        snapshot.ball = self.machine_st.ball.take();
+        snapshot.lifted_heap = mem::replace(&mut self.machine_st.lifted_heap, vec![]);
+
+        snapshot
+    }
+
+    fn absorb_snapshot(&mut self, mut snapshot: MachineState) {
+        self.machine_st.hb = snapshot.hb;
+        self.machine_st.e = snapshot.e;
+        self.machine_st.b = snapshot.b;
+        self.machine_st.b0 = snapshot.b0;
+        self.machine_st.s = snapshot.s;
+        self.machine_st.tr = snapshot.tr;
+        self.machine_st.pstr_tr = snapshot.pstr_tr;
+        self.machine_st.num_of_args = snapshot.num_of_args;
+
+        self.machine_st.fail = snapshot.fail;
+        self.machine_st.trail = mem::replace(&mut snapshot.trail, vec![]);
+        self.machine_st.pstr_trail = mem::replace(&mut snapshot.pstr_trail, vec![]);
+
+        self.inner_heap = self.machine_st.heap.take();
+        self.inner_heap.truncate(0);
+        
+        self.machine_st.heap = snapshot.heap.take();
+        self.machine_st.mode = snapshot.mode;
+        self.machine_st.and_stack = snapshot.and_stack.take();
+        self.machine_st.or_stack = snapshot.or_stack.take();
+        self.machine_st.registers = mem::replace(&mut snapshot.registers, vec![]);
+        self.machine_st.block = snapshot.block;
+
+        self.machine_st.ball = snapshot.ball.take();
+        self.machine_st.lifted_heap = mem::replace(&mut snapshot.lifted_heap, vec![]);
+    }
+
     fn propagate_exception_to_toplevel(&mut self, snapshot: MachineState) {
         let ball = self.machine_st.ball.take();
 
-        self.machine_st.absorb_snapshot(snapshot);
+        self.absorb_snapshot(snapshot);
         self.machine_st.ball = ball;
 
         let h = self.machine_st.heap.h;
@@ -617,7 +683,7 @@ impl Machine {
                 };
 
                 let attr_goals = self.attribute_goals();
-
+                
                 if !(self.machine_st.b > 0) {
                     if bindings.is_empty() {
                         let space = if requires_space(&attr_goals, ".") {
@@ -632,7 +698,7 @@ impl Machine {
                             println!("true.");
                         }
 
-                        self.machine_st.absorb_snapshot(snapshot);
+                        self.absorb_snapshot(snapshot);
                         return;
                     }
                 } else if bindings.is_empty() && attr_goals.is_empty() {
@@ -664,7 +730,7 @@ impl Machine {
                         }
                         ContinueResult::Conclude => {
 			    print!(" ...\r\n");
-                            self.machine_st.absorb_snapshot(snapshot);
+                            self.absorb_snapshot(snapshot);
                             return;
                         }
                     };
@@ -676,12 +742,12 @@ impl Machine {
                                 return;
                             } else {
 				print!("false.\r\n");
-                                self.machine_st.absorb_snapshot(snapshot);
+                                self.absorb_snapshot(snapshot);
                                 return;
                             }
                         }
                         EvalSession::Error(err) => {
-                            self.machine_st.absorb_snapshot(snapshot);
+                            self.absorb_snapshot(snapshot);
                             self.throw_session_error(err, (clause_name!("repl"), 0));
                             return;
                         }
@@ -712,7 +778,7 @@ impl Machine {
                 }
             },
             EvalSession::Error(err) => {
-                self.machine_st.absorb_snapshot(snapshot);
+                self.absorb_snapshot(snapshot);
                 self.throw_session_error(err, (clause_name!("repl"), 0));
                 return;
             }
@@ -725,7 +791,7 @@ impl Machine {
             _ => println!("true.")
         }
 
-        self.machine_st.absorb_snapshot(snapshot);
+        self.absorb_snapshot(snapshot);
     }
 
     pub(super) fn run_query(&mut self, alloc_locs: &AllocVarDict) {
@@ -1048,7 +1114,13 @@ impl MachineState {
                 CodePtr::VerifyAttrInterrupt(_) => {
                     self.p = CodePtr::Local(self.attr_var_init.cp);
 
-                    if !self.verify_attr_stepper(indices, policies, code_repo, prolog_stream) {
+                    let instigating_p = CodePtr::Local(self.attr_var_init.instigating_p);
+                    let instigating_instr = code_repo.lookup_instr(false, &instigating_p).unwrap();
+
+                    if instigating_instr.as_ref().is_head_instr() {
+                        let cp = self.p.local();
+                        self.run_verify_attr_interrupt(cp);
+                    } else if !self.verify_attr_stepper(indices, policies, code_repo, prolog_stream) {
                         if self.fail {
                             break;
                         }

src/prolog/machine/system_calls.rs

@@ -17,6 +17,8 @@ use crate::prolog::ordered_float::OrderedFloat;
 use crate::prolog::read::{readline, PrologStream};
 use crate::prolog::rug::Integer;
 
+use crate::ref_thread_local::RefThreadLocal;
+
 use indexmap::{IndexMap, IndexSet};
 
 use std::collections::VecDeque;
@@ -679,7 +681,8 @@ impl MachineState {
                                             let c = self.int_to_char_code(&n, "atom_codes", 2)?;
                                             chars.push(c as char);
                                         }
-                                        &Addr::Con(Constant::CharCode(c)) => chars.push(c as char),
+                                        &Addr::Con(Constant::CharCode(c)) =>
+                                            chars.push(c as char),
                                         _ => {
                                             let err = MachineError::type_error(
                                                 ValidType::Integer,
@@ -1018,8 +1021,14 @@ impl MachineState {
                             tail
                         };
 
+                        let trail_ref = match old_addr {
+                            Addr::HeapCell(h) => TrailRef::AttrVarHeapLink(h),
+                            Addr::Lis(l) => TrailRef::AttrVarListLink(l1 + 1, l),
+                            _ => unreachable!()
+                        };
+
                         self.heap[l1 + 1] = HeapCellValue::Addr(tail);
-                        self.trail(TrailRef::AttrVarLink(l1 + 1, old_addr));
+                        self.trail(trail_ref);
                     }
                 }
             }
@@ -1041,7 +1050,7 @@ impl MachineState {
                                 };
 
                                 self.heap[h + 1] = HeapCellValue::Addr(tail);
-                                self.trail(TrailRef::AttrVarLink(h + 1, Addr::Lis(l)));
+                                self.trail(TrailRef::AttrVarListLink(h + 1, l));
                             }
                             _ => unreachable!(),
                         }
@@ -1255,6 +1264,19 @@ impl MachineState {
                     _ => self.fail = true,
                 }
             }
+            &SystemClauseType::Maybe => {
+                let result = {
+                    let mut rand = RANDOM_STATE.borrow_mut();
+
+                    if rand.bits(1) == 0 {
+                        true
+                    } else {
+                        false
+                    }
+                };
+
+                self.fail = result;
+            }
             &SystemClauseType::OpDeclaration => {
                 let priority = self[temp_v!(1)].clone();
                 let specifier = self[temp_v!(2)].clone();
@@ -1664,7 +1686,7 @@ impl MachineState {
                 return Ok(());
             }
             &SystemClauseType::ReturnFromAttributeGoals => {
-                self.deallocate();
+                self.deallocate();                
                 self.p = CodePtr::Local(LocalCodePtr::TopLevel(0, 0));
                 return Ok(());
             }
@@ -1834,6 +1856,31 @@ impl MachineState {
             }
             &SystemClauseType::SetBall =>
                 self.set_ball(),
+            &SystemClauseType::SetSeed => {
+                let seed = self.store(self.deref(self[temp_v!(1)].clone()));
+
+                let seed = match seed {
+                    Addr::Con(Constant::Integer(n)) =>
+                        n,
+                    Addr::Con(Constant::CharCode(c)) =>
+                        Integer::from(c),
+                    Addr::Con(Constant::Rational(r)) => {
+                        if r.denom() == &1 {
+                            r.numer().clone()
+                        } else {
+                            self.fail = true;
+                            return Ok(());
+                        }
+                    }
+                    _ => {
+                        self.fail = true;
+                        return Ok(());
+                    }
+                };
+
+                let mut rand = RANDOM_STATE.borrow_mut();
+                rand.seed(&seed);
+            }
             &SystemClauseType::SkipMaxList =>
                 if let Err(err) = self.skip_max_list() {
                     return Err(err);