From 0458a7052793e4414fd008696119b6a3064928cb Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Tue, 1 May 2018 10:11:27 -0600
Subject: [PATCH] handle non-list cases in '', add '' as a built-in (re: issue
 #31)

---
 src/prolog/ast.rs                        |  2 +-
 src/prolog/builtins.rs                   |  7 +++++--
 src/prolog/machine/machine_state_impl.rs | 13 ++++++++-----
 src/prolog/macros.rs                     |  6 ++++++
 src/tests.rs                             |  4 ++++
 5 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/prolog/ast.rs b/src/prolog/ast.rs
index b1208c0a..318586fd 100644
--- a/src/prolog/ast.rs
+++ b/src/prolog/ast.rs
@@ -537,7 +537,7 @@ impl fmt::Display for Constant {
             &Constant::Atom(ref atom) =>
                 write!(f, "{}", atom),
             &Constant::Char(c) =>
-                write!(f, "{}", c as u8),
+                write!(f, "'{}'", c as u8),
             &Constant::EmptyList =>
                 write!(f, "[]"),
             &Constant::Number(ref n) =>
diff --git a/src/prolog/builtins.rs b/src/prolog/builtins.rs
index bb88d6bf..2f5fbc9c 100644
--- a/src/prolog/builtins.rs
+++ b/src/prolog/builtins.rs
@@ -644,6 +644,7 @@ fn get_builtins() -> Code {
          keysort_execute!(), // keysort/2, 484.
          acyclic_term_execute!(), // acyclic_term/1, 485.
          cyclic_term_execute!(), // cyclic_term/1, 486.
+         skip_max_list_execute!() // '$skip_max_list', 487.
     ]
 }
 
@@ -763,7 +764,8 @@ pub fn build_code_and_op_dirs() -> (CodeDir, OpDir)
     code_dir.insert((clause_name!("keysort"), 2), CodeIndex::from((484, builtin.clone())));
     code_dir.insert((clause_name!("acyclic_term"), 1), CodeIndex::from((485, builtin.clone())));
     code_dir.insert((clause_name!("cyclic_term"), 1), CodeIndex::from((486, builtin.clone())));
-
+    code_dir.insert((clause_name!("$skip_max_list"), 4), CodeIndex::from((487, builtin.clone())));
+    
     (code_dir, op_dir)
 }
 
@@ -826,7 +828,8 @@ pub fn builtin_module() -> Module
                                             (clause_name!("sort"), 2),
                                             (clause_name!("keysort"), 2),
                                             (clause_name!("acyclic_term"), 1),
-                                            (clause_name!("cyclic_term"), 1)]);
+                                            (clause_name!("cyclic_term"), 1),
+                                            (clause_name!("$skip_max_list"), 4)]);
 
     for arity in 0 .. 63 {
         module_decl.exports.push((clause_name!("call"), arity));
diff --git a/src/prolog/machine/machine_state_impl.rs b/src/prolog/machine/machine_state_impl.rs
index 71397049..cedff74d 100644
--- a/src/prolog/machine/machine_state_impl.rs
+++ b/src/prolog/machine/machine_state_impl.rs
@@ -1759,8 +1759,9 @@ impl MachineState {
                 },
                 HeapCellValue::Addr(Addr::Con(Constant::EmptyList)) =>
                     return CycleSearchResult::PartialOrProperList(steps, hare),
-                HeapCellValue::Addr(Addr::HeapCell(hc)) if hc == hare =>
-                    return CycleSearchResult::PartialOrProperList(steps, hare),
+                HeapCellValue::Addr(ref hc @ Addr::HeapCell(_))
+                    if Addr::HeapCell(hare) == self.store(self.deref(hc.clone())) =>
+                      return CycleSearchResult::PartialOrProperList(steps, hare),
                 HeapCellValue::Addr(ref sc @ Addr::StackCell(..))
                     if *sc == self.store(self.deref(sc.clone())) =>
                       return CycleSearchResult::PartialOrProperList(steps, hare),
@@ -1787,7 +1788,7 @@ impl MachineState {
   ======================================================================================
       ?N, -Xs0 : N = 0, Xs = Xs0.
       ?N, +Xs0 : Xs0 is a proper or partial list, Xs0 = [X1, X2, ..., XN | Xs], N = Max,
-                 if |Xs0| >= Max, or, Xs = [] and N = |Xs0|.
+                 if |Xs0| >= Max, or, Xs = Xs0 and N = |Xs0|.
 */
     pub(super) fn skip_max_list(&mut self) {
         let max = self.store(self.deref(self[temp_v!(2)].clone()));
@@ -1814,8 +1815,10 @@ impl MachineState {
                                     self.finalize_skip_max_list(0, Addr::Con(Constant::EmptyList)),
                                 CycleSearchResult::PartialOrProperList(n, hc) =>
                                     self.finalize_skip_max_list(n, Addr::HeapCell(hc)),
-                                CycleSearchResult::NotList =>
-                                    self.fail = true
+                                CycleSearchResult::NotList => {
+                                    let xs0 = self[temp_v!(3)].clone();
+                                    self.finalize_skip_max_list(0, xs0);
+                                }
                             }
                         }
                     }
diff --git a/src/prolog/macros.rs b/src/prolog/macros.rs
index 799f666d..2cd018bc 100644
--- a/src/prolog/macros.rs
+++ b/src/prolog/macros.rs
@@ -763,6 +763,12 @@ macro_rules! cyclic_term_execute {
     )
 }
 
+macro_rules! skip_max_list_execute {
+    () => (
+        Line::Control(ControlInstruction::CallClause(ClauseType::SkipMaxList, 4, 0, true))
+    )
+}
+
 macro_rules! return_from_clause {
     ($lco:expr, $machine_st:expr) => {{
         if $lco {
diff --git a/src/tests.rs b/src/tests.rs
index b970af2a..4066300e 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -1644,4 +1644,8 @@ fn test_queries_on_skip_max_list() {
     assert_prolog_failure!(&mut wam, "?- Xs = [a,b|Xs], '$skip_max_list'(3, 5, X, Xs0).");
     assert_prolog_failure!(&mut wam, "?- X = [a,b|Y], Y = [c,d|X], '$skip_max_list'(4, 5, X, Xs0).");
     assert_prolog_failure!(&mut wam, "?- X = [a,b|Y], Y = [c,d|X], '$skip_max_list'(4, 3, X, Xs0).");
+
+    // tests on non lists.
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 9, non_list, Xs).",
+                           [["Xs = non_list", "N = 0"]]);
 }
-- 
2.54.0