From 97d8d079476a7f2f4ef03170cc7449edfdda9a9b Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Sat, 11 Apr 2020 23:08:15 -0600
Subject: [PATCH] recognize empty lists as partial strings (#328)

---
 src/prolog/machine/system_calls.rs | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/prolog/machine/system_calls.rs b/src/prolog/machine/system_calls.rs
index 9083e275..f1004193 100644
--- a/src/prolog/machine/system_calls.rs
+++ b/src/prolog/machine/system_calls.rs
@@ -1196,7 +1196,21 @@ impl MachineState {
                 }
             }
             &SystemClauseType::IsPartialString => {
-                let mut heap_pstr_iter = self.heap_pstr_iter(self[temp_v!(1)]);
+                let addr = self.store(self.deref(self[temp_v!(1)]));
+
+                match addr {
+                    Addr::EmptyList => {
+                        return return_from_clause!(self.last_call, self);
+                    }
+                    Addr::AttrVar(_) | Addr::HeapCell(_) | Addr::StackCell(..) => {
+                        self.fail = true;
+                        return Ok(());
+                    }
+                    _ => {
+                    }
+                }
+
+                let mut heap_pstr_iter = self.heap_pstr_iter(addr);
 
                 while let Some(_) = heap_pstr_iter.next() {}
 
-- 
2.54.0