From 7c8076e433d8bd95a91a9392bdbc2b5ba019542d Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Wed, 12 Dec 2018 13:10:10 -0500
Subject: [PATCH] Added Cyc_is_list()

---
 include/cyclone/runtime.h |  1 +
 runtime.c                 | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index 2f7668c5..69aa1fee 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -421,6 +421,7 @@ double MRG32k3a (double seed);
 int equal(object, object);
 object equalp(object, object);
 object Cyc_has_cycle(object lst);
+object Cyc_is_list(object lst);
 //object Cyc_is_boolean(object o);
 #define Cyc_is_boolean(o) (make_boolean(o == boolean_f || o == boolean_t))
 #define Cyc_is_pair(o) ((is_object_type(o) && ((list) o)->tag == pair_tag) ? boolean_t : boolean_f)
diff --git a/runtime.c b/runtime.c
index 6f84676d..204d9c34 100644
--- a/runtime.c
+++ b/runtime.c
@@ -779,6 +779,39 @@ object Cyc_has_cycle(object lst)
   }
 }
 
+/**
+ * Predicate - is the object a proper list?
+ * Based on `Cyc_has_cycle` so it is safe to call on circular lists.
+ */
+object Cyc_is_list(object lst)
+{
+  object slow_lst, fast_lst;
+  if ((lst == NULL)){
+    return boolean_t;
+  } else if (is_value_type(lst)) {
+    return boolean_f;
+  } else if (is_object_type(lst) && type_of(lst) != pair_tag) {
+    return boolean_f;
+  }
+  slow_lst = lst;
+  fast_lst = cdr(lst);
+  while (1) {
+    if (fast_lst == NULL)
+      return boolean_t;
+    if (Cyc_is_pair(fast_lst) == boolean_f)
+      return boolean_f; // Improper list
+    if ((cdr(fast_lst)) == NULL)
+      return boolean_t;
+    if (Cyc_is_pair(cdr(fast_lst)) == boolean_f)
+      return boolean_f; // Improper
+    if (slow_lst == fast_lst)
+      return boolean_t; // Cycle; we have a list
+
+    slow_lst = cdr(slow_lst);
+    fast_lst = cddr(fast_lst);
+  }
+}
+
 /**
  * Write string representation of a double to a buffer.
  * Added code from Chibi Scheme to print a ".0" if the