From a68a67376127cb9accf26c4ed43438f188eb24c8 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Thu, 6 Mar 2025 21:42:50 -0800
Subject: iterator API: reject objects that don't make sense.

* lib.c (iter_more): Do not return t for unrecognized
objects, but thrown an exception. Do return t for
conses, which is necessary since they are iterators
for lists. Also, as a special case, we return t for
struct objects that don't have an iter-more method.
This is needed for the documented fast protocol.
Iterator objects implementing the fast protocol
still get iter-more invoked on them. The client
usually doesn't know that the iterator implements
the fast protocol, and so calls iter-more, which
unconditionally has to returns true.
(iter_item): Do not fall back on car(iter) for all
unhandled objects. Only conses are handled
via car. All unrecognized objects trigger an
exception.
(iter_step): Do not try to handle list-like
objects via cdr, only lists.
Improve the diagnostic for hitting the end of
an improper list: diagnostic shows the cons
cell rather than just the terminating atom.

* tests/012/iter.tl: Some test cases validating
that the functions error out for strings and
vectors. Much more coverage is possible here but
doesn't seem worth it; e.g. that the functions
reject a buffer, regex, function, ...
---
 lib.c             | 35 +++++++++++++++++++++--------------
 tests/012/iter.tl | 24 ++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 14 deletions(-)

diff --git a/lib.c b/lib.c
index ed0a47fe..1e127e88 100644
--- a/lib.c
+++ b/lib.c
@@ -1287,6 +1287,8 @@ static val iter_dynamic(struct seq_iter *si_orig)
 
 val iter_more(val iter)
 {
+  val self = lit("iter-more");
+
   switch (type(iter)) {
   case NIL:
     return nil;
@@ -1295,6 +1297,8 @@ val iter_more(val iter)
   case NUM:
   case BGNUM:
   case FLNUM:
+  case CONS:
+  case LCONS:
     return t;
   case COBJ:
     if (iter->co.cls == seq_iter_cls)
@@ -1307,15 +1311,19 @@ val iter_more(val iter)
       val iter_more_meth = get_special_slot(iter, iter_more_m);
       if (iter_more_meth)
         return funcall1(iter_more_meth, iter);
+      return t; /* needed for supporting fast procotol */
     }
-    /* fallthrough */
   default:
-    return t;
+    break;
   }
+
+  unsup_obj(self, iter);
 }
 
 val iter_item(val iter)
 {
+  val self = lit("iter-item");
+
   switch (type(iter)) {
   case NIL:
     return nil;
@@ -1324,6 +1332,9 @@ val iter_item(val iter)
   case BGNUM:
   case FLNUM:
     return iter;
+  case CONS:
+  case LCONS:
+    return car(iter);
   case COBJ:
     if (iter->co.cls == seq_iter_cls)
     {
@@ -1338,8 +1349,10 @@ val iter_item(val iter)
     }
     /* fallthrough */
   default:
-    return car(iter);
+    break;
   }
+
+  unsup_obj(self, iter);
 }
 
 val iter_step(val iter)
@@ -1359,7 +1372,8 @@ val iter_step(val iter)
     {
       val next = cdr(iter);
       if (next && !consp(next))
-        uw_throwf(type_error_s, lit("~a: ~s is not a cons"), self, next, nao);
+        uw_throwf(type_error_s, lit("~a: ~s is improperly terminated"),
+                  self, iter, nao);
       return next;
     }
   case COBJ:
@@ -1379,17 +1393,10 @@ val iter_step(val iter)
     }
     /* fallthrough */
   default:
-    {
-      val next = cdr(iter);
-      if (next) {
-        seq_info_t sinf = seq_info(next);
-        if (sinf.kind != SEQ_LISTLIKE)
-          uw_throwf(type_error_s, lit("~a: ~s is improperly terminated"),
-                    self, iter, nao);
-      }
-      return next;
-    }
+    break;
   }
+
+  unsup_obj(self, iter);
 }
 
 val iter_reset(val iter, val obj)
diff --git a/tests/012/iter.tl b/tests/012/iter.tl
index 9b12d49c..94a50f34 100644
--- a/tests/012/iter.tl
+++ b/tests/012/iter.tl
@@ -130,3 +130,27 @@
      (equal l0 l1) t
      (equal l1 l2) nil
      (equal (cdr l1) l2) t)))
+
+(mtest
+  (iter-more "") :error
+  (iter-more "a") :error
+  (iter-more "abc") :error
+  (iter-more #()) :error
+  (iter-more #(1)) :error
+  (iter-more #(1 2 3)) :error)
+
+(mtest
+  (iter-step "") :error
+  (iter-step "a") :error
+  (iter-step "abc") :error
+  (iter-step #()) :error
+  (iter-step #(1)) :error
+  (iter-step #(1 2 3)) :error)
+
+(mtest
+  (iter-item "") :error
+  (iter-item "a") :error
+  (iter-item "abc") :error
+  (iter-item #()) :error
+  (iter-item #(1)) :error
+  (iter-item #(1 2 3)) :error)
-- 
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