seq-begin: bugfix: non-lists don't work.

* lib.c (seq_begin): Do not null out si->inf.obj; it's needed
for accessing hashes and vector-like objects. This bug means
that seq-begin iteration has only worked correctly for lists.
The original motivation was not to have spurious retention of
the head of a lazy list, which is hereby reintroduced. But
iterators can be rewound. Let's just document this away and
leave it as a to-do item.

* txr.1: Document the limitation of seq-begin w.r.t.
lazy lists.

2 files changed, 8 insertions, 1 deletions

diff --git a/lib.c b/lib.c
index 40662d29..522128a6 100644
--- a/lib.c
+++ b/lib.c
@@ -454,7 +454,6 @@ val seq_begin(val obj)
   struct seq_iter *si = coerce(struct seq_iter *, chk_calloc(1, sizeof *si));
   si_obj = cobj(coerce(mem_t *, si), seq_iter_s, &seq_iter_ops);
   seq_iter_init(self, si, obj);
-  si->inf.obj = nil;
   return si_obj;
 }
 
diff --git a/txr.1 b/txr.1
index 689715e3..5377e3b4 100644
--- a/txr.1
+++ b/txr.1
@@ -31952,6 +31952,14 @@ If
 .meta object
 isn't a sequence, an exception is thrown.
 
+Note that if
+.meta object
+is a lazy list, the returned iterator maintains a reference to the
+head of that list during the traversal; therefore, generic iteration
+based on iterators from
+.code seq-begin
+is not suitable for indefinite iteration over infinite lists.
+
 .coNP Function @ seq-next
 .synb
 .mets (seq-next < iter << end-value )