seq_build: seq_pend must be nondestructive for lists.

Let's have both a seq_pend and seq_nconc, where seq_nconc
can reuse the pieces of list passed to it.

* lib.h (struct seq_build_ops): New member, nconc.
(seq_build_ops_init): Add nconc parameter and initializer.
(seq_nconc): Function declared.

* lib.c (seq_build_list_pend): Switch to list_collect_append,
otherwise mappend behaves destructively.
(seq_build_list_nconc): New function.
(sb_vec_ops, sb_str_ops, sb_buf_ops, sb_struct_ops,
sb_carray_ops): Use seq_build_generic_pend for nconc
operation.
(sb_list_ops): Use new seq_build_list_nconc for nconc
operation.
(sb_finished_ops): Use null pointer for nconc operation.
(seq_nconc): New function.

1 files changed, 4 insertions, 2 deletions

diff --git a/lib.h b/lib.h
index f254ce6b..e3816473 100644
--- a/lib.h
+++ b/lib.h
@@ -483,12 +483,13 @@ typedef struct seq_build {
 struct seq_build_ops {
   void (*add)(struct seq_build *, val);
   void (*pend)(struct seq_build *, val);
+  void (*nconc)(struct seq_build *, val);
   void (*finish)(struct seq_build *);
   void (*mark)(struct seq_build *);
 };
 
-#define seq_build_ops_init(add, pend, finish, mark) \
-  { add, pend, finish, mark }
+#define seq_build_ops_init(add, pend, nconc, finish, mark) \
+  { add, pend, nconc, finish, mark }
 
 extern const seq_kind_t seq_kind_tab[MAXTYPE+1];
 
@@ -768,6 +769,7 @@ val iter_reset(val iter, val obj);
 void seq_build_init(val self, seq_build_t *bu, val likeobj);
 void seq_add(seq_build_t *bu, val item);
 void seq_pend(seq_build_t *bu, val items);
+void seq_nconc(seq_build_t *bu, val items);
 val seq_finish(seq_build_t *bu);
 NORETURN val throw_mismatch(val self, val obj, type_t);
 INLINE val type_check(val self, val obj, type_t typecode)