New: length-list-<, length-<

These are functions for testing whether a list or
sequence is shorter than a given integer. This is cheaper
than calculating the length of lists, which is in
some cases impossible if they are infinite.

A length-str-< function already exists, useful
with lazy strings.

length-< uses length-list-< or length-str-<
as appropriate

* lib.[ch] (length_list_lt, length_lt): New functions.

* eval.c (eval_init): length-list-< and length-<
intrinsics registered.

* tests/012/seq.tl: New tests.

* txr.1: Documented.

1 files changed, 28 insertions, 0 deletions

diff --git a/lib.c b/lib.c
index 0fd695a5..515d5439 100644
--- a/lib.c
+++ b/lib.c
@@ -4857,6 +4857,19 @@ val length_list(val list)
   return bn_len;
 }
 
+val length_list_lt(val list, val len)
+{
+  val self = lit("length-list-lt");
+  cnum le = c_num(len, self);
+
+  while (consp(list) && le > 0) {
+    list = cdr(list);
+    le--;
+  }
+
+  return tnil(le > 0);
+}
+
 static val length_proper_list(val list)
 {
   cnum len = 0;
@@ -13147,6 +13160,21 @@ val length(val seq)
   }
 }
 
+val length_lt(val seq, val len)
+{
+  switch (type(seq)) {
+  case NIL:
+    return if3(plusp(len), t, nil);
+  case CONS:
+  case LCONS:
+    return length_list_lt(seq, len);
+  case LSTR:
+    return length_str_lt(seq, len);
+  default:
+    return lt(length(seq), len);
+  }
+}
+
 val sub(val seq, val from, val to)
 {
   switch (type(seq)) {