New whilet macro.

* eval.c (while_s): New symbol variable.
(me_whilet): New static function.
(eval_init): Initialize while_s. Use while_s in registration
of while macro. Register new whilet macro.

* txr.1: whilet is documented.

* tl.vim, txr.vim: Regenerated.

1 files changed, 63 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 745520fe..3bb05aae 100644
--- a/txr.1
+++ b/txr.1
@@ -10029,6 +10029,69 @@ is if the
 operator is used to terminate the implicit anonymous block,
 and is given an argument, which becomes the result value.
 
+.coNP Macro @ whilet
+.synb
+.mets (whilet ({ sym | >> ( sym << init-form )}+)
+.mets \ \  < body-form *)
+.syne
+.desc
+The
+.code whilet
+macro provides a construct which combines iteration with variable
+binding.
+
+The evaluation of the form takes place as follows. First, fresh bindings are
+established for
+.metn sym -s
+as if by the
+.code let*
+operator.
+It is an error for the list of variable bindings to be empty.
+
+After the establishment of the bindings, the the value of the
+.meta sym
+is tested. If the value is
+.codn nil ,
+then
+.code whilet
+terminates. Otherwise,
+.metn body-form -s
+are evaluated in the scope of the variable bindings, and then
+.code whilet
+iterates from the beginning, again establishing fresh bindings for the
+.metn sym -s,
+and testing the value of the last
+.metn sym .
+
+
+All evaluation takes place in an anonymous block, which can be
+terminated with the
+.code return
+operator. Doing so terminates the loop.
+If the
+.code whilet
+loop is thus terminated by an explicit
+.codn return ,
+a return value can be specified. Under normal termination, the return value is
+.codn nil .
+
+.TP* Examples:
+.cblk
+  ;; read lines of text from *std-input* and print them,
+  ;; until the end-of-stream condition:
+
+  (whilet ((line (get-line)))
+    (put-line line))
+
+  ;; read lines of text from *std-input* and print them,
+  ;; until the end-of-stream condition occurs or
+  ;; a line is identical to the character string "end".
+
+  (whilet ((line (get-line))
+           (more (and line (not (equal line "end")))))
+    (put-line line))
+.cble
+
 .coNP Operator/function @ if
 .synb
 .mets (if < cond < t-form <> [ e-form ])