From 14c6036dec37ae37cc0207aff130128528be7d00 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Wed, 23 Oct 2019 22:34:53 -0700
Subject: New macro: ldo.

This is a left-argument-inserting syntactic sugar for do.

* lisplib.c (op_set_entries): Add auto-load entry for ldo.

* share/txr/stdlib/op.tl (ldo): New macro.

* txr.1: Documented.
---
 lisplib.c              |  2 +-
 share/txr/stdlib/op.tl |  3 +++
 txr.1                  | 38 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/lisplib.c b/lisplib.c
index 62b41b3b..e3487496 100644
--- a/lisplib.c
+++ b/lisplib.c
@@ -747,7 +747,7 @@ static val debugger_set_entries(val dlt, val fun)
 static val op_set_entries(val dlt, val fun)
 {
   val name[] = {
-    lit("op"), lit("do"), lit("lop"), lit("ap"), lit("ip"),
+    lit("op"), lit("do"), lit("lop"), lit("ldo"), lit("ap"), lit("ip"),
     lit("ado"), lit("ido"), lit("ret"), lit("aret"),
     lit("opip"), lit("oand"),
     nil
diff --git a/share/txr/stdlib/op.tl b/share/txr/stdlib/op.tl
index ca09e49a..ac09f6df 100644
--- a/share/txr/stdlib/op.tl
+++ b/share/txr/stdlib/op.tl
@@ -159,6 +159,9 @@
 (defmacro lop (:form f :env e . args)
   (sys:op-expand f e args))
 
+(defmacro ldo (op . args)
+  ^(do ,op @1 ,*args))
+
 (defmacro ap (. args)
   ^(apf (op ,*args)))
 
diff --git a/txr.1 b/txr.1
index b560472e..836c7275 100644
--- a/txr.1
+++ b/txr.1
@@ -46466,6 +46466,44 @@ are replaced with hygienic, unique symbols.
 
 .brev
 
+.coNP Macro @ ldo
+.synb
+.mets (ldo < oper << form *)
+.syne
+.desc
+The
+.code ldo
+macro provides a shorthand notation for uses of the
+.code do
+macro which inserts the first argument of the anonymous function
+as the leftmost argument of the specified operator.
+
+The
+.code ldo
+syntax can be understood in terms of these equivalences:
+
+.verb
+  (ldo f)  <-->  (do f @1)
+  (ldo f x)  <-->  (do f @1 x)
+  (ldo f x y)  <-->  (do f @1 x y)
+  (ldo f x @2 y)  <-->  (do f @1 x @2 y)
+.brev
+
+The implicit argument
+.code @1
+is always inserted as the leftmost argument of the operator
+specified by the first form.
+
+.TP* Example:
+
+.verb
+  ;; push elements of l1 onto l2.
+  (let ((l1 '(a b c)) l2)
+    (mapdo (ldo push l2) l1)
+    l2)
+  --> (c b a)
+.brev
+
 .coNP Macros @, ap @, ip @ ado and @ ido.
 .synb
 .mets (ap << form +)
-- 
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