From 128e59c15912c1a598616bf7c091043f95b6984c Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Fri, 28 Mar 2025 22:24:45 -0700
Subject: New function keep: generalized keepqual.

* eval.c (eval_init): Register keep intrinsic.

* lib.[ch] (keep): New function.

* stdlib/compiler.tl (compiler comp-fun-form): Transform
two argument keep to keepqual.

* txr.1: Documented.
---
 eval.c             | 1 +
 lib.c              | 6 ++++++
 lib.h              | 1 +
 stdlib/compiler.tl | 1 +
 txr.1              | 9 +++++++++
 5 files changed, 18 insertions(+)

diff --git a/eval.c b/eval.c
index 11545a12..93576d00 100644
--- a/eval.c
+++ b/eval.c
@@ -7490,6 +7490,7 @@ void eval_init(void)
   reg_fun(intern(lit("keepqual"), user_package), func_n3o(keepqual, 2));
   reg_fun(intern(lit("keep-if"), user_package), func_n4o(keep_if, 2));
   reg_fun(intern(lit("keep-keys-if"), user_package), func_n4o(keep_keys_if, 2));
+  reg_fun(intern(lit("keep"), user_package), func_n5o(keep, 2));
   reg_fun(intern(lit("separate"), user_package), func_n3o(separate, 2));
   reg_fun(intern(lit("separate-keys"), user_package), func_n3o(separate_keys, 2));
   reg_fun(intern(lit("remq*"), user_package), func_n2(remq_lazy));
diff --git a/lib.c b/lib.c
index 68634b4f..ca6c6fe2 100644
--- a/lib.c
+++ b/lib.c
@@ -3616,6 +3616,12 @@ val keep_keys_if(val pred, val seq, val keyfun_in, val mapfun_in)
   return rem_if_impl(notf(pred), seq, keyfun_in, mapfun, lit("keep-keys-if"));
 }
 
+val keep(val obj, val seq, val testfun_in, val keyfun_in, val mapfun_in)
+{
+  val testfun = default_arg(testfun_in, equal_f);
+  return remov(obj, seq, notf(testfun), keyfun_in, mapfun_in);
+}
+
 val separate(val pred, val seq, val keyfun_in)
 {
   val self = lit("separate");
diff --git a/lib.h b/lib.h
index 977fbddf..848a8303 100644
--- a/lib.h
+++ b/lib.h
@@ -896,6 +896,7 @@ val keepql(val obj, val seq, val keyfun);
 val keepqual(val obj, val seq, val keyfun);
 val keep_if(val pred, val seq, val keyfun_in, val mapfun_in);
 val keep_keys_if(val pred, val seq_in, val keyfun_in, val mapfun_in);
+val keep(val obj, val seq, val testfun_in, val keyfun_in, val mapfun_in);
 val separate(val pred, val seq, val keyfun);
 val separate_keys(val pred, val seq_in, val keyfun_in);
 val remq_lazy(val obj, val list);
diff --git a/stdlib/compiler.tl b/stdlib/compiler.tl
index 836d53cd..98232c7b 100644
--- a/stdlib/compiler.tl
+++ b/stdlib/compiler.tl
@@ -1407,6 +1407,7 @@
           ((pos @obj @seq) (set form (rlcp ^(posqual ,obj ,seq) form)))
           ((member @obj @seq) (set form (rlcp ^(memqual ,obj ,seq) form)))
           ((subst @obj @seq) (set form (rlcp ^(subqual ,obj ,seq) form)))
+          ((keep @obj @seq) (set form (rlcp ^(keepqual ,obj ,seq) form)))
           (@(require (chain . @nil)
                      (> olev 5)
                      (can-inline-chain form))
diff --git a/txr.1 b/txr.1
index 42774995..b7e031d9 100644
--- a/txr.1
+++ b/txr.1
@@ -36578,6 +36578,7 @@ is compared to
 .synb
 .mets (remove < key < sequence >> [testfun >> [ keyfun <> [ mapfun ]]])
 .mets (remove-if < predfun < sequence >> [ keyfun <> [ mapfun ]])
+.mets (keep < key < sequence >> [testfun >> [ keyfun <> [ mapfun ]]])
 .mets (keep-if < predfun < sequence >> [ keyfun <> [ mapfun ]])
 .mets (separate < predfun < sequence >> [ keyfun <> [ mapfun ]])
 .mets (remove-if* < predfun < sequence >> [ keyfun <> [ mapfun ]])
@@ -36676,6 +36677,14 @@ will delete, and removes those that
 .code remove-if
 will preserve.
 
+Likewise, the
+.code keep
+function is the predicate-inverting counterpart of
+.codn remove .
+It retains those items which
+.code remove
+will delete and vice versa.
+
 The
 .code separate
 function combines
-- 
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