New condlet macro; small change to iflet/whenlet.

* eval.c (me_iflet_whenlet): Allow the test form
to be an atomic expression instead of bindings.
This allows iflet to be used as the sole target
construct of condlet, while allowing condlet to
have a fallback clause with t. It also means that
an empty list of bindings is allowed (since it
is the atom nil).

* lisplib.c (ifa_set_entries): Add "condlet" to
the autoload names for the ifa module. That's
where we are adding condlet.

* share/txr/stdlib/ifa.tl (sys:if-to-cond): New
macro expander helper function, generalizing the writing
cond-like macros based on if-like operators.
(conda): Rewritten using sys:if-to-cond.
(condlet): New macro.

* txr.1: Documented change in iflet/whenlet.
Documented condlet.

1 files changed, 14 insertions, 6 deletions

diff --git a/eval.c b/eval.c
index c00f6a80..9edc6ae6 100644
--- a/eval.c
+++ b/eval.c
@@ -2961,14 +2961,22 @@ static val me_iflet_whenlet(val form, val env)
   val args = form;
   val sym = pop(&args);
   val lets = pop(&args);
-  val lastlet = last(lets);
 
-  if (nilp(lastlet))
-    eval_error(form, lit("~s: empty binding list"), sym, nao);
+  if (atom(lets)) {
+    return apply_frob_args(list(if3(sym == iflet_s, if_s, when_s),
+                                lets, args, nao));
+  } else {
+    val lastlet = last(lets);
 
-  return list(let_star_s, lets,
-              cons(if3(sym == iflet_s, if_s, when_s),
-                   cons(car(car(lastlet)), args)), nao);
+    if (nilp(lastlet))
+      eval_error(form, lit("~s: empty binding list"), sym, nao);
+
+
+
+    return list(let_star_s, lets,
+                cons(if3(sym == iflet_s, if_s, when_s),
+                     cons(car(car(lastlet)), args)), nao);
+  }
 }
 
 static val me_dotimes(val form, val env)