From a68618a101a34c6b494dad4ce57c3d39507c1827 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sat, 19 Nov 2016 21:04:49 -0800
Subject: Diagnose undefined functions in trace.

* share/txr/stdlib/trace.tl (sys:trace): If symbol-function
returns nil, throw exception.

* txr.1: Document the behavior, along with return values
of trace and untrace, and the tolerance of untrace to
bad arguments.
---
 share/txr/stdlib/trace.tl |  4 +++-
 txr.1                     | 24 +++++++++++++++++++++++-
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/share/txr/stdlib/trace.tl b/share/txr/stdlib/trace.tl
index 951b67f8..33a1bf08 100644
--- a/share/txr/stdlib/trace.tl
+++ b/share/txr/stdlib/trace.tl
@@ -16,7 +16,9 @@
       (each ((n names))
         (unless [sys:*trace-hash* n]
           (let* ((name n)
-                 (prev (symbol-function n))
+                 (prev (or (symbol-function n)
+                           (throwf 'eval-error
+                                   "~s: ~s does not name a function" 'trace n)))
                  (hook (lambda (. args)
                          (let ((abandoned t)
                                (sys:*trace-level* (succ sys:*trace-level*)))
diff --git a/txr.1 b/txr.1
index 8b1b31eb..8e1d2dca 100644
--- a/txr.1
+++ b/txr.1
@@ -48586,11 +48586,19 @@ When
 is called with one or more arguments, it considers each
 argument to be the name of a global function. For each
 function, it turns on tracing, if it is not already turned on.
+If an argument denotes a nonexistent function, or is invalid
+function name syntax,
+.code trace
+terminates by throwing an exception, without processing the
+subsequent arguments, or undoing the effects already applied
+due to processing the previous arguments.
 
 When
 .code trace
 is called with no arguments, it lists the names of functions
-for which tracing is currently enabled.
+for which tracing is currently enabled. In other cases it
+returns
+.codn nil .
 
 When
 .code untrace
@@ -48603,6 +48611,13 @@ When
 is called with no arguments, it disables tracing for all
 functions.
 
+The
+.code untrace
+macro always returns
+.code nil
+and silently tolerates arguments which are not names of functions
+currently being traced.
+
 Tracing a function consists of printing a message prior to entry into the
 function indicating its name and arguments, and another message upon leaving
 the function indicating its return value, which is syntactically correlated
@@ -48638,6 +48653,13 @@ will not show the destructured internal macro arguments, but only the
 two arguments passed to the expander function: the whole form, and the
 environment.
 
+The
+.code trace
+and
+.code untrace
+functions return
+.codn nil .
+
 .SH* INTERACTIVE LISTENER
 
 .SS* Overview
-- 
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