From 1af772303bf4eafb632a95bf4015a7736275e9e7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Mon, 26 Jul 2010 16:36:24 +0200
Subject: [PATCH] Import unbound variable reports in the VM.

* libguile/vm-engine.c (VM_NAME)[vm_error_unbound]: Add comment.

* libguile/vm-i-system.c (variable_ref): Attempt provide the name of X
  in FINISH_ARGS.
---
 libguile/vm-engine.c   | 2 ++
 libguile/vm-i-system.c | 9 ++++++---
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/libguile/vm-engine.c b/libguile/vm-engine.c
index 7f4641a07..ff41ce4b6 100644
--- a/libguile/vm-engine.c
+++ b/libguile/vm-engine.c
@@ -140,6 +140,8 @@ VM_NAME (SCM vm, SCM program, SCM *argv, int nargs)
     goto vm_error;
 
   vm_error_unbound:
+    /* At this point FINISH_ARGS should be a one-element list containing
+       the name of the unbound variable.  */
     err_msg  = scm_from_locale_string ("VM: Unbound variable: ~s");
     goto vm_error;
 
diff --git a/libguile/vm-i-system.c b/libguile/vm-i-system.c
index 3af63082a..958f8b9c3 100644
--- a/libguile/vm-i-system.c
+++ b/libguile/vm-i-system.c
@@ -307,10 +307,13 @@ VM_DEFINE_INSTRUCTION (26, variable_ref, "variable-ref", 0, 1, 1)
 {
   SCM x = *sp;
 
-  if (!VARIABLE_BOUNDP (x))
+  if (SCM_UNLIKELY (!VARIABLE_BOUNDP (x)))
     {
-      finish_args = scm_list_1 (x);
-      /* Was: finish_args = SCM_LIST1 (SCM_CAR (x)); */
+      SCM var_name;
+
+      /* Attempt to provide the variable name in the error message.  */
+      var_name = scm_module_reverse_lookup (scm_current_module (), x);
+      finish_args = scm_list_1 (scm_is_true (var_name) ? var_name : x);
       goto vm_error_unbound;
     }
   else