fix(library/vm/vm): curr_fn() may not be available

library/tools/debugger/cli.lean

@@ -197,8 +197,10 @@ do
     cmd_loop s ["s"]
 
 meta def bp_reached (s : state) : vm bool :=
-do fn    ← vm.curr_fn,
-   return $ s^.fn_bps^.any (λ p, p^.is_prefix_of fn)
+(do fn    ← vm.curr_fn,
+    return $ s^.fn_bps^.any (λ p, p^.is_prefix_of fn))
+<|>
+return ff
 
 meta def in_active_bps (s : state) : vm bool :=
 do sz ← vm.call_stack_size,

src/library/tactic/vm_monitor.cpp

@@ -375,7 +375,11 @@ vm_obj vm_get_options(vm_obj const & /*s*/) {
 }
 
 vm_obj vm_curr_fn(vm_obj const & /*s*/) {
-    return mk_vm_success(to_obj(get_vm_state_being_debugged().curr_fn()));
+    if (auto fn = get_vm_state_being_debugged().curr_fn()) {
+        return mk_vm_success(to_obj(*fn));
+    } else {
+        return mk_vm_failure();
+    }
 }
 
 vm_obj vm_obj_to_string(vm_obj const & o, vm_obj const & /*s*/) {

src/library/vm/vm.cpp

@@ -3034,6 +3034,13 @@ optional<vm_decl> vm_state::get_decl(name const & n) const {
         return optional<vm_decl>();
 }
 
+optional<name> vm_state::curr_fn() const {
+    if (m_fn_idx == g_null_fn_idx)
+        return optional<name>();
+    else
+        return optional<name>(m_decl_map.find(m_fn_idx)->get_name());
+}
+
 #if defined(LEAN_MULTI_THREAD)
 static name * g_profiler      = nullptr;
 static name * g_profiler_freq = nullptr;

src/library/vm/vm.h

@@ -687,7 +687,7 @@ public:
 
     optional<vm_decl> get_decl(name const & n) const;
 
-    name curr_fn() const { return m_decl_map.find(m_fn_idx)->get_name(); }
+    optional<name> curr_fn() const;
 
     void invoke_fn(name const & fn);
     void invoke_fn(unsigned fn_idx);

tests/lean/run/basic_monitor3.lean

@@ -44,12 +44,18 @@ meta def basic_monitor : vm_monitor nat :=
   step := λ sz, do
     csz ← vm.call_stack_size,
     if sz = csz then return sz
-    else do
+    else
+      do {
       fn  ← vm.curr_fn,
       pos ← pos_info fn,
       vm.trace (to_fmt "[" ++ csz ++ "]: " ++ to_fmt fn ++ " @ " ++ pos),
       display_args,
-      return csz }
+      return csz
+      }
+      <|>
+      return csz -- curr_fn failed
+}
+
 
 run_command vm_monitor.register `basic_monitor