(keyword_symbols_constant_flag): New variable.

(syms_of_data): Set up Lisp variable.
(set_internal): Obey it and give error for :-symbols.
(Fmakunbound): Likewise.

1 files changed, 15 insertions, 2 deletions

diff --git a/src/data.c b/src/data.c
index c9eaee7a9b2..5b3a33bddfa 100644
--- a/src/data.c
+++ b/src/data.c
@@ -68,6 +68,10 @@ Boston, MA 02111-1307, USA.  */
 extern double atof ();
 #endif /* !atof */
 
+/* Nonzero means it is an error to set a symbol whose name starts with
+   colon.  */
+int keyword_symbols_constant_flag;
+
 Lisp_Object Qnil, Qt, Qquote, Qlambda, Qsubr, Qunbound;
 Lisp_Object Qerror_conditions, Qerror_message, Qtop_level;
 Lisp_Object Qerror, Qquit, Qwrong_type_argument, Qargs_out_of_range;
@@ -611,7 +615,9 @@ DEFUN ("makunbound", Fmakunbound, Smakunbound, 1, 1, 0, "Make SYMBOL's value be
      register Lisp_Object symbol;
 {
   CHECK_SYMBOL (symbol, 0);
-  if (NILP (symbol) || EQ (symbol, Qt))
+  if (NILP (symbol) || EQ (symbol, Qt)
+      || (XSYMBOL (symbol)->name->data[0] == ':'
+          && keyword_symbols_constant_flag))
     return Fsignal (Qsetting_constant, Fcons (symbol, Qnil));
   Fset (symbol, Qunbound);
   return symbol;
@@ -960,7 +966,9 @@ set_internal (symbol, newval, bindflag)
   register Lisp_Object valcontents, tem1, current_alist_element;
 
   CHECK_SYMBOL (symbol, 0);
-  if (NILP (symbol) || EQ (symbol, Qt))
+  if (NILP (symbol) || EQ (symbol, Qt)
+      || (XSYMBOL (symbol)->name->data[0] == ':'
+          && keyword_symbols_constant_flag))
     return Fsignal (Qsetting_constant, Fcons (symbol, Qnil));
   valcontents = XSYMBOL (symbol)->value;
 
@@ -2833,6 +2841,11 @@ syms_of_data ()
   staticpro (&Qchar_table);
   staticpro (&Qbool_vector);
 
+  DEFVAR_BOOL ("keyword-symbols-constant-flag", &keyword_symbols_constant_flag,
+    "Non-nil means it is an error to set a keyword symbol.\n\
+A keyword symbol is a symbol whose name starts with a colon (`:').");
+  keyword_symbols_constant_flag = 1;
+
   defsubr (&Seq);
   defsubr (&Snull);
   defsubr (&Stype_of);