This is *not* a proposed change for 22.1; more of a question about a
feature, or lack of it.
Having the possibility of marking a symbol as constant (not in the
`defconst' sense, but really constant) seems so potentially useful and
so trivial to implement (vide the attached patch), that I can only
suppose there's some reason, or perceived problem, not to have done it
till now.
Could someone please explain the rationale for not having true
constants in elisp?
Thanks,
/L/e/k/t/u
Index: src/eval.c
===================================================================
RCS file: /cvsroot/emacs/emacs/src/eval.c,v
retrieving revision 1.250
diff -u -2 -r1.250 eval.c
--- src/eval.c 13 Jul 2005 05:31:35 -0000 1.250
+++ src/eval.c 13 Jul 2005 13:49:29 -0000
@@ -956,4 +956,37 @@
}
}
+
+DEFUN ("set-constant-p", Fset_constant_p, Sset_constant_p, 2, 2, 0,
+ doc: /* Set constant state of SYMBOL to FLAG.
+When FLAG is non-nil, SYMBOL becomes constant; otherwise, it is variable.
+Keywords, ie symbols satisfying `keywordp', are always constant. */)
+ (symbol, flag)
+ Lisp_Object symbol, flag;
+{
+ CHECK_SYMBOL (symbol);
+
+ if (EQ (Fkeywordp (symbol), Qt))
+ error ("Cannot change constant state of keyword `%s'",
+ SDATA (SYMBOL_NAME (symbol)));
+
+ XSYMBOL (symbol)->constant = EQ (flag, Qt);
+
+ return symbol;
+}
+
+DEFUN ("constantp", Fconstantp, Sconstantp, 1, 1, 0,
+ doc: /* Return t if SYMBOL is a constant.
+If so, its value cannot be changed.
+The `constant'ness of a symbol can be changed with `set-constant-p'. */)
+ (symbol)
+ Lisp_Object symbol;
+{
+ CHECK_SYMBOL (symbol);
+
+ if (XSYMBOL (symbol)->constant)
+ return Qt;
+ return Qnil;
+}
+
DEFUN ("let*", FletX, SletX, 1, UNEVALLED, 0,
@@ -3561,4 +3594,6 @@
defsubr (&Sdefconst);
defsubr (&Suser_variable_p);
+ defsubr (&Sset_constant_p);
+ defsubr (&Sconstantp);
defsubr (&Slet);
defsubr (&SletX);
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