Add "const" function, which is the inverse of "nonconst"

contrib/remind-conf-mode/remind-conf-mode.el

@@ -164,7 +164,7 @@
 (defconst remind-builtin-functions
   (sort
    (list "_" "abs" "access" "adawn" "adusk" "ampm" "ansicolor" "args" "asc"
-         "baseyr" "catch" "catcherr" "char" "choose" "coerce" "columns" "current" "date"
+         "baseyr" "catch" "catcherr" "char" "choose" "coerce" "columns" "const" "current" "date"
          "datepart" "datetime" "dawn" "day" "daysinmon" "defined" "dosubst"
          "dusk" "easterdate" "escape" "eval" "evaltrig" "filedate" "filedatetime"
          "filedir" "filename" "getenv" "hebdate" "hebday" "hebmon" "hebyear"

man/remind.1.in

@@ -3038,7 +3038,7 @@ Equivalent to \fBday(trigdate())\fR.
 .B $Tm (read-only)
 Equivalent to \fBmonnum(trigdate())\fR.
 .TP
-.B $Tu (read-only)
+.B $Tu (read-only, DATE type)
 Equivalent to \fBtriguntil()\fR.
 .TP
 .B $Tw (read-only)
@@ -3400,7 +3400,12 @@ characters occupy two columns.  \fBcolumns(str)\fR takes all of that
 into account.  Note that if \fBRemind\fR was compiled without Unicode support,
 \fBcolumns(str)\fR returns a type mismatch error.
 .RE
-
+.TP
+.B const(x_arg)
+Evaluates \fIarg\fR and returns it, but also ensures that the result
+is marked "constant".  Use with caution; incorrect use could result
+in unwanted reminders being purged in Purge Mode.  See also the section
+"NON-CONSTANT EXPRESSIONS"
 .TP
 .B current()
 Returns the current date and time as a DATETIME object.  This may be the

src/expr.c

@@ -568,6 +568,10 @@ eval_builtin(expr_node *node, Value *locals, Value *ans, int *nonconst)
         /* All went well; copy the result destructively */
         (*ans) = info.retval;
 
+        /* Special case of const cunction */
+        if (!strcmp(f->name, "const")) {
+            *nonconst = 0;
+        }
         /* Don't allow retval to be destroyed! */
         info.retval.type = ERR_TYPE;
     }

src/funcs.c

@@ -258,6 +258,7 @@ BuiltinFunc Func[] = {
     {   "choose",       2,      NO_MAX, 1,          NULL, FChoose }, /*NEW-STYLE*/
     {   "coerce",       2,      2,      1,          FCoerce, NULL },
     {   "columns",      0,      1,      0,          FColumns, NULL },
+    {   "const",        1,      1,      1,          FNonconst, NULL },
     {   "current",      0,      0,      0,          FCurrent, NULL },
     {   "date",         3,      3,      1,          FDate, NULL },
     {   "datepart",     1,      1,      1,          FDatepart, NULL },

tests/test.cmp

@@ -20253,6 +20253,7 @@ Timed
 Untimed
 
 Purge Test
+../tests/purge_dir/f3.rem(62): Attempt to redefine built-in function: `const'
 ../tests/purge_dir/f3.rem(76): `/': Division by zero
 ../tests/purge_dir/f3.rem(76): `/': Division by zero
 F1
@@ -24460,6 +24461,7 @@ char
 choose
 coerce
 columns
+const
 current
 date
 datepart