* txr.1: Document -C option.

* txr.c (opt_compat): New global variable.
(help): Describe -C option.
(txr_main): Process -C, and set opt_compat.
Ensure -C does not clump.

* txr.h (opt_compat): Declared.

4 files changed, 40 insertions, 3 deletions

diff --git a/ChangeLog b/ChangeLog
index c572feaf..a0be3ea0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,16 @@
 2014-09-02  Kaz Kylheku  <kaz@kylheku.com>
 
+	* txr.1: Document -C option.
+
+	* txr.c (opt_compat): New global variable.
+	(help): Describe -C option.
+	(txr_main): Process -C, and set opt_compat.
+	Ensure -C does not clump.
+
+	* txr.h (opt_compat): Declared.
+
+2014-09-02  Kaz Kylheku  <kaz@kylheku.com>
+
 	* eval.c (eval_init): Update registration of lisp-parse and read
 	to account for new parameter.
 
diff --git a/txr.1 b/txr.1
index 736dc5d8..daf9a3a2 100644
--- a/txr.1
+++ b/txr.1
@@ -191,6 +191,17 @@ Evaluates a TXR Lisp expression and prints its value.  Can be specified more
 than once. The query-file argument becomes optional if -p is used at least
 once.
 
+.IP "-C number"
+Requests TXR to behave in a manner that is compatible with the specified
+version of TXR. This makes a difference in situations when a release of
+TXR breaks backward compatibility. If some version N+1 deliberately introduces
+a change which is backward incompatible, then -C N can be used to request
+the old behavior.  The requested value of N can be too low, in which case TXR
+will complain and exit with an unsuccessful termination status. This indicates
+that TXR refuses to be compatible with such an old version. Users requiring
+the behavior of that version will have to install an older version of TXR which
+supports that behavior, or even that exact version.
+
 .IP --help
 Prints usage summary on standard output, and terminates successfully.
 
diff --git a/txr.c b/txr.c
index 9963fd30..16a2cbe7 100644
--- a/txr.c
+++ b/txr.c
@@ -59,6 +59,7 @@ const wchli_t *version = wli(TXR_VER);
 const wchar_t *progname = L"txr";
 static const char *progname_u8;
 static val progpath = nil;
+int opt_compat;
 
 /*
  * Can implement an emergency allocator here from a fixed storage
@@ -119,6 +120,8 @@ static void help(void)
 "                       option, instead of the query-file argument.\n"
 "                       This allows #! scripts to pass options through\n"
 "                       to the utility.\n"
+"-C number              Request backward-compatible behavior to the\n"
+"                       specified version of TXR.\n"
 "--help                 You already know!\n"
 "--version              Display program version\n"
 "--license              Display software license\n"
@@ -368,7 +371,7 @@ int txr_main(int argc, char **argv)
       return license();
 
     if (memqual(arg, list(lit("-a"), lit("-c"), lit("-f"),
-                          lit("-e"), lit("-p"), nao)))
+                          lit("-e"), lit("-p"), lit("-C"), nao)))
     {
       val opt = chr_str(arg, one);
 
@@ -383,6 +386,7 @@ int txr_main(int argc, char **argv)
 
       switch (c_chr(opt)) {
       case 'a':
+      case 'C':
         {
           val optval = int_str(arg, nil);
 
@@ -393,8 +397,17 @@ int txr_main(int argc, char **argv)
             return EXIT_FAILURE;
           }
 
-          opt_arraydims = c_num(optval);
-          opt_print_bindings = 1;
+          if (opt == chr('a')) {
+            opt_arraydims = c_num(optval);
+            opt_print_bindings = 1;
+          } else {
+            if ((opt_compat = c_num(optval)) < 97) {
+              format(std_error, lit("~a: compatibility with versions "
+                                    "lower than 97 not supported by version ~a\n"),
+                     prog_string, auto_str(version), nao);
+              return EXIT_FAILURE;
+            }
+          }
         }
         break;
       case 'c':
@@ -489,6 +502,7 @@ int txr_main(int argc, char **argv)
         case 'e':
         case 'p':
         case 'f':
+        case 'C':
         case 'D':
           format(std_error, lit("~a: option -~a does not clump\n"),
                  prog_string, opch, nao);
diff --git a/txr.h b/txr.h
index bfc01b04..201bd385 100644
--- a/txr.h
+++ b/txr.h
@@ -33,5 +33,6 @@ extern int opt_gc_debug;
 extern int opt_vg_debug;
 #endif
 extern int opt_derivative_regex;
+extern int opt_compat;
 extern const wchli_t *version;
 extern const wchar_t *progname;