support <:]q> (pop Q-Register) for getting a success/failure boolean

* Could be used to check whether the stack is currently empty,
  although peeking would be cumbersome: `:]q"S [q !...! | !...! '`
* This is from DEC TECO-11.

1 files changed, 14 insertions, 3 deletions

diff --git a/src/qreg-commands.c b/src/qreg-commands.c
index 9407e6c..1d2766f 100644
--- a/src/qreg-commands.c
+++ b/src/qreg-commands.c
@@ -99,18 +99,29 @@ teco_state_popqreg_got_register(teco_machine_main_t *ctx, teco_qreg_t *qreg,
 {
 	teco_state_expectqreg_reset(ctx);
 
-	return ctx->flags.mode == TECO_MODE_NORMAL &&
-	       !teco_qreg_stack_pop(qreg, error) ? NULL : &teco_state_start;
+	if (ctx->flags.mode > TECO_MODE_NORMAL)
+		return &teco_state_start;
+
+	if (!teco_machine_main_eval_colon(ctx))
+		return !teco_qreg_stack_pop(qreg, error) ? NULL : &teco_state_start;
+	teco_expressions_push(teco_bool(teco_qreg_stack_pop(qreg, NULL)));
+	return &teco_state_start;
 }
 
-/*$ "]" "]q" pop
+/*$ "]" "]q" ":]q" pop
  * ]q -- Restore Q-Register
+ * :]q -> Success|Failure
  *
  * Restore Q-Register <q> by replacing its contents
  * with the contents of the register saved on top of
  * the Q-Register push-down stack.
  * The stack entry is popped.
  *
+ * When colon-modified, \fB]\fP returns a success boolean
+ * (-1) if there was a register to pop.
+ * If the stack was empty, a failure boolean (0) is returned
+ * instead of throwing an error.
+ *
  * In interactive mode, the original contents of <q>
  * are not immediately reclaimed but are kept in memory
  * to support rubbing out the command.