Emacs and external programs which support several modes

[Dmitry Dzhus]

Hello hackers.

Suppose that Emacs can provide means of interaction with program Foo in
modes A or B. Mode A is more «basic» than B. User is allowed to select
the mode she prefers. Mode A is always supported, mode B is not. Emacs
can check if mode B is supported after the interaction process started.

What should Emacs do in case mode B was selected by user but is not
supported?

Shall Emacs..

..give a notification that mode B is not supported, switch to mode A and
proceed?

or

..give a notification that mode B is not supported and kill the
interaction process?

What is the common practice to handle such case?

(Foo=GDB,
 A=all-stop mode (when one thread stops, all others stop, too),
 B=non-stop mode (when one threads stops, others may continue execution))
-- 
Happy Hacking.

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