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Re: Making a function than can only be used interactively

From: Stefan Monnier
Subject: Re: Making a function than can only be used interactively
Date: Mon, 04 Jul 2022 17:18:22 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

> Another debate is that although one can declare a non-interactive function,
> and an interactive function that can run non-interactively; there is no
> construct that can define a purely interactive function.

That's because an "interactive function" is just a normal function
together with some auxiliary info to tell `call-interactive` how to call
it "interactively".  Internally `call-interactively` will end up calling
the function via `funcall`, i.e. "non-interactively".  So at
a low-level, technically you just can't have a function that can be
called interactively and not non-interactively.  You can try and kludge
it up above if you really want to (like we've seen in a few different
ways), but we're back to the question: what's the benefit?

> Does a function know whether it was run from lisp code or from the user in
> an Emacs session?

Trying to behave differently depending on who/how a function was called
goes against the design principle of functions, so it tends to be kludgy
and unreliable, like `called-interactively-p`.

BTW, here's another way to make a function that "can't" be called

    (defun foo (a b c &optional extra)
       (list ... 'dont-you-dare-call-me-non-interactively))
      (unless (eql extra 'dont-you-dare-call-me-non-interactively)
        (error "foo called non-interactively"))

You can make it "more robust" with something like:

    (defalias 'foo
      (let ((witness (make-symbol "dont-you-dare-call-me-non-interactively")))
        (lambda (a b c &optional extra)
            (list ... witness))
          (unless (eql extra witness)
            (error "foo called non-interactively"))

But again: is it really worth the trouble?  What is there to gain?
Instead of beating the undesired callers with a stick, why not try and
convince them to do something else with  carrot?


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