Re: Shift selection using interactive spec

[Richard Stallman]

    This is an interesting feature.  Just as a first argument of
    `interactive' specifies command's input, a second argument would
    specify its output.  I think this pair of input/output arguments
    of `interactive' makes sense.

The hard part is to figure out a good design for it.

It is important to support both returning a single value and returning
a list of values.  There could be a character at the beginning of the
interactive output string which specifies that it's a list of values.
Otherwise it is one value.

But what do we use after that?

It could be a format string to format the values.
Is that general enough?

And what about using Lisp code for how to format the value?
How would that Lisp code refer to the value itself?
Perhaps that Lisp code should take the form of a lambda expression.

But how do we specify whether it wants a list of values or a single
value?  I don't see any clean and natural way to specify that.