When I run fc -l (history) from a zsh script that isn't interactive I get
the message
 not interactive shell

Why is it that fc -l needs an interactive shell? One can put entries in the
history and one can use vared to cycle through them, so what is it about an
interactive shell that is important here?

Related to this, why can't an interactive shell be toggled? Suppose I run a
non-interactive program and later want to attach to it say for debugging. At
that point, I attach interactive tty and it becomes interactive. So what are
the issues involved in turning a non-interactive program (from zsh's
definition) to an interactive one?

Thanks.