Debugger API, compatibility between R7 and R8

[Luke Gorrie]

In an old thread, Gunilla Arendt <gunilla@REDACTED> writes:

> This means that it should now be a lot easier to write a new
> interpreter front-end, but it also means that 2.0 is not compatible
> at all with the older versions...

It is indeed a nice and easy interface. One suggestion: perhaps some
more functions in `int' that send messages to dbg_iserver should
ensure that it's already started (by using e.g. safe_cast instead of
cast).

I had a bug where I was calling int:subscribe() first of all, which
casts to dbg_iserver without checking that it's already started (and
gen_server:cast traps/hides the send-to-unregistered-name error). I
worked around by calling int:interpreted() first, which does ensure
that the server is running, as a side-effect - but I think
subscribe() should too.

Cheers,
Luke