[etoys-dev] Current project serialization format?
Ted at vpri.org
Wed Dec 2 15:13:59 EST 2009
As the author of the current ImageSegment-based project
storing (with Dan Ingalls), and as the author of the two infamous
error messages when loading a old project, I need to point out a few
> What I really want to do is
> to provide enough information so that a project loader can tell whether
> it will be able to load a project and if not, why, instead of our old
> friend "Reading an instance of GobblyGook. Which modern class should it
> translate to?" which is my second-favorite questions to ask users. (of
> course my absolute favorite is when project saving invites you to "stop
> and take a look" at some blocks it encountered - both are completely and
> utterly incomprehensible questions to users that it's pointless to ask
> them in the first place)
(I certainly hope that those two infamous error messages are not my
legacy to history... )
The only important questions is, "Would you rather have an error
message when you are trying to save a project, or when you are trying
to load it?" The frustration and anger of not being able to save the
work that you just did is unspeakable. On the other hand, not being
able to load an ancient project just means that you need to go get a
few definitions before you can proceed. There is no comparison of
the urgency of saving vs loading.
Fixed-format systems like SISS, and many XML-based systems, cannot
cover everything. You can always create some data structure or code
in your project that cannot be written out. There is no universal
format. A universal format is not possible. The only thing we can
do is provide enough levels of indirection so that anything can be
expressed in a saved project. (Maybe a universal format is possible,
and maybe we should invent it.)
I am glad that I sit next to Yoshiki, because when I can't write out
my window in our new LObject world, I just turn to Yoshiki and ask
him to fix SISS. I highly recommend this way of working to all
future project creators. A bit impractical, though.
The current ImageSegement-with-address-space project saving has a
much wider range of what it can save. It can save any data
structure, period. It keeps the names of instance variables in the
entire tree of superclasses. A project can survive a huge set of
changes in instance variables in classes.
Yes, there is a problem with rare "naughty blocks" (a highly
technical term of art), but I will fix that if there is demand.
Please note that SISS can't store ANY blocks at all.
One goal of SISS-style formats is that a human can read it. In an
emergency, a person can look in the file and see what objects are
there. Having readable text in the file also helps other people
write translators to other systems.
Hypercard's file format is ascii and is human-readable. Bill
Atkinson did this deliberately, and used it to debug code and repair
stacks by hand. I watched him do this, and my conclusion is that it
was a total waste of time! Every minute he spent poking around in
the innards of a stack was a waste. It is much better to make a
binary format that works completely, and never look at it again. I
followed this technique with the ImageSegement based saving, and it
has worked well.
If we insist on a text-based file format, parsing will play a vital
role. I suggest that we get Alex Warth involved.
Once again, let me ask, "Would you rather have an error message when
you are trying to save a project, or when you are trying to load it?"
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