[armedbear-devel] Gray streams support: plan of attack

Erik Huelsmann ehuels at gmail.com
Sun Jan 10 19:54:13 UTC 2010

Today on IRC, Ville and I had a long discussion on how to get Gray
streams supported in ABCL. The outcome is a long description, but I
think it's good to write it down, so everybody following trunk
understands what we're doing. Of course, if you have any feedback,
please send it!

The problem

The issue with our current support for Gray streams is that we have
almost everything in place except for one important thing: without
hacking TYPEP and STREAMP, we're not able to fully support
user-defined streams. However, hacking TYPEP is extremely fragile, as
the compiler may assume things about certain types in the CL package.
So, we don't want to go that route.

On the other hand, it's not possible to derive classes from built-in
classes, which means that in the current situation where STREAM is a
built-in class, we cannot get the type of FUNDAMENTAL-STREAM right at

Since we need to change things anyway, given the above, it would be
nice to implement some of the CL streams in Lisp, particularly the
ones which are not required to get ABCL booted.

The plan

We found out that if one declares an empty structure class - meaning a
structure without slots - this class can be used in the superclasse
list of defclass. What we're now doing is this:

* Make STREAM an empty structure class
* Create a SYSTEM-STREAM structure class which will contain slots in
the future (but none for now)
* Derive all internal stream classes from SYSTEM-STREAM
* Derive FUNDAMENTAL-STREAM from the empty STREAM structure class
(which is now possible with a normal DEFCLASS), making it a real

At a later stage, we want to move code from all the non-boot-essential
streams to Lisp. This should allow for easier maintenance and more
compact (yet fully comprehensible) code.



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