[Sbcl-devel] Gray stream question and patch

[Pete K. <pet...@ka...>]

Is there any reason why OUTPUT-STREAM-P is not specialized for a
FUNDAMENTAL-INPUT-STREAM?  And likewise, is there any reason why
INPUT-STREAM-P is not specialized for a FUNDAMENTAL-OUTPUT-STREAM in
gray-streams.lisp?  In the current SBCL codebase, INPUT-STREAM-P is
only specialized on a FUNDAMENTAL-INPUT-STREAM, while OUTPUT-STREAM-P
is only specialized on a FUNDAMENTAL-OUTPUT-STREAM.

As a result, if someone wants to subclass either of these streams (as
they were designed to be), he/she must also implement an additional
method which could just as well have been implemented by the
superclass.  I.e., if I subclass FUNDAMENTAL-INPUT-STREAM, I'm forced
to implement a specialized OUTPUT-STREAM-P, which simply returns NIL.
Could this not be specialized for the parent class?

After reading the original Gray proposal, it seems his intent was that
default implementations for OUTPUT-STREAM-P and INPUT-STREAM-P would
be specialized for the FUNDAMENTAL-INPUT/OUTPUT-STREAMs, here is a
quote from the original proposal:

     STREAMP  object                     [Generic Function]
     INPUT-STREAM-P  stream              [Generic Function]
     OUTPUT-STREAM-P  stream             [Generic Function]
   
       These three existing predicates may optionally be
       implemented as generic functions for implementations
       that want to permit users to define streams that are
       not STANDARD-OBJECTs.  Normally, the default methods
       provided by classes FUNDAMENTAL-INPUT-STREAM and
       FUNDAMENTAL-OUTPUT-STREAM are sufficient.

And then later in the proposal are his implementations for both
INPUT-STREAM-P and OUTPUT-STREAM-P.  Notice that both have been
specialized for both FUNDAMENTAL-INPUT-STREAM as well as 
FUNDAMENTAL-OUTPUT-STREAM:

     (defmethod INPUT-STREAM-P ((stream fundamental-input-stream)) t)
     (defmethod INPUT-STREAM-P ((stream fundamental-output-stream))
       ;; allow the two classes to be mixed in either order
       (typep stream 'fundamental-input-stream))
     (defmethod OUTPUT-STREAM-P ((stream fundamental-output-stream)) t)
     (defmethod OUTPUT-STREAM-P ((stream fundamental-input-stream))
       (typep stream 'fundamental-output-stream))

SBCL only implements the two of the above methods which forces a user
to implement the unimplemented methods themselves.  Here is the
corresponding SBCL code:

     (defmethod INPUT-STREAM-P ((stream fundamental-input-stream)) t)
     (defmethod OUTPUT-STREAM-P ((stream fundamental-output-stream)) t)

Is there any reason why SBCL cannot be modified to provide the default
implementations for the two missing methods?  I've searched the web
for any other info on why these two methods might not have been added
and was not able to find anything of significance.  I have included a
patch that adds the methods as well as two additional test cases to
verify the functionality.

Thanks,
Pete