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bit fields in defstruct

Can someone tell me why Common Lisp defstruct doesn't support bit fields?
This is a Zetalisp feature that I make heavy use of.  The equivalent feature
in Common Lisp would look something like:

	(defstruct thingie
	  (foo 'default-foo-value)
	  ((low-bar  (byte 8  0))
	   (mid-bar  (byte 8  8) #x00 :read-only t) ;filled by constructor
	   (high-bar (byte 8 12) #x1a))
	  (baz 'default-baz-value))

This makes THINGIE a vector with 4 elements (counting the structure name);
the second element is an integer with default value #x1a0000.  The structure
has 5 access functions:


The access function for -BAR slots expands to a LDB, and the SETF macro
expands to a DPB.  The knowledge that three of the slots of a THINGIE are
bit fields of the same integer is hidden from the rest of the program.

The reason I need this feature is I am building thousands of instances of a
particular structure, and the difference between representing half a dozen
bit fields with one integer versus six integers is significant in terms of
memory cost.  I don't think a user should build his own access functions for
bit fields; I think defstruct should do that for him.

-- Dave

PS:  I suppose if defstruct were highly optimizing one could declare six
fields of :TYPE BIT and expect them to be packed into one word.  This
doesn't solve everything though; in some cases (like when communicating
with external programs) one wants to specify the exact postion of each bit
field in a word.  Also, one might want to have overlapping bit fields in the
same word, such as three 1-bit fields and a 3-bit field that covers them.