Reply to reply to DESTRUCTURING C.H.

[Guy.Steele at CMU-10A]

Unfortunately there is a bad interaction between destructuring
of DEFSTRUCT structures and defaulting of components.
Consider a slightly modified version of GJC's example:

	(DEFSTRUCT FOO A B (C 'TANG))	;FOO is an astronaut (see Chineual)

Now when one writes #{FOO A: 1 B: 2} I assume that means the same
as #,(MAKE-FOO A '1 B '2), on the Aunt Agatha principle: I can't
think of what else it might usefully mean.  Assuming this to be true,
then we find that writing
	(DSETQ #{FOO A: X B: Y} Z)
is identical to writing
	(DSETQ #,(MAKE-FOO A 'X B 'Y) Z)
which, because of the defaulting mechanism of DEFSTRUCT, is the same as
	(DSETQ #,(MAKE-FOO A 'X B 'Y C 'TANG) Z)
which is the same as
	(DSETQ #{FOO A: X B: Y C: TANG} Z)
because of the defaulting of the C slot, which happens at READ time
when the structure is constructed.  Therefore, like it or not,
the variable TANG gets setq'd as well as X and Y!  I don't think you
can get rid of this effect without either making DSETQ not really
data-directed, or throwing away the component-defaulting mechanism
of DEFSTRUCT (which would be unaccetable, I think).
--Guy