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``Update functions'' in Scheme.
Date: Tue, 10 May 88 22:37:46 edt
From: lyn@BASEL.AI.MIT.EDU (Franklyn Turbak)
> Common LISP's SETF won't work in this example. SETF does a syntactic
> transformation on its first operand. It doesn't evaluate the first operand.
> Compiling ACT-FOOLISHLY-ON will give the error that no SETF method is defined
> for "accessor."
This is exactly what I *DON'T* like about a "macrological" SETF - you
can't abstract over it! If in fact we want to think about and manipulate
the abstract relationship between accessors and mutators, a text-based
model is simply not sufficient.
I agree. That's one reason I'm watching this discussion so closely.
> (LDB (BYTE 4 2) (AREF MY-ARRAY 0))
> returns four bits from MY-ARRAY's first element (a number).
> (SETF (LDB (BYTE 4 2)) (AREF MY-ARRAY 0))
> sets the four bits in MY-ARRAY's first element (a number).
> This is a case which can't be modeled as simply turning LDB into a call to
> SET-LDB. (AREF MY-ARRAY 0) may fetch a number from MY-ARRAY and pass it to
> SET-LDB, but that won't change the number that's stored in MY-ARRAY.
This seems to be another version of Bard's "composition" example - is
it possible to get a functional SETF-like frob to work in
"higher-order" cases? Like Bard, I'd be interested in any ideas on this
Question: The LDB example is "one order higher" than a straight SETQ. Are
there any examples that are more than one order higher?