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Re: Simplified slot access in goops
From: |
Panicz Maciej Godek |
Subject: |
Re: Simplified slot access in goops |
Date: |
Tue, 2 Dec 2008 20:30:11 +0100 |
Neil wtote:
>> I think this is a nice interface; but I'm less sure about the
>> implementation. Why do you need to use primitive-eval at all?
>>
>> Isn't CL's with-slots a bit like this? It might help to look at how
>> that is implemented.
Well, I've been having similar doubts, so I came up with something
which turned out to be similar to CLOS's with-slots. The previous solution
indeed required primitive-eval (or something similar), because it
wasn't just a syntax transformation -- that is, it invoked the class-slots
and class-of functions
The workaround requires the user to enumerate all the slots that
he or she is going to access. I wrote two macros, so you can write
(continuing the example from the previous letter):
(with-object-slots (o a b c)
(set! a 5)
(set! b (* a a))
(set! c (+ a b))
(list a b c))
=> (2 25 30)
you don't have to use the original slot names;
you can also use the `with-object-slots' macro supplying aliases:
(with-object-slots (o (x a) (y b) (z c))
(set! x (+ y z))
(list x y z))
=> (55 25 30)
the only problem is that you can't mix both ways, ie.
! (with-object-slots (o a (y b)) (+ a y))
would be incorrect (it is possible to implement such a macro,
but for now this one should suffice). It's still ok to write
(with-object-slots (o (a a) (y b)) (+ a y))
=> 80
The second macro, let-slots allows to make aliases for slots of more
than one object, for instance:
(define p (make C))
(let-slots ((o a b c)
(p (pa a) (pb b) (pc c)))
(set! a 5)
(set! pa a)
(+ pa a))
Everything should work just fine.
The implementation is based solely on R5RS macro system and the
make-procedure-with-setter, that is afaik specific to guile
Clinton wrote:
> with-slots uses symbol-macrolet[0] to bind each of the variable names to
> symbol macros within its body. I don't know of anything in Guile that is
> equivalent.
R5RS pattern language allows to create syntactic aliases for practically
any form. Some time ago I've found the following `let-alias' macro that does
just this:
(use-syntax (ice-9 syncase))
(define-syntax let-alias
(syntax-rules ()
((_ ((id alias) ...) body ...)
(let-syntax ((helper (syntax-rules ()
((_ id ...) (begin body ...)))))
(helper alias ...)))))
;; And here's the code for the aforementioned with-object-slots and let-slots:
(use-modules (oop goops))
(define slot-ref (make-procedure-with-setter slot-ref slot-set!))
(define-syntax with-object-slots (syntax-rules ()
((_ (object (alias1 slot1) ...) expr1 ...)
(let-alias ((alias1 (slot-ref
object (quote slot1))) ...)
expr1 ...))
((_ (object prop1 ...) expr1 ...)
(let-alias ((prop1 (slot-ref object
(quote prop1))) ...)
expr1 ...))))
(define-syntax let-slots (syntax-rules ()
((_ () expr1 ...)
(begin expr1 ...))
((_ (binding1 binding2 ...) expr1 ...)
(with-object-slots binding1
(let-slots (binding2 ...)
expr1 ...)))))
It is also possible to define the `with-slots' macro exactly
as it is in CLOS (but I think let-slots is fine)
Cheers
M.