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Re: Simplified slot access in goops

From: Andy Wingo
Subject: Re: Simplified slot access in goops
Date: Sat, 28 Aug 2010 13:07:04 -0700
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux)

Hi Maciek,

I know its been a couple years, but you raised an interesting point that
Guile wasn't really able to handle at that point. Your implementation of
`in' (commonly known as `with-slots', I believe) follows the mail.

On Thu 27 Nov 2008 12:36, "Maciek Godek" <address@hidden> writes:

> (use-modules (oop goops))
> (use-syntax (ice-9 syncase))

With Guile 2.0 (coming shortly!), there is no more need to use ice-9

> Now suppose we have an instance of a class:
> (define-class C ()
>   a b c)
> (define o (make C))
> Using the "in" macro, we can write:
> (in o
>   (set! a 5)
>   (set! b (1+ a))
>   (set! c (+ a b))
>   (+ a b c))
> => 22

The problem with this macro is that `o' can be of any type at all, but
Guile needs to know at least whether a given identifier is a macro or a
local variable or what, at expansion time.

So for that reason, with-slots usually takes a list of slots, explicitly:

  (with-slots o (a b c)
    (set! a 5)
    (set! b (1+ a))
    (set! c (+ a b))
    (+ a b c))

It would be possible to pass the class instead, but then the class would
need to be known at compile-time, and you run into a number of issues
there. Anyway, here's that with-slots definition:

  (define-syntax with-slots
    (syntax-rules ()
      ((_ obj (slot ...) b b* ...)
           ((slot (identifier-syntax
                    (id (slot-ref obj 'slot))
                    ((set! id val) (slot-set! obj 'slot val))))
         b b* ...))))
Here we use the new settable identifier-syntax.

Note that slot-ref / slot-set isn't the most efficient way to reference
slots; the best thing is to use accessors, which JIT-compile (sorta) to
struct-ref opcodes, but we don't know that accessors exist for these
slots. You can define a with-accessors macro though, that does assume
accessors are available.

In fact with more assumptions, it's possible to compile to struct-ref /
struct-set yourself -- see the examples in the "Syntax" chapter of "The
Scheme Programming Language" 4th edition (available online) for an
illuminating take on the issue.



The original definition:

> (define-syntax let-alias
>   (syntax-rules ()
>     ((_ ((id alias) ...) body ...)
>      (let-syntax ((helper (syntax-rules ()
>                           ((_ id ...) (begin body ...)))))
>        (helper alias ...)))))
> (define slot (make-procedure-with-setter slot-ref slot-set!))
> (define-syntax in (syntax-rules ()
>                   ((_ object expr ...)
>                    (primitive-eval
>                     (list 'let-alias
>                           (map (lambda(s)(list(car s)(list 'slot object (list 
> 'quote (car s)))))
>                                (class-slots (class-of object)))
>                           (quote expr) ...)))))


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