Re: Making every goops object applicable
Hi, Krister Svanlund krister.svanl...@gmail.com skribis: Apparently this works by some flag being set by applicable-strukt in libguile for the object and that flag is checked during application, calling the 'procedure slot if it's set with some optimization assuming that 'procedure is the first slot. There’s also a vtable flag that determines whether a struct is applicable: #define SCM_STRUCT_APPLICABLE_P(X)(SCM_STRUCT_VTABLE_FLAG_IS_SET ((X), SCM_VTABLE_FLAG_APPLICABLE)) And indeed, the struct’s procedure is the first slot: #define scm_applicable_struct_index_procedure 0 /* The procedure of an applicable struct. Only valid if the struct's vtable has the applicable flag set. */ For instance, every struct whose vtable is applicable-struct-vtable (defined in struct.c) is applicable. Then you can investigate by looking at the indices defined in struct.h: scheme@(guile-user) (struct-vtable? class) $2 = #t scheme@(guile-user) (struct-ref class 1) ; scm_vtable_index_flags $3 = 12291 scheme@(guile-user) (logand $3 4) ; SCM_VTABLE_FLAG_APPLICABLE_VTABLE $4 = 0 ; → not applicable scheme@(guile-user) (logand (struct-ref applicable-struct-vtable 1) 4) $7 = 4 ; → applicable So you could fiddle with the flags of a class to make its instances applicable: scheme@(guile-user) (define (applicable-struct? s) (logand 4 (struct-ref (struct-vtable (struct-vtable s)) 1))) scheme@(guile-user) (applicable-struct? current-input-port) $27 = 4 scheme@(guile-user) (define (applicable-struct-procedure s) (struct-ref s 0)) scheme@(guile-user) (define-class appclass (class) (foo)) scheme@(guile-user) (struct-set! appclass 1 (logior (struct-ref appclass 1) 4)) $32 = 12295 scheme@(guile-user) (define-class foo () (bar #:init-value (lambda args (pk 'apply args))) #:metaclass appclass) $33 = #appclass foo 16e0d20 scheme@(guile-user) (define f (make foo)) scheme@(guile-user) (applicable-struct? f) $34 = 4 scheme@(guile-user) (applicable-struct-procedure f) $35 = #procedure 1863060 at current input:51:0 args scheme@(guile-user) (f 1 2 3) ;;; (apply (1 2 3)) $36 = (1 2 3) But there should certainly be a higher-level facility. :-) Thanks, Ludo’.
Re: Making every goops object applicable
Krister Svanlund krister.svanl...@gmail.com writes: For example an instance of a class inheriting a class that inherits applicable-struct that defines 'procedure is not applicable. Looking at the code, it is clear that in order for a GOOPS instance to be applicable, it is not enough for applicable-struct to be a superclass. It is also necessary for the _metaclass_ of its class to have the SCM_VTABLE_FLAG_APPLICABLE_VTABLE bit set. One such metaclass is already built-in: applicable-struct-class, which is the metaclass for applicable-struct, but if you'd like to define your own metaclass for applicable objects, you can set the bit manually on the metaclass as described by Ludovic. However, keep in mind that the presence of that bit in the metaclass indicates that the first slot of all instances _must_ be the procedure slot. Mark
Re: Making every goops object applicable
Mark H Weaver m...@netris.org writes: Krister Svanlund krister.svanl...@gmail.com writes: For example an instance of a class inheriting a class that inherits applicable-struct that defines 'procedure is not applicable. Looking at the code, it is clear that in order for a GOOPS instance to be applicable, it is not enough for applicable-struct to be a superclass. It is also necessary for the _metaclass_ of its class to have the SCM_VTABLE_FLAG_APPLICABLE_VTABLE bit set. One such metaclass is already built-in: applicable-struct-class, which is the metaclass for applicable-struct, So the solution to Krister's problem is to make sure that every class he's interested in is defined as (define-class whatnot (supers...) ... #:metaclass applicable-struct-class) Right? but if you'd like to define your own metaclass for applicable objects, you can set the bit manually on the metaclass as described by Ludovic. Would this be needed, if the above works? However, keep in mind that the presence of that bit in the metaclass indicates that the first slot of all instances _must_ be the procedure slot. Might the (define-class ... #:metaclass applicable-struct-class) ensure that, by some kind of of compute-slots customisation...? I took a look but unfortunately couldn't see this in the code. Neil
Re: Making every goops object applicable
Hi Neil! Neil Jerram n...@ossau.homelinux.net writes: Mark H Weaver m...@netris.org writes: Krister Svanlund krister.svanl...@gmail.com writes: For example an instance of a class inheriting a class that inherits applicable-struct that defines 'procedure is not applicable. Looking at the code, it is clear that in order for a GOOPS instance to be applicable, it is not enough for applicable-struct to be a superclass. It is also necessary for the _metaclass_ of its class to have the SCM_VTABLE_FLAG_APPLICABLE_VTABLE bit set. One such metaclass is already built-in: applicable-struct-class, which is the metaclass for applicable-struct, So the solution to Krister's problem is to make sure that every class he's interested in is defined as (define-class whatnot (supers...) ... #:metaclass applicable-struct-class) Right? That was indeed my suggestion, though your comments below raise an additional complication that I hadn't considered: some additional magic will be needed to ensure that the 'procedure' slot comes first. but if you'd like to define your own metaclass for applicable objects, you can set the bit manually on the metaclass as described by Ludovic. Would this be needed, if the above works? No, but since this is part of an effort to implement Python 3 in Guile, it's possible that they may need to create their own metaclass for other reasons. However, keep in mind that the presence of that bit in the metaclass indicates that the first slot of all instances _must_ be the procedure slot. Might the (define-class ... #:metaclass applicable-struct-class) ensure that, by some kind of of compute-slots customisation...? I guess so, but I'm not convinced that this is the right approach. It now seems reasonably clear that GOOPS has poor support for applicable objects, so we should think about a more proper solution that's easy to use. I don't see why applicable objects should need a custom metaclass. In general, it seems to me that features that require custom metaclasses do not compose well. Subclasses of applicable-struct should just work without users resorting to these hacks. As I've suggested before, the most elegant solution I can think of is to simply make a two-argument generic function called something like 'generic-apply'. I suspect that there's some clever way to make this very fast using the MOP, but I have yet to work out the details. Unfortunately my copy of AMOP is 50 miles away at the moment. Suggestions solicited, and if anyone wants to take the lead on this, I'd be grateful. If not, I'll try to come up with something. Thanks, Mark
Making every goops object applicable
In our work to look into how Python 3 could be implemented for Guile we have figured out that the only way to make a goops object applicable is to have it inherit applicable-struct. This does not always work the way it could be expected, for example when inheriting from several classes. Apparently this works by some flag being set by applicable-strukt in libguile for the object and that flag is checked during application, calling the 'procedure slot if it's set with some optimization assuming that 'procedure is the first slot. Is this correct and if so, is there any particular reasoning behind this rather than just having all classes that has the slot 'procedure be applicable? Yours, Krister Svanlund
Fwd: Making every goops object applicable
-- Forwarded message --
From: Krister Svanlund krister.svanl...@gmail.com
Date: Tue, May 15, 2012 at 2:45 AM
Subject: Re: Making every goops object applicable
To: Mark H Weaver m...@netris.org
On Tue, May 15, 2012 at 12:16 AM, Mark H Weaver m...@netris.org wrote:
Krister Svanlund krister.svanl...@gmail.com writes:
In our work to look into how Python 3 could be implemented for Guile
we have figured out that the only way to make a goops object
applicable is to have it inherit applicable-struct. This does not
always work the way it could be expected, for example when inheriting
from several classes.
Can you elaborate on what doesn't work as expected?
For example an instance of a class inheriting a class that inherits
applicable-struct that defines 'procedure is not applicable.
Apparently this works by some flag being set by
applicable-strukt in libguile for the object and that flag is
checked during application, calling the 'procedure slot if it's set
with some optimization assuming that 'procedure is the first slot.
Is this correct and if so, is there any particular reasoning behind
this rather than just having all classes that has the slot 'procedure
be applicable?
I see at least two problems with your suggestion: (1) it would be very
slow to search the list of slots for a slot named 'procedure' on every
application. (2) a user might create a class with a slot named
'procedure' without intending that the slot should be used in this way.
(1) Given that slots aren't dynamic at runtime (as they are in Python for
example) and there is already some magic being done by applicable-struct
I have a hard time seeing how looking up a slot for application can't be
done in almost constant time (one look-up and then storing a pointer to the
position of the slot for future applications) and only a tiny bit slower
than the current implementation. And after all, Guile is, judging by it's
documentation, focused on being an extension language where the heavy
lifting is supposed to be done in C code.
(2) This I'm not entirely sure how to fix but I have the impression that
this could be done analogous to the initialization method of the
meta-object protocol, but I assume this could require some severe internal
changes and in fact slow down applications in general in Guile. My thought
is that you simply have a metaclass that has a method ('apply or similar)
that specifies what will happen if an instance of any of it's classes is
being used as a function.
I'm mostly interested in getting all possible input about how good/bad of
an idea this is so please prove (in the looses sense of the word) that I'm
wrong. When I have some free time this summer I will probably make an
attempt at doing this anyhow, I just want to know what I'm getting myself
into.
Yours,
Krister Svanlund
