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Re: make-vtable
From: |
Ludovic Courtès |
Subject: |
Re: make-vtable |
Date: |
Thu, 15 Feb 2007 09:45:27 +0100 |
User-agent: |
Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux) |
Hi,
Kevin Ryde <address@hidden> writes:
> address@hidden (Ludovic Courtès) writes:
>>
>> I'm not sure the indirection in `scm_init_stacks ()' is needed since it
>> uses STACK_LAYOUT for both VTABLE and SCM_STACK_TYPE,
>
> Not sure what you mean there.
We have:
VTABLE -> SCM_STACK_TYPE -> stack objects
where `x -> y' means "`y' is an instance of `x'". When creating stack
objects with (roughly) `(make-struct stack-type)', VTABLE is _not_
consulted at all. The layout of stack objects is determined only by
that specified in SCM_STACK_TYPE. Thus, VTABLE is redundant.
All this would be clearer if we had a `struct-vtable' type such that
`(make-struct struct-vtable)' would yield a new vtable (just like `(make
<class>)' yields a new GOOPS class). Like `<class>', `struct-vtable'
would terminate the "reflective tower" (i.e., its vtable is itself).
Actually, such a `struct-vtable' stealthily appears in
`make-vtable-vtable', under the name of REQUIRED_VTABLE_FIELDS: We could
really have a `struct-vtable' whose layout is REQUIRED_VTABLE_FIELDS and
then `make(-vtable)+' could be simply implemented in terms of
`make-struct' (just like `make-class' uses `make').
I guess I should try implementing this theory one of these days. ;-)
> At any rate, below is where I'm up to so far with trying to make the
> docs a bit easier. It includes my proposed make-vtable. Could be
> possible to leave that out, but perhaps those like me who managed to
> never understand structs can see if it makes the understanding easier
> :-).
Thanks for working on this!
> A "structure" is a first class data type which holds Scheme values or C
> words in slots numbered 0 upwards. A "vtable" represents a structure
> type, giving read/write permissions on slots, whether they're Scheme
> values or uninterpreted words, and giving an optional print function
> for the structure (for use by `write' etc).
Perhaps a word saying the struct fields are laid out in a contiguous
memory area, which makes interaction with C much easier (using C arrays
or some such).
> Vtable Vtables
> ..............
>
> As noted above, vtables are structures and such a structure is itself
> described by a vtable. Such a "vtable of a vtable" can be created with
> `make-vtable-vtable' below and used to build sets of related vtables,
> possibly with extra application fields.
>
> This second level of vtable can be a little confusing. An example,
> and indeed a typical use, is Guile's own record system (*note
> Records::). Currently record types are implemented as vtables, and
> those vtables have an extra slot holding the list of field names for
> that type (as passed to `make-record-type').
While the rest looks good, I remain skeptical about this part. And a
manual that claims to be confusing does not inspire confidence. ;-)
Thanks,
Ludovic.
- make-vtable, Kevin Ryde, 2007/02/12
- Re: make-vtable, Neil Jerram, 2007/02/18
- Re: make-vtable, Ludovic Courtès, 2007/02/18
- Re: make-vtable, Neil Jerram, 2007/02/18
- Re: make-vtable, Kevin Ryde, 2007/02/18
- Re: make-vtable, Neil Jerram, 2007/02/18
- Re: make-vtable, Ludovic Courtès, 2007/02/19