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Smart variables, dumb variables
From: |
Marius Vollmer |
Subject: |
Smart variables, dumb variables |
Date: |
13 Aug 2002 22:06:50 +0200 |
User-agent: |
Gnus/5.09 (Gnus v5.9.0) Emacs/21.2 |
Hi,
currently, our global variables are strictly passive containers.
Accessing a variable is just a memory reference. I think we need to
keep this for performance reasons.
However, we might also want to have more fancy variables. One might
want to get a notification when a variable is set, or one might want
to fetch the real value from some indirect location.
For example, Emacs has the ability to forward references to Elisp
variables to C variables. C code can use a normal "int" variable,
say, and Elisp code can use that variable as well. Translating
between Elisp representation and C representation of values is done
transparently, when Elisp accesses the variable. RMS convinced me
that that would be a nice feature for Guile as well, in general.
One might say that it is probably better to not use variables for
this, but rather have a more abstract interface using setter and
getter functions. But when designed carefully. the need to use
stters/getters for things that are in all respects just variables can
also feel awkward. A good example are the forwarding variables of
Emacs. It shouldn't really be visible to Elisp code that the
variables are forwarded to C variables (although they don't allow
non-integer values, thus making them different from other Elisp
variables, hmm).
I'd like to propose a slight modification to our variables that would
allow us to cleanly mess with what variables exactly are in the
future.
I would add a 'flags' field to variable smob (there is room for it),
with the following flags defined initially:
- SCM_F_VARIABLE_DUMB
When set, the variable is 'dumb' and can be referened and set
with SCM_DUMB_VARIABLE_REF and SCM_DUMB_VARIABLE_SET,
respectively. When it is not set, you need to call
scm_variable_ref and scm_variable_set_x.
- SCM_F_VARIABLE_READONLY
When set, the variable can not be set. You are not allowed to
use SCM_DUMB_VARIABLE_SET and scm_variable_set_x will result in
an error.
A future execution model (that is slowly forming in my head) will make
it so that most variable flags can be tested at compile time. Thus, a
compiler can emit more efficient code for dumb variables.
One possible next step would be to somehow involve Goops in this and
turn scm_variable_ref into a generic function, the way scm_sum is
already.
Opinions?
--
GPG: D5D4E405 - 2F9B BCCC 8527 692A 04E3 331E FAF8 226A D5D4 E405
- Smart variables, dumb variables,
Marius Vollmer <=
- Re: Smart variables, dumb variables, tomas, 2002/08/14
- Re: Smart variables, dumb variables, Marius Vollmer, 2002/08/14
- Re: Smart variables, dumb variables, tomas, 2002/08/14
- Re: Smart variables, dumb variables, Marius Vollmer, 2002/08/14
- Re: Smart variables, dumb variables, rm, 2002/08/14
- Re: Smart variables, dumb variables, Marius Vollmer, 2002/08/14
- Re: Smart variables, dumb variables, rm, 2002/08/15
- Re: Smart variables, dumb variables, Lynn Winebarger, 2002/08/15
- Re: Smart variables, dumb variables, rm, 2002/08/15
- Re: Smart variables, dumb variables, Rob Browning, 2002/08/15