Submitter's name: Pascal Costanza
Submitter's email address: pc_at_p-cos.net
Type of issue: Enhancement
Priority: Major
R6RS component: Records
Version of the report: 5.91
One-sentence summary of the issue: R6RS Scheme should add applicable
record instances.
Full description of the issue:
This is a proposal to add a notion of "applicable record instances"
to R6RS Scheme. An applicable record instance is a record instance
that has a distinct "applicable" field that is supposed to contain a
procedure. Such an applicable instance can be used as the value in
the first position of an expression, or as the first parameter to
apply, and when such an expression is evaluated this will effectively
call the procedure stored in the distinct "applicable" field of the
record instance (or raise an appropriate exception if that field
doesn't contain a procedure).
Rationale:
Applicable record instances are useful when a program needs to
associate procedures with data. They enable storing both procedures
and data directly in the same instances rather than linking them via
hash tables, association lists, etc., or alternatively making it
inconvenient to call such procedures.
Such applicable record instances are especially useful to build a
more full-fledged object system on top. For example, the Common Lisp
Object System, or better its concept of generic functions, is
essentially built in terms of such applicable instances, there called
"funcallable instances" [1]. The downside of a lack of applicable
instances can for example be observed in the (Scheme-based)
implementation of Tiny CLOS. The source code of Tiny CLOS has to jump
through a few unpleasent hoops in order to get the same effect [2].
See also [3] for another object system for Scheme with a different
flavor based on T that also requires a notion of applicable record
instance, there simply called instances (cf. Sections 3.1 and 3.2 in
that paper).
Note that there is no need that the procedure stored in the distinct
field of an applicable record instance has a special reference to its
enclosing instance (for example, like a self/this/current reference
in OOP languages). It's sufficient that such a procedure closes over
the respective record instance.
I currently see two ways how such applicable record instances can be
added to R6RS Scheme. One way would be to add a declaration for
fields, similar to the mutable / immutable declarations, roughly like
this:
(define-record-type some-record
(fields (mutable applicable proc)))
or:
(define-record-type some-record
(fields (mutable-apply proc)))
A disadvantage of such a design would be a requirement to handle
record types inheriting from each other that declare more than one
applicable field.
Alternatively, a predefined record type could be added that
predeclares an applicable field which other record types can be
derived from. A disadvantage here would be that it has to be decided
whether such a field is mutable or not (which in the general case it
should be).
There are probably better alternatives.
Pascal
[1]
http://www.lisp.org/mop/concepts.html#funcallable-instances
[2]
http://community.schemewiki.org/?tiny-clos
[3] Norman Adams and Jonathan Rees, Object-Oriented Programming in
Scheme,
http://library.readscheme.org/servlets/cite.ss?pattern=Rees-
Rees-88
--
Pascal Costanza, mailto:pc_at_p-cos.net, http://p-cos.net
Vrije Universiteit Brussel, Programming Technology Lab
Pleinlaan 2, B-1050 Brussel, Belgium
Received on Mon Sep 18 2006 - 15:50:35 UTC