Formal comment #90 (simplification)

Record layers are not orthogonal
Reported by:	William D Clinger

Component:	records
Version:	5.91

The procedural record layer (r6rs records procedural) is strictly more
expressive than the syntactic layers, (r6rs records explicit) and
(r6rs records implicit), because the procedural layer can create new
subtypes given only a record-type-descriptor, but the two syntactic
layers cannot.

This is entirely appropriate, assuming the syntactic layers are
provided only for convenience in simple cases, and the procedural
layer is intended to be used for all complex cases.

Assuming that assumption, however, the two syntactic layers could be
simplified further. Since they are not intended to be fully general,
they ought to be made easier to use for simple cases.

Although simplification of the syntactic layers is one solution to
this problem, I would suggest a more radical solution: omit both
syntactic layers from the R6RS. Both could be described by a SRFI and
provided by portable reference implementations that build on the
procedural layer.

Omitting the syntactic layers from the report would have the added
benefit of permitting other syntactic alternatives on an equal footing
with those described in the draft R6RS. No portability would be lost;
the procedural layer would be the only universal record layer, just as
in the draft R6RS.

RESPONSE:

For the reasons given below, the editors have decided not to simplify or
drop the syntactic layers and not to generalize them to allow syntactic
extension of dynamically generated record types.

The syntactic layers are designed to allow expansion- or compile-time
determination of record characteristics, including field offsets, so that,
for example, record accesses can be be compiled efficiently to simple
memory indirects.  This property would be lost if the parent were not
generally known until run time, so the restriction requiring that the
parent be the name of a previously defined record type is well motivated
and not intended as a simplification.

While the syntactic layers could be simplified, particularly with respect
to the constructor mechanism, significant demand for the features of the
present mechanism, particularly the general construction mechanism, was
expressed during the review process for SRFI 76: R6RS Records, upon which
the present meechanism is based.

The syntactic layers could be eliminated.  Standardizing on the surface
syntax will facilitate communication via code, however, and because the
syntactic layers can be implemented more efficiently, they will facilitate
the development of efficient portable libraries that define and use record
types.

Another way to solve the problem would be to eliminate the procedural
layer.  If this were done, the syntactic layers would become fully general
in the sense that they would be able to extend any definable record type. 
The procedural interface remains useful, however, e.g., for writing
portable interpreters and expanders, so it should be retained along with
the syntactic layers.