Formal comment #210 (simplification)

Why are simple conditions so different from records
Reported by: 	Andre van Tonder
Version: 	5.92

Component : Condition objects

Summary:

The interface for simple conditions is different from that of records
but redundantly reproduces much of the functionality of
records. Consider making the interfaces more similar or even dropping
simple conditions as a separate concept and instead using a record
hierarchy for them.  Discussion:

Simple conditions, like record types, have named fields and single
inheritance. As such, the API for defining these types, constructing
instances and accessing fields has to reproduce much of the same
functionality. However, the interface is different. For example,

    * Record type field names are for informational purposes only,
      whereas condition field names are actual symbolic labels that
      are used for lookup.

    * Make-condition-type takes a list of symbols, whereas
      make-record-type-descriptor takes a vector of symbols.

    * Define-condition-type has a subset of the functionality of
      define-record-type. Yet the syntax of the former is different
      from that of the latter.

In fact, I see no good reason why the ability to have custom
constructors, accessors, explicit and implicit naming APIs, control
over introspection, opacity, finality, etc., should be denied a
designer of a condition hierarchy given that these features are
already available for other user-defined types.

It seems that simple conditions could perfectly well be a record type
hierarchy under a common root /record/ type.

An API for compound conditions would of course still have to be
separate.

Recommendation:

Consider making interfaces of simple conditions similar to that of
records. Better yet, drop simple conditions and consider implementing
the condition hierarchy as a hierarchy of record types with common
parent /record type/ condition. In this way, all of the infrastructure
available for records, including custom constructors, explicit and
implicit naming interfaces, introspection, etc., would become
available for new condition types.

Compound conditions could remain largely as they currently are, and so
can the convenience syntax (condition ----), etc.


RESPONSE:

The next draft will follow the suggestion, and specify simple
condition types as record types.