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Cleanup Issue `DATA-TYPES-HIERARCHY-UNDERSPECIFIED`

Status: Passed, Aug 88 X3J13
Category: CHANGE
References: CLtL 33-35 (data types) CLtL 312 (DEFSTRUCT :INCLUDE option)

Problem Description

The types HASH-TABLE, READTABLE, PACKAGE, PATHNAME, STREAM, and RANDOM-STATE currently are not required to be disjoint from CONS, SYMBOL, ARRAY, NUMBER, or CHARACTER. The same is true of DEFSTRUCT types. With the arrival of CLOS, lack of disjointness will be inconvenient because one cannot reliably use generic function dispatch on these types.

Proposal (DISJOINT)

Remove the third and antepenultimate bulleted items from section 2.15 is CLtL and replace with the following:

* The types CONS, SYMBOL, ARRAY, NUMBER, CHARACTER, HASH-TABLE, READTABLE, PACKAGE, PATHNAME, STREAM, RANDOM-STATE, and any single other type created by DEFSTRUCT [or DEFCLASS] are pairwise disjoint.

Also, in the discussion of the DEFSTRUCT :INCLUDE option, it is an error for the type given to the :INCLUDE option to be CONS, SYMBOL, ARRAY, NUMBER, CHARACTER, HASH-TABLE, READTABLE, PACKAGE, PATHNAME, STREAM, or RANDOM-STATE.

Rationale

It would be useful to extend sequence functions to operate on streams or hash tables, for example, through the CLOS generic function mechanism. This is not possible under the current specification.

Current Practice

Some implementations in effect use DEFSTRUCT to define data structures such as hash tables and random-states.

Cost to Implementors

Small or nonexistent. The main reason this disjointness was not specified in CLtL was the possibility that structures might not be easily distinguishable: a stupid concern over a slight efficiency. This implementation freedom apparently has not been exploited in practice.

Cost to Users

None.

Cost of Non-Adoption

CLOS will be less useful.

Benefits

CLOS will be more useful.

Aesthetics

This makes the type system simpler and easier to understand.

Discussion

This suggestion originated in the CLOS committee.

Edit History

24-Mar-88, version 1 by Steele
4-Sep-88, version 2 by X3J13 June 88