Issue DATA-TYPES-HIERARCHY-UNDERSPECIFIED

[gls@Think.COM]

Issue:         DATA-TYPES-HIERARCHY-UNDERSPECIFIED

References:    CLtL 33-35 (data types)
	       CLtL 312 (DEFSTRUCT :INCLUDE option)

Category:      CHANGE

Edit history:  24-Mar-88, version 1 by Steele

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 (DATA-TYPES-HIERARCHY-UNDERSPECIFIED: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, explicitly forbid
the type given to the :INCLUDE option to be CONS, SYMBOL, ARRAY, NUMBER,
CHARACTER, HASH-TABLE, READTABLE, PACKAGE, PATHNAME, STREAM, or
RANDOM-STATE.

Test Cases/Examples:

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.

Esthetics:

This makes the type system simpler and easier to understand.

Discussion:

This suggestion originated in the CLOS committee.