Issue DATA-TYPES-HIERARCHY-UNDERSPECIFIED Writeup

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

		 4-Sep-88, version 2 by X3J13 June 88


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, 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.


Esthetics:


This makes the type system simpler and easier to understand.


Discussion:


This suggestion originated in the CLOS committee.