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Issue DEFSTRUCT-COPIER-ARGUMENT-TYPE Writeup

Issue:        DEFSTRUCT-COPIER-ARGUMENT-TYPE

Forum: X3J13

References: DEFSTRUCT (X3J13/92-102, pp8-5..6)

Category: CLARIFICATION

Edit history: 07-Jul-93, Version 1 by Pitman

Status: Proposal RESTRICT passed 11-1 on letter ballot 93-306.

Problem Description:

The description of the :COPIER defstruct option doesn't say that the

consequences of applying the defined copier to an object not of the

structure type are undefined.

Proposal (DEFSTRUCT-COPIER-ARGUMENT-TYPE:RESTRICT):

Change the first sentence of the second paragraph of the description

of the :COPIER to:

The automatically defined copier function is a function of one

\term{argument}, which must be of the structure type being defined.

In addition, the last sentence can be changed to the following:

If the DEFSTRUCT :TYPE option was not used, the following

equivalence applies:

(<copier> x) = (copy-structure (the \param{structure-name} x))

Test Case:

(DEFSTRUCT S1 A B C)

(DEFSTRUCT S2 A B C)

(COPY-S2 (MAKE-S1 :A 3)) => ??

Before this proposal, it is possible to interpret the definition to say

that #S(S1 :A 3) might be returned here. This proposal makes it clear

that the consequences are in fact undefined.

Rationale:

This gives implementational freedom in a situation where no reasonable

programmer would say that the behavior should be well-defined.

Current Practice:

LispWorks 3.1.0 signals an error in the test case (even in low safety code).

Symbolics Genera 8.3 returns #S(S1 :A 3) in the test case.

Cost to Implementors:

None. This change is upward compatible.

Cost to Users:

HOPEFULLY none. It would be really ugly to imagine anyone taking advantage

of the room that there is now.

Cost of Non-Adoption:

The spec is less tight.

Benefits:

Aesthetics.

Editorial Impact:

Very small.

Aesthetics:

Mostly the spec (and any resulting code) will be slightly cleaner this way.

Discussion:

This change is in response to comment Margolin #8.

Pitman doubts that anyone will actually abuse the loophole that

this comment is about, but think's it's reasonable to tighten things

up just in case.


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