Issue RETURN-VALUES-UNSPECIFIED Writeup

Issue:        RETURN-VALUES-UNSPECIFIED

References:   CLOSE (p 332), IN-PACKAGE (p 183), RENAME-PACKAGE (p 184),

	      TRACE (p 440), UNTRACE (p 440), INSPECT (p 442), 

	      SET-SYNTAX-FROM-CHAR (p 361),

	      LOCALLY (p 156), PROVIDE (p 188), REQUIRE (P 188)


Related issues: REQUIRE-PATHNAME-DEFAULTS

                CLOSED-STREAM-OPERATIONS

Category:     CLARIFICATION

Edit history: 26-Aug-88, Version 1 by Chapman

	      19-Sept-88, Version 2 by Chapman

		 6-Oct-88, Version 3 by Masinter

		 7-Oct-88, Version 4 by Masinter

		26-Nov-88, Version 5 by Masinter

 		 9-Dec-88, Version 6 by Masinter



Problem Description:

 

The descriptions of CLOSE, IN-PACKAGE, RENAME-PACKAGE, TRACE, UNTRACE,

INSPECT, SET-SYNTAX-FROM-CHAR, LOCALLY, PROVIDE, and REQUIRE 

are not clear about the values returned from those constructs.

 

Proposal (RETURN-VALUES-UNSPECIFIED:SPECIFY)

 

Clarify that the return values for the listed constructs are as follows:

 

CLOSE -- T

IN-PACKAGE -- the new package, i.e. the value of *PACKAGE* after the

execution of IN-PACKAGE.

RENAME-PACKAGE -- the renamed package.

TRACE (when called with arguments) -- implementation-dependent.

UNTRACE -- implementation-dependent.

INSPECT -- implementation-dependent.

SET-SYNTAX-FROM-CHAR -- T

LOCALLY -- the return values of the last form of its body, i.e. the body is

	surrounded by an implicit PROGN.

PROVIDE -- implementation-dependent.

REQUIRE -- implementation-dependent.

 

(NB: the issue CLOSED-STREAM-OPERATIONS proposes modifying

the return value of CLOSE on closed streams. The issue 

REQUIRE-PATHNAME-DEFAULTS proposes removing 

REQUIRE and PROVIDE. Those proposals would take 

priority over this one.)


Rationale:

 

This clarification allows users to know when they can and can not

count on the values returned from these constructs. 

 

Current Practice:


Varies; the choices made here are consistent with many but

not all implementations.

  

Cost to Implementors:

 

Small.


Benefits:

 

This clarification will assist users in writing portable code.

 

Cost to users:

 

Small; it seems unlikely that there is much code that currently

depends on the return values of these functions; such code isn't

portable.


Aesthetics:

 

Specified is better than not, when it makes sense.

 

Discussion:

 

PROVIDE and REQUIRE are not likely to appear except in the "top level" of

files, and so their return value is possibly moot.  Another proposal would

eliminate them from the language, and then their return value would definitely

be moot!


There is some sentiment for leaving unspecified the values of the

debugging/environment features such as TRACE and UNTRACE,

for the same reasons that so much else of their behavior is unspecified.


Notes from Oct 88 X3J13:


Except for LOCALLY, why bother?


That just causes portability problems. Don't want to leave it

unspecified -unless- someone can cite a reason to do so.


"If many weren't defined, maybe we should leave 'em, but

since nearly all -are- defined, let's just go ahead and

round out the set."


Most text books she's seen show CLOSE returning NIL.

One text book shows it returning T.

Since some people like to think of T as success and NIL

as failure, maybe it should always return T.

(See Issue: CLOSED-STREAM-OPERATIONS.)