Issue REQUIRE-PATHNAME-DEFAULTS Writeup

Issue:         REQUIRE-PATHNAME-DEFAULTS

References:    *MODULES*, PROVIDE, REQUIRE, pp 188-191

               LOAD, pp 426-427

Category:      CHANGE

Edit history:  Version 1 by Pierson 9/13/88

               Version 2 by Pierson 9/19/88, change PROVIDE stuff per comments

               Version 3 by Pierson 10/17/88, remove PROVIDE locaction specs.

               Version 4 by Pierson 10/31/88, remove from language

               Version 5 by Pierson 11/15/88, cleanup, fix discussion

               Version 6 by Pierson 12/9/88, remove *MODULES* as well


Problem description:


PROVIDE and REQUIRE are a dual-purpose pair of functions that attempt

to provide multi-file Common Lisp programs with a single mechanism to

detect and correct incorrect load sequences.  These functions were

also designed to be used for general file inclusion in Common Lisp.

Unfortunately, the file loading feature of REQUIRE is specified such

that it is inherently non-portable and environment dependent.


Proposal (REQUIRE-PATHNAME-DEFAULTS:ELIMINATE):


Remove PROVIDE, REQUIRE, and *MODULES* from the Common Lisp standard.


Test Cases/Examples:


(PROVIDE 'fft)


Would not be Common Lisp.


(REQUIRE 'fft)


Would not be Common Lisp.


Rationale:


The file loading feature of REQUIRE is non-portable.  The remaining

functionality of PROVIDE and REQUIRE (pushing and testing *MODULES*)

can easily be implemented by user code.  Since some implementations

will retain the automatic module loading features of REQUIRE and some

won't, use of REQUIRE will almost always make code less portable.


Current practice:


All implementations currently support some sort of file loading via

single-argument REQUIRE.  In general, the Lisp Machine implementations

invoke the system module building/loading facility while the Unix

implementations simply try to load a file in the current directory.


Cost to Implementors:


Implementations will have to move PROVIDE and REQUIRE to their package

for implementation extensions and change their documentation to

indicate that PROVIDE and REQUIRE are non-standard.  This is a fairly

small change.


Cost to Users:


Non-portable programs that rely on PROVIDE and REQUIRE will probably

be unaffected since implementations will probably maintain their

existing functionality.  Since the current behavior is decidedly

non-portable, portable programs have to aviod or special-case PROVIDE

and REQUIRE anyway.


Cost of non-Adoption:


PROVIDE and REQUIRE will continue as impediments to portability.


Benefits:


The non-portability of PROVIDE and REQUIRE will be made obvious.


Aesthetics:


This simplifies the language by removing an environment-dependent

feature. 


Discussion:


The cleanup committee tried to come up with a proposal to restrict

PROVIDE and REQUIRE to the portable subset of their functionality.

This failed because several implementors objected that it compelled

them to significantly reduce the functionality they provided users in

order to create a trivial feature which any user could easily write

for herself.


Fahlman, Gregor, Grey, Loosemore, Moon, Pierson, Pitman, Steele, and

Zacharias have expressed support for removing PROVIDE and REQUIRE from

the language, at least as the lesser of several evils.


JonL would much rather see PROVIDE and REQUIRE remain in the language

as a safety net behind any implementation-specific system building

facility.  Pierson likes the safety net idea, but doesn't think it's

workable without forbidding REQUIRE from loading files.


Pitman suggested that PROVIDE and REQUIRE should be depricated rather

than removed entirely.  Pierson agrees, but notes that Larry wants us

to deal with deprication versus elimination as a separate global topic.


Several people have expressed a desire not to break existing user

code.  If accepted, this proposal should not break existing code

because all implementations are expected to retain their current

PROVIDE and REQUIRE functionality as an extension to Common Lisp.