Issue: LOAD-TRUENAME (Version 4)

[Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>]

New version to accomodate Moon's comments.

I changed the Example a bit.
I added a paragraph to the Discussion.
Everything else is the same.

I'm hoping this is a final version.
 -kmp

-----
Issue:        LOAD-TRUENAME
Forum:	      Cleanup
References:   LOAD (p426), PROVIDE (p188), REQUIRE (p188),
	      Issue REQUIRE-PATHNAME-DEFAULTS
Category:     ADDITION
Edit history: 13-Mar-89, Version 1 by Pitman
	      29-Mar-89, Version 2 by Moon (add -PATHNAME vars)
	      10-Apr-89, Version 3 by Pitman (clarify v2)
	      11-Apr-89, Version 4 by Pitman (merge Moon's v3 comments)

Problem Description:

 It is difficult to construct sets of software modules which work
 together as a unit and which port between different implementations.

 REQUIRE and PROVIDE were intended to provide this level of support
 but have `failed' to be portable in practice.

 Typical user configurations involve a `system definition' file which
 loads the modules of a `system' (collection of software modules).

 Among the specific problems which arise are:

  - File system types may vary. Different file syntax must be used for
    each site.

  - Even with the same Lisp implementation and host file system type,
    the directory in which a software system resides may differ from
    delivery site to delivery site.

  - Multiple `copies' of the same system may reside in different
    directories on the same machine.

Proposal (LOAD-TRUENAME:NEW-PATHNAME-VARIABLES):

 Introduce new variables:

   *LOAD-TRUENAME*					[Variable]

   This special variable is initially NIL, but is bound by LOAD to
   hold the truename of the pathname of the file being loaded.

   *COMPILE-FILE-TRUENAME*				[Variable]

   This special variable is initially NIL, but is bound by 
   COMPILE-FILE to hold the truename of the pathname of the file
   being compiled.
   
   *LOAD-PATHNAME*					[Variable]
  
   This special variable is initially NIL but is bound by LOAD
   to hold a pathname which represents the filename given as the
   first argument to LOAD merged against the defaults.
   That is, (PATHNAME (MERGE-PATHNAMES arg1)).
  
   *COMPILE-FILE-PATHNAME*				[Variable]
  
   This special variable is initially NIL but is bound by COMPILE-FILE
   to hold a pathname which represents the filename given as the
   first argument to COMPILE-FILE merged against the defaults.
   That is, (PATHNAME (MERGE-PATHNAMES arg1)).

Example:

 ------ File SETUP ------
 (IN-PACKAGE "MY-STUFF")
 (DEFMACRO COMPILE-TRUENAME () `',*COMPILE-FILE-TRUENAME*)
 (DEFVAR *MY-COMPILE-TRUENAME* (COMPILE-TRUENAME) "Just for debugging.")
 (DEFVAR *MY-LOAD-PATHNAME* *LOAD-PATHNAME*)
 (DEFUN LOAD-MY-SYSTEM ()
   (DOLIST (MODULE-NAME '("FOO" "BAR" "BAZ"))
     (LOAD (MERGE-PATHNAMES MODULE-NAME *MY-LOAD-PATHNAME*))))
 ------------------------

 (LOAD "SETUP")
 (LOAD-MY-SYSTEM)

Rationale:

 This satisfies the most common instances of the frequently reported
 problem in the Problem Description.

 The ...-TRUENAME* variables are useful to tell the real file being
 loaded.

 The ...-PATHNAME* variables are useful to find information about
 the original link names or logical device names mentioned in the
 pathname to be opened but no longer reflected in the truename.

 Note that it is not adequate to just have the -PATHNAME* variables
 since TRUENAME on these pathnames might not yield the value of the
 -TRUENAME* variables if the file has been deleted or protected
 since the open occurred (in some implementations).

Current Practice:

 Wide variation.

 In some implementations, calling LOAD binds or sets 
 *DEFAULT-PATHNAME-DEFAULTS* so that pathnames named in a file being
 LOADed will default to being `nearby.'

 Some implementations provide special variables that are similar or
 identical to one or both of those proposed.

 Some implementations have a way to represent the pathname for the
 current working directory, and make the default pathname default
 to that, so that loading without specifying a default again tends to
 get `nearby' files.

 None of these techniques is portable, unfortunately, because there
 is no agreement.

Cost to Implementors:

 Very small.

Cost to Users:

 None. This change is upward compatible.

Cost of Non-Adoption:

 Continued difficulty for anyone trying to put a system of modules
 in a form where they can be conveniently delivered using portable code.

Benefits:

 The cost of non-adoption is avoided.

Aesthetics:

 Negligible.

Discussion:

 Touretzky raised the issue most recently on Common-Lisp. A number
 of people immediately jumped on the bandwagon, indicating it was
 important to them, too.

 Pitman made three suggestions in response, of which the above is
 the first. The others included:
  2. Variables *LOAD-TRUENAMES* and *COMPILE-FILE-TRUENAMES* which hold
     lists of the truenames of all files being loaded or compiled,
     respectively, during the dynamic invocation of LOAD and COMPILE-FILE.
 
  3. Variable *LOAD-OR-COMPILE-FILE-TRUENAMES* which holds a list like
    ((LOAD truename) (COMPILE-FILE truename) ...)
    during the dynamic invocation of LOAD and COMPILE-FILE.
 
 Touretzky responded:
 ``I like KMP's proposals.  I like the second one best: have separate
   variables for files being loaded and files being compiled, and use
   them to maintain a stack so we can see the nesting of loads within
   files.''

 Pitman ultimately chose to present the first rather than the second
 because it seemed simpler, easier to explain, and more likely to
 pass at this late date.

 Other suggestions which were considered discarded were:
  a. Provide just variables *LOAD-STREAM* and *COMPILE-FILE-STREAM*.
     Then PATHNAME and TRUENAME could be used to yield the 
     information contained in the -PATHNAME* and -TRUENAME* variables
     of the proposal above.
  b. Like (a), but call both variables *STANDARD-INPUT*. That is,
     say that LOAD and COMPILE-FILE bind *STANDARD-INPUT* to the
     stream being loaded.
 There were a number of pitfalls with this approach which all center
 around the way it invites the user to do other operations besides
 PATHNAME and TRUENAME.  Not only would some people be confused by
 the difference between the characteristics of *LOAD-STREAM* for
 compiled and interpreted files, but also even with interpreted 
 streams, the actual position of the stream pointer at the time of
 execution of the forms contained in the file could vary between
 implementations in a way that became a lurking portability barrier.
 Since the observed user need which spawned this discussion was for
 a way to tell what file was being loaded and not for a way to 
 manipulate the stream, it seemed best to just go with the variables
 that addressed that specific need--fewer pitfalls and more perspicuous
 code are likely to result.