Issue LOAD-TRUENAME Writeup

Status: Passed, Jun 89 X3J13


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.