CLHS: Issue PATHNAME-LOGICAL Writeup

#

## Issue PATHNAME-LOGICAL Writeup

    Status:	Passed, Jun 89 X3J13
    Issue:          PATHNAME-LOGICAL
    Forum:          Cleanup
    References:     Pathnames (pp410-413)
                    OPEN (p.418), WITH-OPEN-FILE (p.422), RENAME-FILE (p.423),
                    DELETE-FILE (p.424), PROBE-FILE (p.424),
                    FILE-WRITE-DATE (p.424), FILE-AUTHOR (p.424), LOAD (p.426),
                    COMPILE-FILE (p.439), DIRECTORY (p.427), PATHNAME (p.413),
                    TRUENAME (p.413), MERGE-PATHNAMES (p.415),
                    MAKE-PATHNAME (p.416), and PARSE-NAMESTRING (p.414).
    Related issues: PATHNAME-CANONICAL-TYPE, PATHNAME-COMPONENT-VALUES,
                    PATHNAME-SUBDIRECTORY-LIST, and PATHNAME-WILD
    Category:       ADDITION
    Edit history:   Version 1, 11-May-89, by Moon
                    Version 2, 18-May-89, by Moon
                    Version 3, 21-Jun-89, by Moon (revise based on discussion
                            in the cleanup committee)
                    Version 4, 23-Jun-89, by Moon (remove backtranslation)

    Problem description:

      Pathname values are not portable, but they are sometimes part of a
      program, for example the names of files containing the program and the
      data used by the program.  Moving large programs between sites would
      be easier if pathname values did not have to be translated.

      Pathname values are nonportable because not all Common Lisp
      implementations use the same operating system and file name syntax varies
      widely among operating systems.  In addition, corresponding files at two
      different sites may have different names even when the operating system
      is the same; for example, they may be on different directories or
      different devices.

      The issue of portable pathname values is separate from the issues of
      portable pathname operations.  See the related issues listed above.
      For inter-issue interactions, see the discussion section below.

      Note that issue PATHNAME-LOGICAL fundamentally depends on issue
      PATHNAME-WILD.  If PATHNAME-WILD:NEW-FUNCTIONS does not pass,
      PATHNAME-LOGICAL cannot pass.

    Proposal (PATHNAME-LOGICAL:ADD):

      1. Define a "logical" file system that looks the same at every site.
      This file system is implemented by translating each logical pathname into
      a physical pathname on a real file system.  The logical pathnames are the
      same at all sites, but the translations are different at each site, thus
      the physical pathnames can be different at each site.

      2a. The syntax of a logical pathname namestring is as follows:

      [ host ":" ] [ ";" ] { directory ";" }* [ name ] [ "." type [ "." version ]]

      2b. Terminology:

      A <word> consists of one or more uppercase letters, digits, and hyphens.

      A <wildcard word> consists of one or more asterisks, uppercase letters,
      digits, and hyphens, including at least one asterisk, with no two
      asterisks adjacent.  Each asterisk matches a sequence of zero or more
      characters.  The <wildcard word> "*" parses into :WILD, the others parse
      into strings.

      In <words> and <wildcard words> lowercase letters are translated to
      uppercase.  The consequences of using other characters are unspecified.

      2c. Logical pathname components:

      The host is a <word> that has been defined as a logical pathname host by
      using SETF of LOGICAL-PATHNAME-TRANSLATIONS.

      There is no device, so the device component of a logical pathname is
      always :UNSPECIFIC.  No other component can be :UNSPECIFIC.

      Each directory is a <word>, a <wildcard word>, or "**" (:WILD-INFERIORS).
      If a semicolon precedes the directories, the directory component is
      relative, otherwise it is absolute.

      The name is a <word> or a <wildcard word>.

      The type is a <word> or a <wildcard word>.

      The version is a positive decimal integer or "NEWEST" (:NEWEST) or "*"
      (:WILD).  The letters in "NEWEST" can be in either alphabetic case.

      The consequences of using any value not specified here as a logical
      pathname component are unspecified.

      The null string "" is not a valid value for any component of a logical
      pathname, since "" is not a <word> and not a <wildcard word>.

      3. Parsing of logical pathname namestrings into logical pathnames
      operates as follows:

      3a. Logical pathname namestrings are recognized by the LOGICAL-PATHNAME
      and TRANSLATE-LOGICAL-PATHNAME functions.  In this case the host portion
      of the logical pathname namestring and its following colon are required.

      3b. The PARSE-NAMESTRING function recognizes a logical pathname
      namestring when the host argument is logical or the defaults argument is
      a logical pathname.  In this case the host portion of the logical
      pathname namestring and its following colon are optional.  If the host
      portion of the namestring and the host argument are both present and do
      not match, an error is signalled.

      The host argument is logical if it is supplied and came from
      PATHNAME-HOST of a logical pathname.  Whether a host argument is logical
      if it is a string equal to a logical pathname host name is
      implementation-defined.

      3c. The MERGE-PATHNAMES function recognizes a logical pathname namestring
      when the defaults argument is a logical pathname.  In this case the host
      portion of the logical pathname namestring and its following colon are
      optional.

      3d. Whether the other functions that coerce strings to pathnames
      (PATHNAME, TRUENAME, PARSE-NAMESTRING in other circumstances than those
      described in point 3b, MERGE-PATHNAMES in other circumstances than those
      described in point 3c, the :DEFAULTS argument to MAKE-PATHNAME,
      PATHNAME-HOST, PATHNAME-DEVICE, PATHNAME-DIRECTORY, PATHNAME-NAME,
      PATHNAME-TYPE, PATHNAME-VERSION, NAMESTRING, FILE-NAMESTRING,
      DIRECTORY-NAMESTRING, HOST-NAMESTRING, ENOUGH-NAMESTRING, OPEN,
      WITH-OPEN-FILE, RENAME-FILE, DELETE-FILE, PROBE-FILE, FILE-WRITE-DATE,
      FILE-AUTHOR, LOAD, DIRECTORY, COMPILE-FILE, ED, DRIBBLE, WILD-PATHNAME-P,
      PATHNAME-MATCH-P, TRANSLATE-PATHNAME, and COMPILE-FILE-PATHNAME)
      recognize logical pathname namestrings is implementation defined.

      4. Some real file systems do not have versions.  Logical pathname
      translation to such a file system ignores the version.  This implies that
      a program cannot rely on being able to store more than one version of a
      file named by a logical pathname.

      5. The type of a logical pathname for a Common Lisp source file is "LISP".
      This should be translated into whatever type is appropriate in a physical
      pathname.

      6. The logical pathname host name "SYS" is reserved for the implementation.
      The existence and meaning of SYS: logical pathnames is
      implementation-defined.

      7. File manipulation functions operate with logical pathnames as follows:

      7a. The functions OPEN (and WITH-OPEN-FILE), RENAME-FILE, DELETE-FILE,
      PROBE-FILE, FILE-WRITE-DATE, FILE-AUTHOR, LOAD, DIRECTORY, COMPILE-FILE,
      ED, DRIBBLE, COMPILE-FILE-PATHNAME, and TRUENAME accept logical pathnames
      and translate them into physical pathnames, as if by calling the
      TRANSLATE-LOGICAL-PATHNAME function.

      7b. PATHNAME of a stream created by OPEN (or WITH-OPEN-FILE) of a logical
      pathname is a logical pathname.

      7c. TRUENAME, PROBE-FILE, and DIRECTORY never return logical pathnames.

      7d. RENAME-FILE with a logical pathname as the second argument returns a
      logical pathname as the first value.

      7e. MERGE-PATHNAMES returns a logical pathname if and only if its first
      argument is a logical pathname or its first argument does not specify a
      host and the default is a logical pathname.

      7f. MAKE-PATHNAME returns a logical pathname if and only if the host is
      logical.  If the :host argument to MAKE-PATHNAME is supplied, the host is
      logical if it came from PATHNAME-HOST of a logical pathname.  Whether a
      :host argument is logical if it is a string equal to a logical pathname
      host name is implementation-defined.

      7g. PARSE-NAMESTRING returns a logical pathname according to points 3b
      and 3d.

      Add these defined names to Common Lisp in support of logical pathnames:

      8. LOGICAL-PATHNAME                                            [Class]

        LOGICAL-PATHNAME is a subclass of PATHNAME.

      9. LOGICAL-PATHNAME pathname                                   [Function]

        Converts the argument to a logical pathname and returns it.  The
        argument can be a logical pathname, a logical pathname namestring
        containing a host component, or a stream for which the PATHNAME
        function returns a logical pathname.  For any other argument,
        LOGICAL-PATHNAME signals an error of type TYPE-ERROR.

      10. TRANSLATE-LOGICAL-PATHNAME pathname &key                   [Function]

        Translates a logical pathname to the corresponding physical pathname.
        The pathname argument is first coerced to a pathname.  If it is not a
        pathname, string, or file stream an error of type TYPE-ERROR is
        signalled.

        If the coerced argument is a physical pathname, it is returned.

        If the coerced argument is a logical pathname, the first matching
        translation (according to PATHNAME-MATCH-P) of the logical pathname
        host is applied, as if by calling TRANSLATE-PATHNAME.  If the result is
        a logical pathname, this process is repeated.  When the result is
        finally a physical pathname, it is returned.

        If no translation matches, an error of type FILE-ERROR is signalled.

        TRANSLATE-LOGICAL-PATHNAME might perform additional translations,
        typically to provide translation of file types to local naming
        conventions, to accomodate physical file systems with limited length
        names, or to deal with special character requirements such as
        translating hyphens to underscores or uppercase letters to lowercase.
        Any such additional translations are implementation defined.  Some
        implementations do no additional translations.

        There are no specified keyword arguments for
        TRANSLATE-LOGICAL-PATHNAME, but implementations are permitted to extend
        it by adding keyword arguments.  There is one specified return value
        from TRANSLATE-LOGICAL-PATHNAME; implementations are permitted to
        extend it by returning additional values.

      11. LOGICAL-PATHNAME-TRANSLATIONS host                         [Function]

        If <host> is not the host component of a logical pathname and not a
        string that has been defined as a logical pathname host name by SETF of
        LOGICAL-PATHNAME-TRANSLATIONS, signals an error of type TYPE-ERROR.
        Otherwise returns the host's list of translations.  Each translation is
        a list of at least two elements: from-wildcard and to-wildcard.  Any
        additional elements are implementation defined.  From-wildcard is a
        logical pathname whose host is <host>.  To-wildcard is a pathname.
        Translations are searched in the order listed, so more specific
        from-wildcards must precede more general ones.

        (SETF (LOGICAL-PATHNAME-TRANSLATIONS host) translations) sets a logical
        pathname host's list of translations.  If <host> is a string that has
        not been previously used as logical pathname host, a new logical
        pathname host is defined, otherwise an existing host's translations are
        replaced.  Logical pathname host names are compared with STRING-EQUAL.

        When setting the translations list, each from-wildcard can be a logical
        pathname whose host is <host> or a logical pathname namestring
        parseable by (PARSE-NAMESTRING string <<host>>), where <<host>>
        represents the appropriate object as defined in point 3b.  Each
        to-wildcard can be anything coercible to a pathname by
        (PATHNAME to-wildcard).  If to-wildcard coerces to a logical pathname,
        TRANSLATE-LOGICAL-PATHNAME will perform repeated translation steps when
        it uses it.

        Implementations can define additional functions that operate on
        logical pathname hosts, for example to specify additional translation
        rules or options.

      12. LOAD-LOGICAL-PATHNAME-TRANSLATIONS host                    [Function]

        If a logical pathname host named <host> (a string) is already defined,
        return NIL.  Otherwise, search for a logical pathname host definition
        in an implementation defined manner.  If none is found, signal an
        error.  If a definition is found, install it and return T.

        The search used by LOAD-LOGICAL-PATHNAME-TRANSLATIONS should be
        documented, as logical pathname definitions will be created by users,
        not only by Lisp implementors.  A typical search technique is to
        look in a certain directory for a file whose name is derived from
        the host name in an implementation-defined fashion.

      13. COMPILE-FILE-PATHNAME pathname &key :output-file           [Function]

        Returns the pathname that COMPILE-FILE would write into, if given the
        same arguments.  If the pathname argument is a logical pathname and the
        :output-file argument is unspecified, the result is a logical pathname.
        If an implementation supports additional keyword arguments to
        COMPILE-FILE, COMPILE-FILE-PATHNAME must accept the same arguments.

    Examples:

      ;A very simple example of setting up a logical pathname host.  No
      ;translations are necessary to get around file system restrictions, so
      ;all that is necessary is to specify the root of the physical directory
      ;tree that contains the logical file system.
      ;The namestring syntax on the right-hand side is implementation-specific.
      (setf (logical-pathname-translations "foo")
            '(("**;*.*.*"              "MY-LISPM:>library>foo>**>")))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "foo:bar;baz;mum.quux.3")
      => MY-LISPM:>library>foo>bar>baz>mum.quux.3

      ;A more complex example, dividing the files among two file servers
      ;and several different directories.  This Unix doesn't support
      ;:WILD-INFERIORS in the directory, so each directory level must
      ;be translated individually.  No file name or type translations
      ;are required except for .MAIL to .MBX.
      ;The namestring syntax on the right-hand side is implementation-specific.
      (setf (logical-pathname-translations "prog")
            '(("RELEASED;*.*.*"        "MY-UNIX:/sys/bin/my-prog/")
              ("RELEASED;*;*.*.*"      "MY-UNIX:/sys/bin/my-prog/*/")
              ("EXPERIMENTAL;*.*.*"    "MY-UNIX:/usr/Joe/development/prog/")
              ("EXPERIMENTAL;DOCUMENTATION;*.*.*"
                                       "MY-VAX:SYS$DISK:[JOE.DOC]")
              ("EXPERIMENTAL;*;*.*.*"  "MY-UNIX:/usr/Joe/development/prog/*/")
              ("MAIL;**;*.MAIL"        "MY-VAX:SYS$DISK:[JOE.MAIL.PROG...]*.MBX")))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "prog:mail;save;ideas.mail.3")
      => MY-VAX:SYS$DISK:[JOE.MAIL.PROG.SAVE]IDEAS.MBX.3

      ;Example translations for a program that uses three files main.lisp,
      ;auxiliary.lisp, and documentation.lisp.  These translations might be
      ;supplied by a software supplier as examples.

      ;For Unix with long file names
      (setf (logical-pathname-translations "prog")
            '(("CODE;*.*.*"             "/lib/prog/")))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "prog:code;documentation.lisp")
      => /lib/prog/documentation.lisp

      ;For Unix with 14-character file names, using .lisp as the type
      (setf (logical-pathname-translations "prog")
            '(("CODE;DOCUMENTATION.*.*" "/lib/prog/docum.*")
              ("CODE;*.*.*"             "/lib/prog/")))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "prog:code;documentation.lisp")
      => /lib/prog/docum.lisp

      ;For Unix with 14-character file names, using .l as the type
      ;The second translation shortens the compiled file type to .b
      (setf (logical-pathname-translations "prog")
            `(("**;*.LISP.*"            ,(logical-pathname "PROG:**;*.L.*"))
              (,(compile-file-pathname (logical-pathname "PROG:**;*.LISP.*"))
                                        ,(logical-pathname "PROG:**;*.B.*"))
              ("CODE;DOCUMENTATION.*.*" "/lib/prog/documentatio.*")
              ("CODE;*.*.*"             "/lib/prog/")))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "prog:code;documentation.lisp")
      => /lib/prog/documentatio.l

      ;For a Cray with 6 character names and no directories, types, or versions.
      (setf (logical-pathname-translations "prog")
            (let ((l '(("MAIN" "PGMN")
                       ("AUXILIARY" "PGAUX")
                       ("DOCUMENTATION" "PGDOC")))
                  (logpath (logical-pathname "prog:code;"))
                  (phypath (pathname "XXX")))
              (append
                ;; Translations for source files
                (mapcar #'(lambda (x)
                            (let ((log (first x))
                                  (phy (second x)))
                              (list (make-pathname :name log
                                                   :type "LISP"
                                                   :version :wild
                                                   :defaults logpath)
                                    (make-pathname :name phy
                                                   :defaults phypath))))
                        l)
                ;; Translations for compiled files
                (mapcar #'(lambda (x)
                            (let* ((log (first x))
                                   (phy (second x))
                                   (com (compile-file-pathname
                                          (make-pathname :name log
                                                         :type "LISP"
                                                         :version :wild
                                                         :defaults logpath))))
                              (setq phy (concatenate 'string phy "B"))
                              (list com
                                    (make-pathname :name phy
                                                   :defaults phypath))))
                        l))))

      ;Sample use of that logical pathname.  All return values
      ;are of course implementation-specific.
      (translate-logical-pathname "prog:code;documentation.lisp")
      => PGDOC

    Rationale:

      1. Large programs can be moved between sites without changing any
      pathnames, provided all pathnames used are logical.  A portable system
      construction tool can be created that operates on programs defined as
      sets of files named by logical pathnames.

      2. Logical pathname syntax was chosen to be easily translated into most
      popular file systems, while still being powerful enough for storing large
      programs.  Although they have hierarchical directories, extended wildcard
      matching, versions, and no limit on the length of names, they can be
      mapped onto a less capable real file file system by translating each
      directory that is used into a flat directory name, doing wildcards in
      Lisp rather than in the file system, treating all versions as :newest,
      and/or using translations to shorten long names.

      Logical pathname words are restricted to non-case-sensitive letters,
      digits, and hyphens to avoid creating problems with real file systems
      that support limited character sets for file naming.  Other characters
      could have been mapped onto such file systems through translations, but
      that didn't seem worth the trouble.  Logical pathnames have to be
      non-case-sensitive or it would be very difficult to map them onto a
      non-case-sensitive file system.

      Features such as :UP and :BACK relative directories and a namestring
      syntax for the root directory were not felt to be necessary in logical
      pathnames.  They could be added later if a need emerges.

      It is not a goal of logical pathnames to be able to represent all
      possible file names.  Their goal is rather to represent just enough file
      names to be useful for storing software.  Real pathnames, in contrast,
      need to provide a uniform interface to all possible file names, including
      names and naming conventions that are not under the control of Common
      Lisp.

      The choice of logical pathname syntax, using colon, semicolon, and
      period, was guided by the goals of being visually distinct from real file
      systems and minimizing the use of special characters.

      The consequences of using any value not specified here as a logical
      pathname component are unspecified, for the benefit of the Explorer.

      3. The LOGICAL-PATHNAME function is separate from the PATHNAME function
      so that the syntax of logical pathname namestrings does not constrain the
      syntax of physical pathname namestrings in any way.  Logical pathname
      syntax must be defined by Common Lisp so that logical pathnames can be
      conveniently exchanged between implementations, but physical pathname
      syntax is dictated by forces outside our control.

      3b,c. Allowing PARSE-NAMESTRING and MERGE-PATHNAMES to recognize logical
      pathname namestrings in these situations provides for natural operations
      on logical pathnames.  Frequently a string containing just a name, or a
      name and a type, will be recognized as a logical pathname by merging it
      against a default containing a logical pathname host and directory.

      3d. Recognition of logical pathname namestrings by PATHNAME and related
      functions is left up to each implementation because some implementations
      definitely require this, other implementations don't want to do this, and
      nobody wants to change.  In any case, Common Lisp historically has avoided
      saying anything about the syntax of the strings accepted by the PATHNAME
      function, and point 3d preserves that position.

      3b,7f. Leaving it implementation defined whether a string, used as the
      host argument to PARSE-NAMESTRING or the :host argument to MAKE-PATHNAME,
      can be recognized as logical pathname host name is for the same reason as
      point 3d.  It allows each implementation to decide whether there is one
      namespace or two.  The correct way to write this is:

        (MAKE-PATHNAME :HOST (PATHNAME-HOST (LOGICAL-PATHNAME "hostname:"))
                       ...)

      4. Logical pathname versions could have been supported on real file
      systems that do not have versions by defining a kind of translation to
      encode the version number in the name.  However, the typical use of
      versions is such that on a file system without versions, people would
      rather just store one version of a file, and not preserve the version
      information by encoding it somehow in the name.  This is different from
      the typical use of types or directories, where the files with different
      values in those components are truly distinct and everything would break
      if you only kept one file.

      5,13. The COMPILE-FILE-PATHNAME function and the specification of "LISP"
      as the type of a logical pathname for a Common Lisp source file together
      provide enough information about compilation for a portable system
      construction tool that uses logical pathnames to work.  Suppose you want
      to call COMPILE-FILE only if the source file is newer than the compiled
      file.  To do that, you have to have a way to know the name of the
      compiled file without actually calling COMPILE-FILE.
      No standard file type for compiler output is proposed, because in some
      implementations the compiler produces one of several file types,
      depending on a variety of implementation-dependent circumstances.
      COMPILE-FILE-PATHNAME provides access to the "default[ing] in a manner
      appropriate to the implementation's file system conventions" mentioned in
      the CLtL documentation of COMPILE-FILE.

      6. The use of the logical pathname host name "SYS" for the implementation
      is current practice.  Standardizing on this name helps users choose
      logical pathname host names that avoid conflicting with
      implementation-defined names.

      7. Accepting logical pathnames for file access is a natural extension
      of the file access functions and makes it easier to program using only
      logical pathnames in situations where that is appropriate.

      8. The LOGICAL-PATHNAME class exists so that methods can distinguish
      logical pathnames from regular pathnames.

      9. See point 3 above.

      10. TRANSLATE-LOGICAL-PATHNAME is the heart of the logical pathname
      feature.  Allowing TRANSLATE-LOGICAL-PATHNAME on a physical pathname,
      simply returning the argument, makes some programs easier to write.
      Additional implementation defined translations make it possible for
      implementations with unusual file systems to offer some help to the user
      in setting up the translations for a logical pathname host, by handling
      some of the work automatically.  Logical pathnames that translate to
      other logical pathnames are a feature that several people have requested.

      11. SETF of LOGICAL-PATHNAME-TRANSLATIONS is a simple way for a user to
      define a new logical pathname host.  Using SETF makes it possible to add
      to or modify the translations of an existing logical pathname host.

      It is always up to the person who writes the translation rules for a
      particular logical pathname host to a particular physical file system to
      make sure that the logical pathnames that are actually going to be used
      translate to valid pathnames for the particular file system, and that
      no two logical pathnames that are supposed to be distinct translate to
      the same physical pathname.

      12. Loading of logical pathname translations from a site-dependent file
      allows software to be distributed using logical pathnames.  The assumed
      model of software distribution is a division of labor between the
      supplier of the software and the user installing it.  The supplier
      chooses logical pathnames to name all the files used or created by the
      software, and supplies examples of logical pathname translations for a
      few popular file systems.  Each example uses an assumed directory and/or
      device name, assumes local file naming conventions, and provides
      translations that will translate all the logical pathnames used or
      generated by the particular software into valid physical pathnames.
      For a powerful file system these translations can be quite simple.  For
      a more restricted file system, it may be necessary to list an explicit
      translation for every logical pathname used, for example when dealing
      with restrictions on the maximum length of a file name.

      The user installing the software decides on which device and/or directory
      to store the files and edits the example logical pathname translations
      accordingly.  If necessary, the user also adjusts the translations for
      local file naming conventions and any other special aspects of the user's
      local file system policy and local Common Lisp implementation.  For
      example, the files might be divided among several file server hosts to
      share the load.  The process of defining site-customized logical pathname
      translations is quite easy for a user of a popular file system for which
      the software supplier has provided an example.  A user of a more unusual
      file system might have to take more time; the supplier can help by
      providing a list of all the logical pathnames used or generated by the
      software.

      Once the user has created a suitable SETF of LOGICAL-PATHNAME-TRANSLATIONS
      form, he can evaluate that form and then load and run the software.  It
      may be necessary to use the translations again, or on another workstation
      at the same site, so it is best to save the SETF form in the standard
      place where it can be found later by LOAD-LOGICAL-PATHNAME-TRANSLATIONS.
      Often a software supplier will include a program for restoring software
      from the distribution medium to the file system, and a program for loading
      the software from the file system into a Common Lisp, and these programs
      will start by calling LOAD-LOGICAL-PATHNAME-TRANSLATIONS to make sure that
      the logical pathname host is defined.

      Note that the SETF of LOGICAL-PATHNAME-TRANSLATIONS form isn't part of
      the program, it's separate.  It's written by the user, not by the
      software supplier.  That separation, and a uniform convention for how to
      do the separation, are the key aspects of logical pathnames.  For small
      programs involving only a handful of files, it doesn't matter much.  The
      real benefits come with large programs with hundreds or thousands of
      files and more complicated situations such as program-generated file
      names or porting a program developed on a system with long file names
      onto a system with a very restrictive limit on the length of file names.

    Current practice:

      Symbolics Genera has had a similar facility for many years.  It is used
      extensively for software distribution by Symbolics and its customers.
      The Genera facility uses the same logical pathname syntax but different
      function names, and is somewhat more complicated.  The extra complexity
      is not necessary in the Common Lisp standard.

      The T.I. Explorer also has a comparable logical pathname facility,
      although the translation mechanism is unfortunately less general than
      proposed here.  The namestring syntax used is slightly different:

        host ":" [{directory "."}* directory ";"] [name] ["." type] ["#" version]

      The newest version is indicated by ">" instead of "newest".

      Macintosh Allegro Common Lisp) has a logical pathname feature which is
      somewhat simpler but aimed at solving the same problems.  It has logical
      directory names, to simplify access to sets of files in differently named
      directories (an especially severe problem on micros where everybody just
      has to have a different pet name for their hard disk).  This isn't really
      the same as simplifying access to different file systems, although of
      course solving the latter automatically solves the former.  In general,
      access to different file systems requires translating names and types,
      not just translating directories.

      Symbolics Genera offers a function for translating from a physical
      pathname back to a logical pathname.  There are a number of problems with
      this, and so it has not been proposed here.  An earlier version specified
      TRANSLATE-LOGICAL-PATHNAME to return enough information to allow the user
      program to perform the backtranslation itself, but that hsd problems
      so it was removed.

      The Genera equivalent of LOAD-LOGICAL-PATHNAME-TRANSLATIONS looks for
      a file named SYS:SITE;hostname.TRANSLATIONS.

      Current practice in Genera, Explorer, and Macintosh has one namespace for
      both logical and physical namestrings.  This proposal allows an
      implementation to choose to have one namespace or to have two separate
      namespaces for namestrings.

    Cost to Implementors:

      This is a fairly complex facility, but its performance is unimportant
      so a straightforward implementation should suffice.  Most of the
      complexity comes in dealing with unusual file systems, such as ones
      that don't allow long file names.

    Cost to Users:

      None.

    Cost of non-adoption:

      Portable software construction and distribution will have to rely on
      implementation-dependent kludges.  Lisp software will continue to be
      difficult to install.

    Performance impact:

      None.

    Benefits:

      Avoid cost of non-adoption.

    Esthetics:

      Improved portability of large programs.

    Discussion:

      Issue PATHNAME-LOGICAL fundamentally depends on issue PATHNAME-WILD.  If
      PATHNAME-WILD:NEW-FUNCTIONS does not pass, PATHNAME-LOGICAL cannot pass.

      If PATHNAME-CANONICAL-TYPE:NEW-CONCEPT passes, it will affect the
      behavior of the function TRANSLATE-PATHNAME and therefore the behavior of
      the function TRANSLATE-LOGICAL-PATHNAME.  When a logical pathname
      translation has from-wildcard and to-wildcard type components that are
      :WILD or omitted, translation of the type will be guided by canonical
      types.  If PATHNAME-CANONICAL-TYPE:NEW-CONCEPT fails to pass, it will
      either have to be done behind the scenes by TRANSLATE-PATHNAME or users
      will have to write more verbose translations that individually specify
      the handling of each file type (as shown in some of the examples here).

Copyright 1996-2001, Xanalys Inc. All rights reserved.