CLHS: Issue PATHNAME-SUBDIRECTORY-LIST Writeup

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## Issue PATHNAME-SUBDIRECTORY-LIST Writeup

    Status: passed, as amended, Jun 89 X3J13
    Issue:          PATHNAME-SUBDIRECTORY-LIST
    References:     Pathnames (pp410-413), MAKE-PATHNAME (p416),
                    PATHNAME-DIRECTORY (p417)
    Related-issues: PATHNAME-COMPONENT-CASE, PATHNAME-COMPONENT-VALUE
    Category:       CHANGE
    Edit history:   18-Jun-87, Version 1 by Ghenis.pasa@Xerox.COM
                    05-Jul-88, Version 2 by Pitman (major revision)
                    28-Dec-88, Version 3 by Pitman (merge discussion)
                    22-Mar-89, Version 4 by Moon (fix based on discussion)
                    19-May-89, Version 5 by Moon (improve based on mail)
                    21-May-89, Version 6 by Moon (final cleanups)
                    17-Jun-89, Version 7 by Moon (add current practice
                            and discussion; minor fixes based on discussion)
                    2-Jul-89, Version 8 by Masinter (add Jun89X3J13 amendment)

    Problem Description:

      It is impossible to write portable code that can produce a pathname
      in a subdirectory of a hierarchical file system. This defeats much of
      the purpose of the pathname abstraction.

      According to CLtL, only a string is a portable value for the directory
      component of a pathname.  Thus in order to denote a subdirectory, the use
      of punctuation characters (such as dots, slashes, or backslashes) would
      be necessary. The very fact that such syntax varies from host to host
      means that although the representation might be "portable", the code
      using that representation is not portable.

      This problem is even worse for programs running on machines on a network
      that can retrieve files from multiple hosts, each using a different OS
      and thus different subdirectory punctuation.

      Related problems:

      - In some implementations "FOO.BAR" might denote the "BAR" subdirectory
        of "FOO", while in other implementations it would denote a top-level
        directory, because "." is not treated as punctuation. To be safe,
        portable programs must avoid all potential punctuation characters.

      - Even in implementations where "." is used for subdirectories,
        "FOO.BAR" may be recognized by some to mean the "BAR" subdirectory of
        "FOO" and by others to mean `a seven character directory name with "."
        as the fourth character.'

      - In fact, CLtL does not even say whether punctuation characters are
        part of the string. eg, is "foo" or "/foo" the directory component for
        a unix pathname "/foo/bar.lisp". Similarly, is "[FOO]" or "FOO" the
        directory component for a VMS pathname "[FOO]ME.LSP"?
        PATHNAME-COMPONENT-VALUE:SPECIFY says punctuation characters are not
        part of the string.

    Proposal (PATHNAME-SUBDIRECTORY-LIST:NEW-REPRESENTATION)

      Remove the "structured" directory feature mentioned on CLtL p.412.

      Allow the value of a pathname's directory component to be a list.  The
      car of the list is one of the symbols :ABSOLUTE or :RELATIVE.
      Each remaining element of the list is a string or a symbol (see below).
      Each string names a single level of directory structure.  The strings
      should contain only the directory names themselves -- no punctuation
      characters.

      A list whose car is the symbol :ABSOLUTE represents a directory path
      starting from the root directory.  The list (:ABSOLUTE) represents
      the root directory.  The list (:ABSOLUTE "foo" "bar" "baz") represents
      the directory called "/foo/bar/baz" in Unix [except possibly for
      alphabetic case -- that is the subject of a separate issue].

      A list whose car is the symbol :RELATIVE represents a directory path
      starting from a default directory.  The list (:RELATIVE) has the same
      meaning as NIL and hence is not used.  The list (:RELATIVE "foo" "bar")
      represents the directory named "bar" in the directory named "foo" in the
      default directory.

      In place of a string, at any point in the list, symbols may occur to
      indicate special file notations. The following symbols have standard
      meanings.  Implementations are permitted to add additional objects of any
      non-string type if necessary to represent features of their file systems
      that cannot be represented with the standard strings and symbols.
      Supplying any non-string, including any of the symbols listed below, to a
      file system for which it does not make sense signals an error of type
      FILE-ERROR.  For example, Unix does not support :WILD-INFERIORS in
      most implementations.

       :WILD           - Wildcard match of one level of directory structure.
       :WILD-INFERIORS - Wildcard match of any number of directory levels.
       :UP             - Go upward in directory structure (semantic).
       :BACK           - Go upward in directory structure (syntactic).

      :ABSOLUTE or :WILD-INFERIORS immediately followed by :UP or :BACK
      signals an error.

      "Syntactic" means that the action of :BACK depends only on the pathname
      and not on the contents of the file system.  "Semantic" means that the
      action of :UP depends on the contents of the file system; to resolve
      a pathname containing :UP to a pathname whose directory component
      contains only :ABSOLUTE and strings requires probing the file system.
      :UP differs from :BACK only in file systems that support multiple
      names for directories, perhaps via symbolic links.  For example,
      suppose that there is a directory
        (:ABSOLUTE "X" "Y" "Z")
      linked to
        (:ABSOLUTE "A" "B" "C")
      and there also exist directories
        (:ABSOLUTE "A" "B" "Q")
        (:ABSOLUTE "X" "Y" "Q")
      then
        (:ABSOLUTE "X" "Y" "Z" :UP "Q")
      designates
        (:ABSOLUTE "A" "B" "Q")
      while
        (:ABSOLUTE "X" "Y" "Z" :BACK "Q")
      designates
        (:ABSOLUTE "X" "Y" "Q")

      If a string is used as the value of the :DIRECTORY argument to
      MAKE-PATHNAME, it should be the name of a toplevel directory and should
      not contain any punctuation characters.  Specifying a string, str, is
      equivalent to specifying the list (:ABSOLUTE str).  Specifying the symbol
      :WILD is equivalent to specifying the list (:ABSOLUTE :WILD-INFERIORS),
      or (:ABSOLUTE :WILD) in a file system that does not support :WILD-INFERIORS.

      The PATHNAME-DIRECTORY function always returns NIL, :UNSPECIFIC, or a
      list, never a string, never :WILD.

      The list returned is not guaranteed to be "freshly consed" -- the
      consequences of modifying this list is undefined.

      In non-hierarchical file systems, the only valid list values for the
      directory component of a pathname are (:ABSOLUTE string) and
      (:ABSOLUTE :WILD).  :RELATIVE directories and the keywords
      :WILD-INFERIORS, :UP, and :BACK are not used in non-hierarchical file
      systems.

      Pathname merging treats a relative directory specially.  Let
      <pathname> and <defaults> be the first two arguments to
      MERGE-PATHNAMES.  If (PATHNAME-DIRECTORY <pathname>) is a list whose
      car is :RELATIVE, and (PATHNAME-DIRECTORY <defaults>) is a list, then
      the merged directory is the value of
        (APPEND (PATHNAME-DIRECTORY <defaults>)
                (CDR  ;remove :RELATIVE from the front
                  (PATHNAME-DIRECTORY <pathname>)))
      except that if the resulting list contains a string or :WILD immediately
      followed by :BACK, both of them are removed.  This removal of redundant
      :BACKs is repeated as many times as possible.
      If (PATHNAME-DIRECTORY <defaults>) is not a list or
      (PATHNAME-DIRECTORY <pathname>) is not a list whose car is :RELATIVE, the
      merged directory is
        (OR (PATHNAME-DIRECTORY <pathname>) (PATHNAME-DIRECTORY <defaults>))

      A relative directory in the pathname argument to a function such as
      OPEN is merged with *DEFAULT-PATHNAME-DEFAULTS* before accessing the
      file system.

    Test Cases/Examples:

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING "[FOO.BAR]BAZ.LSP")) ;on VMS
      => (:ABSOLUTE "FOO" "BAR")

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING "/foo/bar/baz.lisp")) ;on Unix
      => (:ABSOLUTE "foo" "bar")
      or (:ABSOLUTE "FOO" "BAR")
      If PATHNAME-COMPONENT-CASE:KEYWORD-ARGUMENT passes with a default of
      :COMMON, the value is the second one shown.

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING "../baz.lisp")) ;on Unix
      => (:RELATIVE :UP)

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING "/foo/bar/../mum/baz")) ;on Unix
      => (:ABSOLUTE "foo" "bar" :UP "mum")

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING ">foo>**>bar>baz.lisp")) ;on LispM
      => (:ABSOLUTE "FOO" :WILD-INFERIORS "BAR")

      (PATHNAME-DIRECTORY (PARSE-NAMESTRING ">foo>*>bar>baz.lisp")) ;on LispM
      => (:ABSOLUTE "FOO" :WILD "BAR")

    Rationale:

      This would allow programs to deal usefully with hierarchical file
      systems, which are by far the most common file system type.
      This would allow a system construction utility that organizes programs
      by subdirectories to be portable to all implementations that have
      hierarchical file systems.

      Discussion indicated that "Implementations are permitted to add
      additional objects of any non-string type if necessary to represent
      features of their file systems that cannot be represented with the
      standard strings and symbols" is a necessary escape hatch for things like
      home directories and fancy pattern matching.  Implementations should
      limit their use of this loophole and use the standard keyword symbols
      whenever that is possible.

    Current Practice:

      Symbolics Genera implements something very similar to this. The main
      differences are:
       - In Genera, there is no :ABSOLUTE keyword at the head of the list.
         This has been shown to cause some problems in dealing with root
         directories. Genera represents the root directory by a keyword
         symbol (rather than a list) because the list representation
         was not adequately general.
       - Genera has no separate concepts of :UP and :BACK.  Genera
         represents Unix ".." as :UP, but deals with :UP syntactically, not
         semantically.

      On the Explorer, the directory component is a list of strings, not yet
      supporting the symbols specified in proposal PATHNAME-SUBDIRECTORY-LIST.

      Macintosh Allegro Common Lisp 1.2.2 uses a string with punctuation
      characters instead of a list for the directory.

      Lucid Common Lisp 3.0.1 under Unix uses a list for directories of
      somewhat different form from what is proposed in
      PATHNAME-SUBDIRECTORY-LIST.  It uses :ROOT instead of :ABSOLUTE and uses
      ".." instead of :UP.  It does use :RELATIVE.

      Ibuki Common Lisp Release 01/01 uses a list for directories of somewhat
      different form from what is proposed in PATHNAME-SUBDIRECTORY-LIST.  It
      uses :ROOT instead of :ABSOLUTE, uses :PARENT instead of :UP, and omits
      the leading keyword instead of using :RELATIVE.

      IIM uses a list for directories of somewhat different form from what is
      proposed in PATHNAME-SUBDIRECTORY-LIST.  It uses :ABSOLUTE-DIRECTORY
      instead of :ABSOLUTE, uses :SUPER-DIRECTORY instead of :BACK, and omits
      the leading keyword instead of using :RELATIVE.

    Cost to Implementors:

      In principle, nothing about the implementation needs to change except
      the treatment of the directory component by MAKE-PATHNAME and
      PATHNAME-DIRECTORY. The internal representation can otherwise be left
      as-is if necessary.

      Implementations such as Genera, Explorer, Lucid, Ibuki, and IIM that
      already have hierarchical directory handling will have to make an
      incompatible change to switch to what is proposed here.

      For implementations that choose to rationalize this representation
      throughout their internals and any other implementation-specific
      accessors, the cost will be necessarily higher.

    Cost to Users:

      None for portable programs. This change is upward compatible with CLtL.
      Nonportable programs will have to be changed if they use implementation
      dependent hierarchical directory handling and the implementation
      removes support for that when it adds support for this proposal.

    Cost of Non-Adoption:

      Serious portability problems would continue to occur. Programmers would be
      driven to the use of implementation-specific facilities because the need
      for this is frequently impossible to ignore.

    Benefits:

      The serious costs of non-adoption would be avoided.

    Aesthetics:

      This representation of hierarchical pathnames is easy to use and quite
      general. Users will probably see this as an improvement in the aesthetics.

    Discussion:

      This issue was raised a while back but no one was fond of the particular
      proposal that was submitted. This is an attempt to revive the issue.

      The original proposal, to add a :SUBDIRECTORIES component to a
      pathname, was discarded because it imposed an unnatural distinction
      between a toplevel directory and its subdirectories. Pitman's guess is
      the the idea was to try to make it a compatible change, but since most
      programmers will probably want to change from implementation-specific
      primitives to portable ones anyway, that's probably not such a big
      deal. Also, there could have been some programs which thought the
      change was compatible and ended up ignoring important information (the
      :SUBDIRECTORIES component). Pitman thought it would be better if
      people just accepted the cost of an incompatible change in order to
      get something really pretty as a result.

      Some people feel it is unnecessary to standardize the format of
      pathname components such as the directory.

      Moon doesn't like having both :UP and :BACK, but admits that some
      file systems do it one way and some do it the other.  He still thinks
      it would be simpler if we got rid of :BACK and just had :UP.

      To keep it simple, we chose not to add to this issue the functions
      DIRECTORY-PATHNAME-AS-FILE and PATHNAME-AS-DIRECTORY, which convert
      the name of a directory from or to the directory component of a file
      inferior to that directory.  This conversion is system-dependent, for
      example TOPS-20 appends a type field and Unix does not.  Also in some
      systems the root directory has a name and in others it doesn't.  Of
      course these functions signal an error in non-hierarchical file
      systems.  Examples (for Unix, assuming #P print syntax for pathnames):
       (directory-pathname-as-file #P"/usr/bin/sh") => #P"/usr/bin"
       (pathname-as-directory #P"/usr/bin") => #P"/usr/bin"/
      These functions have not been proposed because they are mainly useful
      in conjunction with additional functions for manipulating directories
      (creating, expunging, setting access control) that have not been made
      available in Common Lisp.

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