Common Lisp the Language, 2nd Edition

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23.1.5.5. Discussion of Logical Pathnames

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.

Logical pathname syntax was chosen to be easily translated into the formats of 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, logical pathnames can be mapped onto a less capable real file system by translating each directory that is used into a flat directory name, processing wildcards in the Lisp implementation rather than in the file system, treating all versions as :newest, and 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. (If logical pathnames were case-sensitive, it would be very difficult to map them into a file system that is not sensitive to case in its file names.)

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 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 the operating environments.

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 to make possible a portable system construction tool. Suppose that it is desirable to call compile-file only if the source file is newer than the compiled file. For this to succeed, it must be possible to know the name of the compiled file without actually calling compile-file. In some implementations the compiler produces one of several file types, depending on a variety of implementation-dependent circumstances, so it is not sufficient simply to prescribe a standard logical file type for compiled files; compile-file-pathname provides access to the defaulting that is performed by compile-file ``in a manner appropriate to the implementation’s file system conventions.’’

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.

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 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 and executed a suitable setf form for setting the logical-pathname-translations of the relevant logical host, the software can be loaded and run. 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; 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 is separate and is written by the user, not by the software supplier. That separation and a uniform convention for doing 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.

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