Volume Number: 14 (1998)
Issue Number: 2
Column Tag: Tips & Tidbits

Tips & Tidbits

by Steve Sisak

Unfortunately, the Macintosh doesn't have a system-level mechanism for dealing with the notion of logical paths. Aliases don't work when files get moved, and using paths with device names doesn't work because everybody uses different device names (besides, device names aren't guaranteed to be unique).

As a kluge for MPW Shell, AppleScript and Macintosh Common Lisp, we create a folder in the system folder called "Logical Names" and place aliases to other files and folders in this folder. The name of each alias is then defined, in some application specific way (q.v, below), as a symbol bound to the file found by resolving the alias. This allows us to put aliases like "ProjFolder" or "MoreFilesFolder" in the logical names folder and have a chance of being able to use build scripts, Makefiles, etc. on other folks' machines without having to do a major re-write of the files in question.

Usage

To make use of any of the specific implementations described below, you need first create your logical names folder. This folder must be immediately in your system folder, and it must be named "Logical Names" (note the space).

Then, create some useful aliases in this folder. For example, we typically place the following in our logical names folders:

  TempFolder - an alias to the RAM Disk
  HomeFolder - an alias to the folder containing user folders
  ProjFolder - an alias to the folder containing project folders
  ArchivesFolder - an alias to the folder containing archives
  TransferFolder - an alias to a folder used as transient storage

You may place aliases within folders within the logical names folder, though you should avoid name conflicts between folders.

MPW Shell

This implementation is distributed in the file: UserStartup*!LogicalNames

For the MPW Shell implementation of Logical Names, we use a start-up script to create an environment variable for each alias in the logical names folder. The name of each variable is the same as the name of the alias. The value of each variable is the resolution of the alias. In addition, the LogicalNames variable is set to a list of the installed logical names.

To make use of this script, simply place it in your MPW folder (leaving it named "UserStartup*!LogicalNames").

For example, if you've created the "ArchivesFolder" logical name alias, once you launch MPW the following should report the full path to your archives folder:

  echo {ArchivesFolder}

AppleScript

This implementation is distributed in the file: LogicalNames.as.

For the AppleScript implementation of LogicalNames, we use a function to lookup and resolve a logical name alias given its name. For this implementation only, if the alias is in a sub-folder of the logical names folder, then a partial path must be supplied. LogicalNames.as also supplies a utility function which converts an item into a full path string.

To make use of these functions, you must first load and compile LogicalNames.as using the Script Editor. Then, you must save the compiled script as a Compiled Script to the file "LogicalNames" in your Scripting Additions folder. Now it's in a well-known location. Then, insert the following at the beginning of every script in which you want to use logical names:

  tell application "Finder"
    set LogicalNames to -
      (load script file -
        (name of startup disk & ":" & -
          name of system folder & ":" & -
          "Extensions:Scripting Additions:LogicalNames"))
  end tell

This will bind the LogicalNames variable to the (previously saved) compilation of the LogicalNames.as script, from which you may use the logicalNameGetItem(aName) and fullPathFromItem(anItem) functions.

For example, if you've created the "ArchivesFolder" logical name alias, a script such as the following should report the full path to your archives folder:

  tell application "Finder"
    set LogicalNames to -
      (load script file -
        (name of startup disk & ":" & -
          name of system folder & ":" & -
          "Extensions:Scripting Additions:LogicalNames"))
  end tell
  set myArc to logicalNameGetItem("ArchivesFolder") of LogicalNames
  fullPathFromItem(myArc) of LogicalNames

Macintosh Common Lisp

This implementation is distributed in the file: logical-names.lisp

MCL has a concept similar to logical names in its "logical hosts", but no reasonable built-in mechanisms for defining them in terms of the generalized Mac file system. So, for the MCL implementation of Logical Names, we simply provide functions for defining a logical host for each alias in the logical names folder. The name of each logical host is the same as the name of the alias. The target of each logical host is the resolution of the alias.

To make use of these functions, simply place logical-names.lisp in your MCL folder, then add the following to your init.lisp (or .fasl/.pfsl) file:

  (load "logical-names.lisp")
  (define-all-logical-names)

For example, if you've created the "ArchivesFolder" logical name alias, once you launch MCL the following should report the full path to your archives folder:

  (full-pathname (make-pathname :host "ArchivesFolder"))

Mike Webb, mjw@codewell.com and
Jeff Mallatt, jjm@codewell.com