|Column Tag:||Object-Oriented Languages
Macintosh Common Lisp 3.0
An Old Language is New Again!
By Rich Parker, Modesto, CA
Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.
MCL: The Rumor and The Truth
For a while last year we all thought that Macintosh Common Lisp (MCL) was going to be abandoned. Apple had decided not to continue development of the product and there was no suitable replacement in sight, particularly for the new PowerMac computers. But Apple finally decided to sell the product to Digitool, Inc. Digitool started right off by commencing the development of two new versions of the product; the first is the new 68K version 3.0 (which, unlike the previous version 2.0.1 runs just fine on the PowerMac with the Modern Memory Manager enabled). Digitool is also working on a PowerMac native version of the product, which they say will be released later this year.
Creating the PowerMac version is a fairly major undertaking, mainly because MCL contains an incremental compiler that generates native 68K code. MCL also relies on the memory management facilities of the 68K family, and supports 68K-style floating-point computations. All of this will have to change for a native PPC version. But Digitool promises that the upcoming PowerMac product will be very efficient and fully compatible with even the latest PowerMac machines.
The truth is that MCL 3.0 is extremely efficient right now, even though its running under emulation mode on my PowerMac 6100. It no longer crashes when I have the Modern Memory Manager turned on, and its happy to execute with VM turned on as well.
So What Good Is Lisp?
I see many messages from developers who surf the various online networks and it seems that many of these messages are devoted to discussions of the various problems with the C++ language. In fact, many have expressed that C++ is an abominable language, full of pitfalls and traps.
But Lisp is not without its own detractors (those who bemoan its copious use of parenthesis), but the fact is that few ever complain about limitations of the language. In fact, Lisp (ANSI Common Lisp, that is) is so full-featured and straightforward that the main body of complaints are focused on the breadth of the language, rather than its limitations - its such a large language that learning all of its features is almost impossible. Although the language is said to be quite large, its not because of its syntax; that part is simple. The difficulty lies in learning the hundreds of standard functions, macros, and special forms that are documented in the Common Lisp standard.
Once you spend some time with Lisp (and especially MCL), youll begin to see the tremendous potential of the language. MCL appears to be interpretive, but it is actually compiling each statement you type into native code (68K code, presently) as it is entered. If a particular set of statements is executed a second time, it is not recompiled, but is executed directly. You can compile individual files of source text and store these into fasl files, which load and execute very efficiently.
You can build an application piece by piece, testing as you go, without having to go through the edit, compile, link, execute cycle that is standard with languages like Pascal, C, and C++. MCL is a prototypers dream come true. The language implements the full ANSI Common Lisp standard, including the Common Lisp Object System (CLOS). In addition, MCL contains classes and methods that implement all of the Macintosh user interface features, as well as almost all of the latest features of the Mac OS. The product directly supports interfaces to over 5000 traps, 1000 record definitions, and 13,000 constant definitions, including support for WorldScript, Apple events, Comm Toolbox, QuickDraw GX, QuickTime, and the Apple Open Collaboration Environment (AOCE). It also supports a Foreign Function interface that allows you to interface functions written in other languages to your MCL applications. MCLs current Foriegn Function interface has some limitations (supports only model near) which Digitool reports will be addressed in the PowerPC implementation.
And of course, MCL is a fully object-oriented dynamic language (OODL), so you can create functions that create other functions to your hearts content. When an object becomes unreferenced, it is disposed and the garbage collector will reuse its memory automatically. All of the latest fascinations in the computer science realm are provided with MCL.
The ease with which applications can be prototyped is one of the major features of the product. Once familiar with the language, you can develop an application prototype in a matter of hours instead of days or weeks. Creating windows, nested views, dialogs, menus, and other Macintosh user interface features is a relatively simple chore. And the interactive debugging tools are also a great help in getting an application written and ready to use.
Just to give you an example of what Lisp code looks like, Listing 1 is a simple MCL program that when executed creates a simple modeless dialog window containing a counter value as a static text item, and two push buttons. When pushed, one of the buttons increments the counter and the other decrements the counter. The full source code to accomplish this is as follows:
Listing 1. MCL Counter Code Example
;; define a global variable to hold the count value
(defvar *value* 0)
;; incr-item function called when Increment is clicked
(defun incr-item (item)
(if (<= *value* 999)
(setq *value* (1+ *value*)))
(set-dialog-item-text item (format nil "~3,'0D" *value*)))
;; decr-item function called when Decrement is clicked
(defun decr-item (item)
(if (> *value* 0)
(setq *value* (1- *value*)))
(set-dialog-item-text item (format nil "~3,'0D" *value*)))
;; create the dialog window and all of its subviews
:window-title "Increment & Decrement"
:view-size #@(230 100)
:view-position #@(75 10)
:view-position #@(120 10)
:view-position #@(30 50)
(incr-item (find-named-sibling item 'value))))
:view-position #@(120 50)
(decr-item (find-named-sibling item 'value))))))
The code in listing 1 is fairly self-explanatory. The first two function definitions implement the actions necessary to increment and decrement the counter. In this case, I have chosen to ensure that the counter value is always greater than or equal to zero. It is not allowed to become negative. This is an arbitrary decision - it could easily have been allowed to become negative and the tests would not be necessary in that case. When executed, the counter example appears as shown in Figure 1, with the Decrement button being pushed. The format function in the incr-item and decr-item functions causes the counter value to be displayed with leading zeros and with three digits.
Figure 1. MCL Counter Example During Execution
New Features and a Face Lift
When MCL is first placed into execution it opens a single Listener window into which you can type statements that are to be compiled and executed immediately. An example of the appearance of the Listener window after the Counter example was executed is shown in Figure 2. The question mark is MCLs standard prompt for input. The code for the Counter example was stored in a file and was loaded and executed by choosing the Load File command from the File menu.
Figure 2. Listener Window Appearance
The new version of MCL includes a number of new features and a major face-lift for many of its tool dialogs (many of which were quite ugly in the previous version). The Apropos tool window, which is used to find functions, variables, classes, or macros is shown in Figure 3. Both the appearance and functionality of this tool is greatly enhanced in the new version.
Figure 3. Apropos Tool Window
The Apropos window shown in Figure 3 also shows the results of searching for references to window-related functions. The WINDOW-CLOSE function is highlighted in the list, indicating that it has been chosen by clicking on the entry. At this point, the function can be inspected, callers of the function can be listed, or documentation for the function can be retrieved. If source code exists for the function, then that also can be shown and its methods can be listed.
In addition to the Apropos tool, there is also a new Trace tool window. This tool is convenient for tracing the execution of a number of functions when a program is being debugged. An example of the appearance of this window and also the Listener window is shown in Figure 4.
Figure 4. Trace and Listener Windows
Figure 4 shows the results of tracing a single execution of the incr-item function. The Trace window shows that the incr-item function is to be traced and that its name and arguments are to be printed (in the Listener window) upon entry, and that its value (result) is to be printed when it exits. The results of the trace action are displayed in the Listener window, which is also shown in the figure. Any number of functions can be traced. The actions upon entry to a function can be to print its name and arguments, break (i.e., halt execution and prompt the user to perform some action at the Listener prompt), or perform no action. The actions upon exit from a function can be to print its name and values (result), break to allow the user to perform some action, or do nothing.
Actions at a break can include choosing to display the values of variables, modifying the source code, or any other related action. Other debugging aids, such as stepping the execution of a function, expression by expression, can be invoked by typing commands into the Listener window. Execution can be easily resumed at a break by typing Command-/ into the Listener window.
In addition to the tools just mentioned, MCL also includes an Inspector window. This is used to inspect the internal features of any MCL object or variable. You can either enter the name of the item to be inspected into the Apropos tool window, choose the type of the item from the pop-up menu, and then click the Inspect button; or you can simply type (inspect item ) into the Listener and an Inspector window showing the appropriate features of the item will be displayed. Figure 5 shows the contents of the Inspector window when the variable *value* (from the Counter example) is being inspected.
Figure 5. Inspector Window
Figure 5 shows the contents of the *value* variable when (inspect *value*) is typed into the Listener. Notice that the value is shown in many different forms, any of which might be of interest to you as the user. Other variations of the appearance of the Inspector window are common when different types of MCL objects are being inspected.
MCL 3.0 also provides the ability to change quite a number of preferences. This is accomplished in a new Preferences window. This window is shown in Figure 6.
Figure 6. Preferences Window
The Preferences window shown in Figure 6 includes a pop-up menu at its top that allows Environment (shown), Compiler, and also Printing preferences to be viewed and changed. Each of the preference items is a global variable (commonly written with leading and trailing asterisk characters to identify it as such). Each of the variables takes on a true or false value. The Documentation pane at the bottom of the window describes the selected preference variables function.
Interface Development Toolkit
MCL 3.0, like the previous version, offers a somewhat lame Interface Development Toolkit. This feature allows the development of simple application interfaces that consist of windows or dialogs containing buttons, text fields, and menus. It is clearly a limited facility and is one of the areas where MCL could definitely be improved. While the tool is relatively easy to use and offers a point and click approach to interface design, the number and type of interface elements is very limited. Once an interface is designed, however, you can automatically generate the code to recreate the interface. In that respect, its at least a help.
Processes & Process Scheduling
Version 3.0 of MCL includes the ability to create and schedule multiple processes. This feature facilitates the concurrent execution of computational tasks. When MCL starts up it creates two processes. The first, called Initial, is responsible for processing events. The second, called Listener, is responsible for accepting input from the user in the Listener window (i.e., it runs the read-eval-print loop). Processes can have assigned priorities and processes with the same priority run in a round-robin fashion. Regardless of the priority assigned to a process, it will not run any longer than the time interval specified when the process was created. The status of all existing processes can be displayed by opening the Process window from the Tools menu. The appearance of the Process window is shown in Figure 7.
Figure 7. Process Window
Figure 7 shows that only the Listener and Initial processes currently exist. If the users program had created other processes, thier names and their statistics would have appeared in the Process window as well.
Locks are provided to enable multiple processes to be synchronized. A lock is like a semaphore. When one process obtains a lock, any other process attempting to obtain that lock will block until the lock has been released by the original owner. Processes can also be placed into queues, where they execute in the order in which they were entered into the queue. These are major new features in MCL 3.0.
Saving an Application
MCL 3.0 has a new tool that makes the task of saving an application much easier than the previous save-application function call. The appearance of the Save Application tools window is shown in Figure 8.
Figure 8. Save Application Tool Window
The Application class, Menubar, and Error handler choices are selected from pop-up menus in the Save Application window. Values can be typed into the other corresponding fields. There is provision for the contents of a resource file to be copied into the application. The Disable compiler checkbox ensures that the application will not be able to be used for development purposes.
Digitool will also offer a Redistribution Kit, for an extra charge. It should be available by the time you read this. The kit will offer the ability to truly excise the compiler from the saved application and reduce the size of the executable file significantly. I have no information about the kit, its cost, or any redistribution licensing costs at this time. You should check with either APDA or Digitool for this information.
A Few Problems Have Surfaced
Although the new MCL 3.0 product appears very stable, a few users have complained of problems when using it. Apparently there is some sort of incompatibility with WorldScript II and Kanji Talk, although Digitool is unable to confirm this. Also, a couple of people have reported problems when running the new product with accelerator boards. There dont seem to be any serious problems with standard Mac computers (either 68K or PowerMac), except for the possible conflict with WorldScript.
The Bottom Line
I found MCL version 3.0 to be a great improvement over the previous version (2.0.1). However, there are areas in which it could be improved. The Interface Toolkit really needs to be developed into a much more comprehensive tool. In addition, the existing debugging aids, while helpful, are not very friendly and lack the ease of use that other modern development environments provide.
All in all, the MCL 3.0 product is a very worthwhile addition to your arsenal of tools. Its prototyping abilities are unexcelled and the overall breadth of the product is unequaled in any other environment.
In addition to the product itself, MCL 3.0 comes with over 200 megabytes of example code. It comes with the complete Apple Programmers Toolbox Assistant Quickview application and database on the products CDROM.
The product is available only on CDROM; however, Digitool has placed disk images for the product onto the CDROM disc, so if you dont own a CDROM drive and have a friend who has one, you can copy the disk images and then install the product from floppy disks. However, youll also be missing access to the over 200 megabytes of contributed code, so if youve ever wanted an excuse to buy a CDROM drive, this could be it.
The Redistribution Kit is $400 regular price, or $350 educational price. The MCL complier may also be licensed, as a scripting language in an application for example, for $2000, or $1600 eudcational. MCL is $295, $135 for students. Digitool, Inc. One Main Street, Cambridge, MA 02142, (617)441-5000.