Dan's previous article (Linux Journal #14) explains why Eiffel is a language of great interest. In this article Dan reviews two Linux implementations of Eiffel-3.
The two packages reviewed, ISE Eiffel-3 from Interactive Software Engineering (ISE), of Santa Barbara, California, and TowerEiffel from Tower Technology, of Austin, Texas, have much in common, including:
A short “Ace” file that describes the makeup of a system of classes comprising a program and that contains options to the compiler, most critically, the name of the root class and its creation procedure, and the class directories. Based on the Ace file, the compiler handles all dependency analysis itself.
A command line interface (not evaluated in this review). The graphical interfaces are nice, but not absolutely needed, and you can get around them if you find they are getting in your way.
Between 50KB and 100KB of static runtime binary libraries. Both ISE and Tower Technology plan to offer shared libraries in the ELF binary format when ELF stabilizes.
Eiffel class libraries are delivered in source or sometimes encrypted source form, with one file per class. Lots of classes are present in such a library, offering things such as linked lists, trees, file I/O, iterators, classes for object persistence and transmission, and much more.
Except for the kernel library, which furnishes hooks to the operating system and the runtime binary library, class libraries are planned to be portable across compilers. With the adoption of PELKS, the Proposed Eiffel Kernel Library Standard, all vendors now move toward a common kernel library interface supporting their other class libraries.
The Tower development environment is tailored to lemacs or its successor, xemacs. Neither is included in the distribution, but these editors are are commonly available from ftp sites and CD-ROMs. Look for lemacs-19.10 or xemacs-19.11. You can also use emacs, though the fit is less precise.
Tower is working on a non-emacs integrated development environment, to be in beta test by press time.
Installation was smooth, except for a little difficulty in the license setup. After some exchange of e-mail, a working license code was installed and TowerEiffel was up and running.
Program edit, class browsing, compilation, and debug are initiated from lemacs pull-down menu selections. A nice touch is the “Send performance report to Tower” menu item which e-mails a bug report. I mailed some, and got immediate responses.
The “Browse” menu item gives you library browsing capability, as well as allowing you to work with your own classes. Some very useful views were missing from the menu. Tower agreed, and had already implemented the most difficult of these. More choices will soon be available.
With all optimizations enabled, a small test application compiled from scratch in about one minute. The resulting binary used about 77KB of code space. A rough breakdown of this showed most of the code coming from the runtime library.
With all assertion checking on and no compiler optimizations, the binary file from compiling this application occupied about a megabyte. It still took one minute to compile from scratch.
Compiler errors list to a window, with explanations and references to Eiffel: The Language. If you click on the errors, the related program text appears in another window.
The Tower debugger, egdb, adapted from GNU gdb, is a command-line symbolic debugger. Xemacs or xegdb provide graphical front ends. Breakpoints may be set at any reasonable line of the Eiffel source code. When execution encounters a breakpoint the corresponding program text appears in a window. You can single-step one Eiffel statement at a time, and the cursor in the display window steps to the statement executed. When execution enters C code functions, egdb becomes a C source code debugger. All the other amenities familiar to gdb users are present.
There were a few mysteries about how to display the contents of some objects. Setting some breakpoints was also obscure. Other than that, egdb was straightforward. Source code awareness was good, and the ability to switch over to C mode was also nice, as time-critical or platform dependent parts of Eiffel systems are often implemented in C.
The TowerEiffel tools are rich in run-time options, not all accommodated in the menus. In particular, report generating tools, such as eshort, can produce output in text, LaTeX, plain roff, manpage roff, or LaTeX, or Rich Text Format for use in Word or NeXT. The options are well documented in man pages.
The Tower class library has 102 classes, including the 47 kernel classes, providing the standard Eiffel language hooks into the underlying runtime library and operating system, as well as certain basic amenities, such as ANY, the ancestor of all user classes. Also present here are classes for strings, numbers, arrays, and so on.
Another 22 encrypted classes comprise a subset of the Booch components from Rational (Santa Clara, CA). These provide data structure and support classes giving you containers, lists, searching and sorting, pattern matching, and so on. Tower sells the full set of Booch components in unencrypted form.
Finally, 16 classes provide for storage and transmission of objects, and 10 more provide support functions.
Tower staff were consistently prompt in responding to e-mail, even when the answer to a question was not immediately available. The longest delay between sending mail and receiving a response was about four days, for a non-urgent matter. Most e-mail responses were within 24 hours. I phoned Tower only twice, about matters unrelated to any preceding e-mail. The person I wished to speak with answered the phone himself the first time, and the second, returned my call within a couple of hours.
Interactive Software Engineering is the company of Bertrand Meyer, Eiffel's designer.
Included with the pre-release document set I read were Eiffel: The Language, Eiffel: The Reference, Eiffel: The Libraries, and Eiffel: The Environment, all by Bertrand Meyer.
The Language is the definitive work on Eiffel, required reading for every Eiffel developer. Best read after obtaining some initial familiarity with the language, it is the language standard, and the place where all discussions of the fine points turn for answers.
The remaining books are necessary if you plan to use the ISE package, and are worthwhile reading in their own right. Bertrand Meyer's writing is often entertaining, always very insightful.
Interactive Software Engineering Eiffel-3 version 3.3.4 for Linux arrived on a DAT tape. I took it over to the Linux Journal offices to read it onto disk, then compressed it to six floppies for klister to read. Unpacking was easy, and I filled out the registration form and sent it off by e-mail. In due time the license codes arrived by return e-mail, I installed them and fired up the license manager daemon, and ISE Eiffel-3 was up and running.
Program edit, class browsing, compilation, and debug are performed from ebench or the tools it launches. These static linked Motif applications require no Motif shared libraries.
The user interface is thoughtful and innovative. Many of the things you need to do are initiated by pushbuttons or drag and drop. Dragging is performed without holding down a button, by clicking button three once to drag and again to drop, reducing the carpal tunnel wear and tear. Drag is easily aborted by clicking button 1. Pushbuttons have distinctive icons, and a phrase describing each pushbutton appears in the tool frame when the cursor is over the button. Those who have worked with graphical tools will appreciate that multiple level menus, pop-ups, and pull-downs are few. Instead, most of the controls needed at any point are immediately available on the frames of the tools present on the desktop.
You can edit program text using the native Motif capabilities, or you can click on a button that launches your favorite editor in a subshell. vi is the default editor. If you prefer emacs, you might run it in its own window to avoid starting it every time you want to edit a file.
ISE includes the following tools:
Project tool: Launch other tools, debug programs.
System tool: Edit Ace file; look at system statistics. Provide class names, for launching or targeting class tools.
Class tool: Edit and browse classes.
Feature tool: Closer look at features, including ancestry.
Object tool: Examine objects at debug time.
Explanation tool: Help on compilation errors
The class, one of the more interesting tools, gives you alternate views of a class at the push of a button. Views include:
Text: Editable class text.
Short: Feature definitions and feature header comments.
Short flat: Short, with all ancestors rolled in.
Ancestors: Indented list of all ancestors.
Descendents: Indented list of descendents.
Clients: List of all class clients in system.
Suppliers: List of classes whose client the target class is.
And more. Within each of these views, many things can be clicked and dragged to various tools. The effect is a very good hypertext view of a system, including the supporting classes from the libraries. Users of ctags may know what I mean if I call this “ctags on steroids”.
My small test application took sixteen minutes to compile from scratch with full optimizations and no assertions. The resulting binary file was about 130KB.
With assertions enabled and optimizations off, full compile time from scratch was about six minutes, and the binary file was about 1.7MB.
ISE ebench uses “melting ice technology”, which allows incremental changes to run in an interpreted mode. Only modified classes are recompiled. Changing one class and clicking the Melt button caused only a few seconds of compilation. The entire application can be run in this mode, which runs slower but compiles faster. My test application took 20 seconds to compile from scratch in “melt” mode.
Compiler errors are shown in the project tool, whence you can drag and drop class and feature names into class tools, letting you find the affected program code quickly. For more information, drag and drop the error code itself into an explanation tool, and very likely you'll see a paragraph and page number from The Language.
Debug uses the same drag and drop technique used elsewhere. Breakpoints can be set regardless of whether the system is executing, in any class whose text is accessible to the tools. At each breakpoint the stack frame is shown, and features may be displayed by dragging from the stack frame to an object tool. Once displayed, further references may be followed by dragging from the object tool to itself. For example, to traverse a linked list, drag its root object to an object tool, then repeatedly drag the forward link to the same tool.
A breakpoint can be set only at a routine's entry point. As most Eiffel routines are short, this gives better granularity than a C source debugger able to break only at function entry points, but can sometimes require insertion of feature calls and recompilation. Single stepping is available, but I was unable to discover any indication of which source statement had been reached at a step.
The EiffelBench environment is a very nice piece of work. If you are interested in integrated development environments, ISE Eiffel-3 is worth the purchase price just to get a good look at what they've done here. While not perfect, this environment gives a stable, quick, and user friendly, but not confining, basis for doing projects in Eiffel.
The ISE class library is large, with 752 classes, all in source form. Of these, the ones of most general interest will be the 157 classes in the base hierarchy, comprising the kernel classes, support classes, and containers and data structures. Beyond that there are libraries for building parsers, GUIs under X, Motif, or MS Windows, or for use in EiffelBuild, their Motif GUI builder.
ISE gave a satisfactory response to every significant question I have raised to date. Sometimes this required persistence on my part, and once or twice it required a phone call to follow up the e-mail.
Dan Wilder has been employed as a software engineer since 1975. Dan resides in Seattle, where he divides his time between work, family, and a home Linux system. Any time left is spent ignoring his moss collection, which his neighbors think he calls a lawn. He can be reached via e-mail as firstname.lastname@example.org.