DYLAN
Answers to Frequently Asked Questions

last updated 11 August 1995

This memo answers questions which are frequently asked about the Dylan programming language, and its implementations. If you want to keep up with Dylan news, consider joining the announce-dylan or info-dylan mailing lists, or the comp.lang.dylan newsgroup, described below.

The latest version of this memo is available by anonymous ftp from http://www.cambridge.apple.com.

The Apple Dylan ftp site is cambridge.apple.com:/pub/dylan/

The Dylan WWW page at Carnegie Mellon University is http://legend.gwydion.cs.cmu.edu:8001/dylan/

The Dylan newsgroup is comp.lang.dylan

On Applelink, Dylan files are regularly uploaded to this area: Developer Support:Developer Services:Development Platforms:Dylan Related

On eWorld, Go (cmd-G) to dev service, then click on Tool Chest:Development Platforms:Dylan Related

On CompuServe, Type GO APPLE to get to the Apple support forum.

There are 16 libraries, go into Programmers/Developers Library #15

Power Macintosh Apple Dylan Technology Release
On 8/5/96, Apple announced the availability of the Power Macintosh version of the Apple Dylan Technology Release. The Technology Release is available through ADC (formerly APDA) either as a package or standalone CD. The standalone CD provides an economical way for developers who already have the documentation to obtain this native version. The CD also includes the 68K version released previously. The package includes the CD and 6 manuals. Both items will also be available for sale at the ADC booth in MacWorld Boston.

We are expecting to ship this release on August 7,1996. The ADC order # for the package is M5466Z/A and is listed at $59.95. The ADC order # for the standalone CD is M5475Z/A and is listed at $39.95.

Customers can order: 

	By PHONE:	United States 1-800-282-2732 (Toll-free)
			Canada 1-800-637-0029 (Toll-free)
			International 1-716-871-6555


	By FAX:		APDA Fax number 1-716-871-6511


	By EMAIL:	AppleLink.......ORDER.ADC
			Internet....... ORDER.ADC@applelink.apple.com
                        Web.............http://www.devcatalog.apple.com
International customers may also order by contacting one of the licensed resellers in 30 countries.

[1-2] Where can I get a copy of the language specification?
The first book on the Dylan language was published in 1992. Since then, the language has undergone a great deal of change in response to feedback from potential users and implementors. A substantially revised language reference (The Dylan Interim Reference Manual, or DIRM) was published in June, 1994, and is available as a browsable web document and in various formats (Acrobat, Postscript, RTF) by anonymous ftp. See [1-1] for more information about web and ftp sites.

The current round of language design is now essentially complete, and we are finishing a definitive reference specification known as the Dylan Reference Manual (DRM). A draft will be made available online by Fall 1995.

Of course, we realize that many people want to read about Dylan now! So to fill the gap until the new book is published, an interim Dylan reference book is available by anonymous ftp, in the directory /pub/dylan/dylan-manual/, as well as in the other file areas listed in topic [1-1] above. It is available in both RTF (Rich Text Format) and PostScript formats (214 pages). The interim book is a combination of the original Dylan book, the previously published design notes, and additional previously unpublished design decisions. The document is called "interim" not because the language design is unfinished, but because this is a very rough document intended for use until the new book is ready.

[1-3] Are there any public mailing lists for discussing Dylan?

Yes. The info-dylan mailing list is a forum for discussing all subjects related to Dylan. Each day's messages are gathered into a digest and sent as a single compound message to the list info-dylan-digest. The mailing list is also linked to the Newsgroup comp.lang.dylan.

The announce-dylan mailing list is a moderated list, for major announcements about Dylan (such as the availability of new implementations of the language). All messages sent to announce-dylan are also sent to info-dylan.

To subscribe to info-dylan or announce-dylan send mail to majordomo@cambridge.apple.com. The body of the message should be

"subscribe <list-name>",

where <list-name> is the name of the mailing list you want to subscribe to.

To unsubscribe to one of the mailing lists, send majordomo a message with the body

"unsubscribe <list-name>".

If you would like to subscribe or unsubscribe an address which is different from the return address of the message, include the address after the <list-name>. For complete majordomo instructions, send a message with the body "help".

The name of the digest form of info-dylan is info-dylan-digest. Please do not send administrative requests to the mailing lists! If you have trouble with info-dylan, send mail to sysadmin@cambridge.apple.com.

[1-4] Where else can I get information about Dylan?

The Gwydion Project at Carnegie Mellon University maintains a World Wide Web (WWW) page of general information on Dylan. This is accessible using Mosaic or other web-browsing software. This page contains the Interim Reference Manual as well as the origin al Dylan book in on-line form, annotated with links to the design notes and changes that have been approved. This web page also contains the current FAQ from Apple and other general information of interest to the Dylan community.

The URL is "http://legend.gwydion.cs.cmu.edu:8001/dylan/".

The Gwydion Project maintains an FTP-accessible on-line repository of Dylan libraries and code that the authors wish to share with the rest of the community. The Gwydion site is currently available as:

http://legend.gwydion.cs.cmu.edu:8001/gwydion/index.html ftp://legend.gwydion.cs.cmu.edu/afs/cs/project/gwydion/release/

For further info, contact gwydion-group@cs.cmu.edu.

[1-5] Where should I send comments on the Dylan language?

Most comments can be sent to info-dylan. If you wish to send a private letter to the Apple engineers working on Dylan, you can send a message to dylan-comments@cambridge.apple.com. (AppleLink users can send mail to DYLAN).

[1-6] What should I do if I want to implement Dylan?

Send mail to dylan-comments@cambridge.apple.com. We have put together a program to support implementors, and we want to hear from you. (AppleLink users can send mail to DYLAN).
If you've written an implementation of Dylan and want to release it, please contact us for a trademark license.


[2] General Questions About Dylan and Implementations

[2-1] What is Dylan?
Dylan is a new Object Oriented Dynamic Language (OODL). Dylan combines the features of static and dynamic languages. The goal of the language is to support a high level of programmer productivity, while still allowing the efficient delivery of applications and libraries.

Dylan is consistently object-oriented. It is not based on a particular previously-existing procedural language with some kind of object-oriented extension.

[2-2] What is the target audience for Dylan?
The target audience for Dylan is developers of commercial application software, most of whom are currently using static languages such as C and C++, as well as in-house developers and system integrators who are using languages such as Smalltalk and who need a high-level, very productive language. We expect Dylan will appeal to many other groups of programmers as well, such as educational users who want a very clean object-oriented language design.

[2-3] How does Dylan differ from previous OODLs?
Dylan is designed to allow the powerful and flexible programming techniques and development environments associated with OODLs, while also allowing the small, fast delivered applications currently associated with static languages.

Unlike many dynamic languages, Dylan's design consciously enables the runtime environment to execute without the development environment present. In addition, Dylan will let you selectively turn off dynamic capabilities when they are no longer needed, allowing more efficient compilation.

[2-4] Does Apple have an implementation of Dylan?
Apple is currently working on an implementation of Dylan. Apple's current implementation will be made available in a form of a "technology release" to the Macintosh developer community by the end of 1995. Getting a language broadly used requires time, experimentation and prototyping. The goal of this release is to start promoting the use of the Dylan language as a first step towards building a large community of Dylan developers. We also would like to to receive feedback from the developer community. More details regarding where and how to order the Apple Dylan technology release will be announced on the Dylan mailing lists and the Dylan WWW page later-on during 1995.

The Apple Dylan technology release will include:


The technology release will be hosted on 68K-based Macintosh systems. However, you will be able to produce code targeting both the 680x0 and Power Macintosh systems.

The development environment will let you create projects with all the advantages of a rapid-prototyping environment. Your project is stored in a database, unlike traditional, file-based systems. Customizable browsers will give you a powerful new way to look at and manipulate your application as it executes. You can browse class hierarchies graphically, find all references to a given routine or variable, and inspect objects in your program while it's running.

The incremental compiler will allow you to actually change code in a running program and see the results right away. No more waiting for a long edit, compile, link, execute, debug cycle. This gives you freedom as a programmer to explore various options and rapidly improve your software.

Automatic memory management will free you from the burden of manually allocating, tracking, and deallocating memory usage in your application. This will happens seamlessly and behind the scenes, saving you both programming and debugging time.

Concurrently with the development of the technology release, Apple is working on defining the plans for an Apple Dylan product that will be available in the future. Based on input received from the developer community during the last year, the final product is expected to better address key market requirements such as multi-platform development and deployment. Details about availability of a final Apple Dylan product from Apple will be made available on the Dylan mailing lists and the Dylan WWW page as development continues.

[2-5] How can I apply to be an early seed site for Apple's Dylan implementation?
Send a message to the AppleLink address DYLAN, or the Internet address dylan@applelink.apple.com. The message should include your name, address, phone number, and a brief description of how you plan on using Apple Dylan.

[2-6] What third-party implementations of Dylan are available?

Several third-parties have expressed interest in implementing Dylan.

[a] Marlais is an experimental interpreter written in C. It was created by Brent Benson, and is being extended by Joe Wilson, Patrick Beard, and others. It supports the official algebraic syntax, as well as the older lisp-like syntax. Marlais runs on a variety of unix platforms, as well as Macintosh and Windows. The latest version of Marlais is 0.5.11, and is available from ftp://ftp.cis.ufl.edu/pub/Marlais/.

MacMarlais is the Macintosh port of Marlais, ported by Patrick Beard. It offers an interpreted environment with an integrated editor and interactive listener window. The latest version is available from ftp://ftp.bdt.com/home/beard/.

[b] Harlequin is currently investing in a commercial implementation of Dylan. They are building a PC based Windows 95/NT hosted version of the language. For more information, write to dylanworks@harlequin.com, or check out http://www.harlequin.com/full/dylan.html.

[c] The Gwydion Project at Carnegie Mellon University is building an innovative new software development and maintenance environment around the Dylan language. A part of this work will involve the development of a high-performance Dylan implementation that will produce code for many platforms.

For more information on this project, see the Gwydion project's World Wide Web page: http://legend.gwydion.cs.cmu.edu:8001/gwydion/

[d] As a development tool, the Gwydion project has produced a byte-coded Dylan implementation called "Mindy". Mindy is documented on the Gwydion web page (see above), and is available by anonymous ftp legend.gwydion.cs.cmu.edu in the file: /afs/cs.cmu.edu/project/gwydion/release/mindy.tar.gz
Due to CMU security measures, if you use the ftp 'cd' command, then you must 'cd' to the release directory in one cd command line; for example, do not try to 'cd' to "/afs/cs/project/gwydion/".

[e] Thomas 1.1 is an experimental interpreter written in Scheme. It was created by Matt Birkholz, Jim Miller, and Ron Weiss at the DEC Cambridge Research Lab in 1992. It supports the older lisp-like syntax, and does not incorporate many of the more recent language changes. Thomas is available at the Dylan ftp site at cambridge.apple.com

[2-7] Is Dylan related to the Newton PDA?
Not at this time. If you are looking for more information on Newton development, you need to contact the Newton Developer Relations at NEWTON.DEVS@applelink.apple.com

[2-8] Is Dylan designed to be a replacement for Common Lisp?
No. We see Common Lisp and Dylan filling two distinct needs.

[2-9] Is Dylan a proprietary language?
Clearly Dylan is not a proprietary language. The Dylan language design is in the public domain. Apple wants Dylan to be available on as many computers as possible. To this end, we are encouraging groups outside Apple to implement Dylan. At the same time, it is our intention to work with different Dylan implementors for them to pass a standard test suite to ensure quality and consistency among implementations. Implementations will have to pass the test suite in order to use the name Dylan.


[3] Questions About the Dylan Language Design

[3-1] Is the Dylan language design frozen?
The current language design is complete. At this time, we will only be making small tweaks, or changes necessary to resolve bugs or inconsistencies in the specification.

[3-2] What does the Dylan syntax look like?
The first Dylan book published in 1992 used a parenthesized, lisp-like syntax. The current language definition uses an algebraic syntax, which will be very familiar to users of Pascal or C.

[3-3] Will Dylan have a standard macro system with the algebraic syntax?
Yes. This will be based on substitution at the parse level (like Lisp) rather than text substitution (like C). A description will be included in the next version of the Dylan language reference manual.

[3-4] Are there plans to specify a standard I/O package for Dylan? What about threads?
Threads, I/O, and other language features may be provided as optional libraries in the future. They are not currently part of the language. Because of the great variety of computing platforms from PDA to mainframe we do not want to standardize on these features too strictly.

[3-5] Why is 'make' allowed to return a previously allocated instance, or an object which is an indirect instance of the class passed to make?

We feel that this is a very important abstraction mechanism. A class should have flexibility in how it implements make, as long as the object returned fulfills the protocol of the class.

For example, this allows a library to document a single abstract class which is supported by several undocumented implementation classes. The abstract class can choose which implementation class to instantiate based on the additional arguments to make. This allows optimizations which are transparent to the clients of the library.

The default method for make of a user-defined class returns a fresh direct instance of the requested class.

[3-6] The Dylan'92 manual doesn't require implementations to optimize tail-recursion. Was this an intentional omission, or an editorial oversight?

It was an editorial oversight. Dylan implementations are required to be properly tail recursive.

[3-7] The Dylan'92 manual doesn't say much about modules. Will this be specified in the future?
The new language document includes a complete module specification.

[3-8] Can the 'method' special form be used to create closures?
Yes.

[3-9] I don't understand how setter variables work. Is 'setter' a special form?
This has been clarified in the new language design. The new design is included in the interim Dylan reference book.

[3-10] What kind of object is used to return multiple values?
When a function returns multiple values, the return values are not stored in a wrapper object; they are returned directly. For example, if a function returns "the values 4 and 5", it returns two integers. It does not return a data structure which contains two integers.

Returning multiple values is similar to calling a function with more than one argument. When passing multiple objects as arguments to a function, the objects do not have to be stored in a single data structure before they are passed.

[3-11] Is the specification of sealing complete?
Yes. The new language design includes a complete specification of sealing. This design is included in the interim Dylan reference book.

[3-12] Will Dylan include 'eval'?
Some implementations may choose to support eval, but we do not have plans to add it to the language standard. We feel that the delivery of applications which are space efficient requires the separation of development time activity from runtime activity.

[3-13] Will Dylan include an application framework?
We recognize the value of application frameworks, especially cross-platform application frameworks. Unfortunately, because of the great variation in computing platforms, a single application framework will not be part of the Dylan language. On each platform, there should either be a Dylan-specific application framework, or Dylan should be able to use application frameworks written in other languages.

[3-14] Will Dylan interface to other languages?
We recognize that seamless integration with other languages, especially C and C++, is essential. We are working on addressing this issue. The solutions may not be part of the Dylan language proper. Apple Dylan supports Creole, allowing seamless access to existing C code and APIs. The Dylan implementations from Harlequin and CMU provide similar functionality.