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Successful Scheme

Programming language still going strong after 25 years

Summary
What's so great about Scheme? In this week's Regular Expressions, Cameron Laird and Kathryn Soraiz introduce the ever-changing scripting Lisp dialect that has influenced REBOL, Forth, and Guile. (1,100 words)


  REGULAR EXPRESSIONS  

By Cameron Laird and Kathryn Soraiz
Thought up any great programming schemes lately? The attendees at the Workshop on Scheme and Functional Programming 2000 -- held in Montreal on Sept. 17 -- have. The recent conference means the time is ripe to introduce the Scheme programming language and mention a bit of what's new in the Scheme world.

Scheme goes to school
Scheme, for the most part, is a simplified Lisp: one with extremely regular syntax, lexical interpretation, and semantics. That means it's easy to teach computers how to interpret Scheme, and to teach humans how to program in it.

That duality has made Scheme an academic language in two senses. Its regularity renders it a good construction material for research into new language structures and verification mechanisms; its powerful simplicity invites its use as a language for instruction.

In fact, one of Scheme's greatest advantages is the pedagogic classic, Structure and Interpretation of Computer Programs (SICP). Also known as The Wizard Book or The Purple Book because of its unique cover, SICP has been lauded since its inception in 1984 for its cogent, well-organized explanations of the fundamental abstractions of computing. SICP presents its algorithms and data in Scheme.

Scheme's place in the world of languages
Consequently, Scheme has been the mother tongue for many of the last generation's best computing students. However, Scheme is not stuck in the classroom or its own past. It's a very productive language for application development, and modern Schemes include extensive libraries that give it the Web, GUI, and networking capabilities that most programs need.

Many Scheme aspects appear in other languages and are familiar to loyal readers of Regular Expressions. Scheme shares its grounding in denotational semantics with REBOL, the subject of our last column. Scheme is the first prominent dialect of Lisp to demonstrate static lexical scoping, and is also properly tail-callable. The spirit of its coding is much like Forth's, although the two languages are at odds regarding garbage collection and the hardware-dependence of values. Guile, introduced in our November 1998 column, is itself a specialization of Scheme, crafted for portability and embeddability.

Papers at the Montreal workshop touched on several other topics that we consider important. Two speakers demonstrated the interoperability between Scheme as a scripting language and legacy objects coded in either C or Java; another explained advanced models for distributed computing.

The pieces of the Scheme world are relatively common, but Scheme's combination of them is uniquely potent. Scheme supports a variety of coding styles, including an imperative one and an object-oriented messaging-passing one. Scheme is particularly known for its referential transparency and functional flavor. In that context, functional programming concerns the safety and verifiability of coding in terms of procedures, which are free of side effects. As we'll explain next year, when Regular Expressions takes a more extended look at the subject, we also believe functional programming will be helpful in exploiting future computing architectures.

Scheming enthusiasts
Scheme's fans can be very enthusiastic. They use the language for the full range of applications that scripting generally covers. David Rush, inventor of the CryptoGnome network-oriented version control system, wrote to tell us, "I also use [Scheme] for all kinds of prototyping and home hackery because it wastes very little of my time with obfuscatory semantics. The very things that make Scheme a good academic language are what make it a good RL [real-life] language. It's very hard to argue against uncluttered syntax, orthogonal semantics, first-class continuations and hygienic macros."

While the Guile dialect of Scheme aims to connect to all GNU-backed projects, three other projects could potentially bring Scheme to wider audiences.

First, Scheme is the extension language of JEmacs, the reimplementation of Emacs in Java.

Second, Bruce R. Lewis's open source Beautiful Report Language (BRL) is an interesting system for server-side Web scripting in Scheme. Among the beautiful aspects that Lewis claims make BRL "the best environment for developing Web/database applications" is Scheme's simplicity: Web developers don't need to pick up any of the historical baggage that weighs down other scripting languages.

Finally, Workshop Program committeeman and longtime Scheme instructor Matthias Felleisen has ambitions for what he describes as the first "programming environment [PE] especially fined tuned for beginning programmers." While Pascal, APL, BASIC, and many other languages have served in introductory coursework, in each case the implementers "produced programming environments for knowledgeable programming novices. None of their PEs smoothes people into a real, large language through several stages. In my experience, that's critical. Beginners make mistakes. PEs explain mistakes--based on a programmed understanding of the language and a user model."

Felleisen hopes to take such a programming environment and coordinated curriculum "nationwide" within the next few years. He and his coworkers see this kind of computer course as part of a liberal arts or general education, because it focuses on problem solving more than computing jargon.

Scheme hasn't yet enjoyed the intense advocacy that characterizes the social life of some other languages. Rush said, "It's mainly [for] people who are busy getting code written." Still, the Usenet newsgroup comp.lang.scheme has a good reputation for its gravity and helpfulness, and a growing number of Webpages are pertinent to Scheme. The Resources section below can help you find further information.

About the author
Cameron Laird and Kathryn Soraiz manage their own software consultancy, Phaseit, from just outside Houston. Visit Cameron's Scripting Languages & Techniques discussion in the Unix forum, hosted on ITworld.com.

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