Scheme -- consistent, elegant, concise, and easy to learn

Scheme

Scheme started as an experiment in programming language design by challenging some fundamental design assumptions. It emerged from MIT in the mid-1970's. It is a dialect of Lisp invented by Guy Lewis Steele Jr. and Gerald Jay Sussman. Originally called Schemer, it was shortened to Scheme because of a 6 character limitation on file names. Scheme is a small, exceptionally clean language which is fun to use. The language was designed to have very few, regular constucts which compose well to support a variety of programming styles including functional, object-oriented, and imperative. Scheme was influenced by lambda calculus, which is one of the first functional ``programming languages'' in the world, Algol, from which Scheme inherited statical scope and Lisp, from which Scheme inherited the syntax.

Features

Scheme is a statically (= lexically) scoped and weakly typed programming language which is very well suited for transformational programming (that is, it is easy to write programs that write programs), scripting and for developing and implementing programming languages--among other things. Most of the Scheme implementations are interactive (that is, interpreters), but there are also compilers available.

The procedures in Scheme are first class procedures, which means that they can be given as arguments to other procedures, they can be returned as the value of a procedure, they can be bound to variables or fields of other objects etc. The arguments of Scheme procedures are passed by value.

Syntactically Scheme is very simple and beautiful. The syntax consists of parentheses and prefix notation, which means that the operator comes first and then the operands.

Dialects and standards

The first description of Scheme was written by Gerald Jay Sussman and Guy Lewis Steele Jr., "Scheme: an interpreter for extended lambda calculus". MIT Artificial Intelligence Memo 349, December 1975. A revised report appeared in 1978, which described the evolution of the language as its MIT implementation was upgraded to support an innovative compiler (Rabbit). Three distinct projects began in 1981 and 1982 to use variants of Scheme for courses at MIT, Yale, and Indiana University.

As Scheme became more widespread, local dialects began to diverge until students and researchers occasionally found it difficult to understand code written at other sites. Fifteen representatives of the major implementations of Scheme therefore met in October 1984 to work toward a better and more widely accepted standard for Scheme.

Their report (The revised revised report on Scheme, or an uncommon Lisp [RRRS]) was published at MIT and Indiana University in the summer of 1985. Further revision took place in the spring of 1986 (The revised^3 report on the algorithmic language Scheme [R3RS]), and in the spring of 1988 (The revised^4 report on the algorithmic language Scheme [R4RS]). The present report reflects further revisions agreed upon in a meeting at Xerox PARC in June 1992 (R5RS). Have a look at the Scheme Standards documents section of The Internet Scheme Repository for more details.

Learning (with) Scheme

Scheme is a wonderful choice as an introductory language: its clear syntax makes it possible to concentrate on the programming problems at hand instead of wasting a lot of time on teaching the syntax. It is also possible to teach several different programming paradigms, e.g. functional, imperative and object oriented (message passing). Scheme is also a very small language and is therefore easy to learn. Hal Abelson's, Jerry Sussman's and Julie Sussman's Structure and Interpretation of Computer Programs (MIT Press, 1984; ISBN 0-262-01077-1), an excellent computer science text used in introductory courses at MIT is one of the bibles of the LISP/Scheme world. It is a good example of the usefulness of Scheme in teaching programming concepts. Also called Wizard Book because of the wizard on the jacket. Also, less commonly, known as the Purple Book.

Another online book is The Scheme Programming Language, 2nd Edition which is a introduction to Scheme, but not to programming in general.

Schemer.org is a very comprehensive web resource for all things Scheme.

Implementations

These days, there is a huge number of Scheme implementations available. Some people even say there are more implementations than Scheme programmers.

Guile

Guile is an interpreter for Scheme, packaged for use in a wide variety of environments. Guile implements Scheme as described in R5RS, providing clean and general data and control structures. Guile goes beyond the rather austere language presented in R5RS, extending it with

Guile can run interactively, reading expressions from the user, evaluating them, and displaying the results, or as a script interpreter, reading and executing Scheme code from a file. However, Guile is also packaged as an object library, allowing other applications to easily incorporate a complete Scheme interpreter. An application can use Guile as an extension language, a clean and powerful configuration language, or as multi-purpose "glue", connecting primitives provided by the application. It is easy to call Scheme code from C code and vice versa, giving the application designer full control of how and when to invoke the interpreter. Applications can add new functions, data types, control structures, and even syntax to Guile, creating a domain-specific language tailored to the task at hand, but based on a robust language design.

Guile's module system allows one to break up a large program into manageable sections with well-defined interfaces between them. Modules may contain a mixture of interpreted and compiled code; Guile can use either static or dynamic linking to incorporate compiled code. Modules also encourage developers to package up useful collections of routines for general distribution.

Last Modified: 2005-10-22 12:38:08
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