1 Overview

This manual is a detailed description of the MIT/GNU Scheme runtime system. It is intended to be a reference document for programmers. It does not describe how to run Scheme or how to interact with it — that is the subject of the MIT/GNU Scheme User’s Manual.

This chapter summarizes the semantics of Scheme, briefly describes the MIT/GNU Scheme programming environment, and explains the syntactic and lexical conventions of the language. Subsequent chapters describe special forms, numerous data abstractions, and facilities for input and output.

Throughout this manual, we will make frequent references to standard Scheme, which is the language defined by the document Revised^4 Report on the Algorithmic Language Scheme, by William Clinger, Jonathan Rees, et al., or by IEEE Std. 1178-1990, IEEE Standard for the Scheme Programming Language (in fact, several parts of this document are copied from the Revised Report). MIT/GNU Scheme is an extension of standard Scheme.

These are the significant semantic characteristics of the Scheme language:

Variables are statically scoped

Scheme is a statically scoped programming language, which means that each use of a variable is associated with a lexically apparent binding of that variable. Algol is another statically scoped language.

Types are latent

Scheme has latent types as opposed to manifest types, which means that Scheme associates types with values (or objects) rather than with variables. Other languages with latent types (also referred to as weakly typed or dynamically typed languages) include APL, Snobol, and other dialects of Lisp. Languages with manifest types (sometimes referred to as strongly typed or statically typed languages) include Algol 60, Pascal, and C.

Objects have unlimited extent

All objects created during a Scheme computation, including procedures and continuations, have unlimited extent; no Scheme object is ever destroyed. The system doesn’t run out of memory because the garbage collector reclaims the storage occupied by an object when the object cannot possibly be needed by a future computation. Other languages in which most objects have unlimited extent include APL and other Lisp dialects.

Proper tail recursion

Scheme is properly tail-recursive, which means that iterative computation can occur in constant space, even if the iterative computation is described by a syntactically recursive procedure. With a tail-recursive implementation, you can express iteration using the ordinary procedure-call mechanics; special iteration expressions are provided only for syntactic convenience.

Procedures are objects

Scheme procedures are objects, which means that you can create them dynamically, store them in data structures, return them as the results of other procedures, and so on. Other languages with such procedure objects include Common Lisp and ML.

Continuations are explicit

In most other languages, continuations operate behind the scenes. In Scheme, continuations are objects; you can use continuations for implementing a variety of advanced control constructs, including non-local exits, backtracking, and coroutines.

Arguments are passed by value

Arguments to Scheme procedures are passed by value, which means that Scheme evaluates the argument expressions before the procedure gains control, whether or not the procedure needs the result of the evaluations. ML, C, and APL are three other languages that pass arguments by value. In languages such as SASL and Algol 60, argument expressions are not evaluated unless the values are needed by the procedure.

Scheme uses a parenthesized-list Polish notation to describe programs and (other) data. The syntax of Scheme, like that of most Lisp dialects, provides for great expressive power, largely due to its simplicity. An important consequence of this simplicity is the susceptibility of Scheme programs and data to uniform treatment by other Scheme programs. As with other Lisp dialects, the read primitive parses its input; that is, it performs syntactic as well as lexical decomposition of what it reads.