3.1.2 Overview

This chapter gives an overview of Scheme’s semantics. The purpose of this overview is to explain enough about the basic concepts of the language to facilitate understanding of the subsequent chapters of the report, which are organized as a reference manual. Consequently, this overview is neither a complete introduction to the language, nor precise in all respects, nor normative in any way.

Following Algol, Scheme is a statically scoped programming language. Each use of a variable is associated with a lexically apparent binding of that variable.

Scheme has latent as opposed to manifest types. Types are associated with objects (also called values) rather than with variables. (Some authors refer to languages with latent types as untyped, weakly typed or dynamically typed languages.) Other languages with latent types are Python, Ruby, Smalltalk, and other dialects of Lisp. Languages with manifest types (sometimes referred to as strongly typed or statically typed languages) include Algol 60, C, C#, Java, Haskell, and ML.

All objects created in the course of a Scheme computation, including procedures and continuations, have unlimited extent. No Scheme object is ever destroyed. The reason that implementations of Scheme do not (usually!) run out of storage is that they are permitted to reclaim the storage occupied by an object if they can prove that the object cannot possibly matter to any future computation. Other languages in which most objects have unlimited extent include C#, Java, Haskell, most Lisp dialects, ML, Python, Ruby, and Smalltalk.

Implementations of Scheme must be properly tail–recursive. This allows the execution of an iterative computation in constant space, even if the iterative computation is described by a syntactically recursive procedure. Thus with a properly tail–recursive implementation, iteration can be expressed using the ordinary procedure–call mechanics, so that special iteration constructs are useful only as syntactic sugar.

Scheme was one of the first languages to support procedures as objects in their own right. Procedures can be created dynamically, stored in data structures, returned as results of procedures, and so on. Other languages with these properties include Common Lisp, Haskell, ML, Ruby, and Smalltalk.

One distinguishing feature of Scheme is that continuations, which in most other languages only operate behind the scenes, also have “first–class” status. First–class continuations are useful for implementing a wide variety of advanced control constructs, including non–local exits, backtracking, and coroutines.

In Scheme, the argument expressions of a procedure call are evaluated before the procedure gains control, whether the procedure needs the result of the evaluation or not. C, C#, Common Lisp, Python, Ruby, and Smalltalk are other languages that always evaluate argument expressions before invoking a procedure. This is distinct from the lazy–evaluation semantics of Haskell, or the call–by–name semantics of Algol 60, where an argument expression is not evaluated unless its value is needed by the procedure.

Scheme’s model of arithmetic provides a rich set of numerical types and operations on them. Furthermore, it distinguishes exact and inexact number objects: Essentially, an exact number object corresponds to a number exactly, and an inexact number object is the result of a computation that involved rounding or other errors.