1.3. Typographical and Notational Conventions

Often, the value of a procedure or syntactic form is said to be unspecified. This means that an implementation is free to return any Scheme object as the value of the procedure or syntactic form. Do not count on this value being the same across implementations, the same across versions of the same implementation, or even the same across two uses of the procedure or syntactic form. Some Scheme systems routinely use a special object to represent unspecified values. Printing of this object is often suppressed by interactive Scheme systems, so that the values of expressions returning unspecified values are not printed.

Scheme expressions usually evaluate to a single value, although the multiple values mechanism described in Section 5.7 allows an expression to evaluate to zero or more than one value. To simplify the presentation, this book usually refers to the result of an expression as a single value even if the expression may in fact evaluate to zero or more than one value.

This book sometimes says "it is an error" or "an error will be signaled" when describing a circumstance in violation of the rules of Scheme. Something that is an error is not valid in Scheme, and the behavior of a Scheme implementation in such a case is not specified. A signaled error results in the invocation of an implementation-dependent error handler, which typically results in an error message being printed and a reset of the interactive programming system or entry into a debugging subsystem.

The typographic conventions used in this book are straightforward. All Scheme objects are printed in a typewriter typeface, just as they are to be typed at the keyboard. This includes syntactic keywords, variables, constant objects, Scheme expressions, and example programs. An italic typeface is used to set off syntax variables in the descriptions of syntactic forms and arguments in the descriptions of procedures. Italics are also used to set off technical terms the first time they appear. In general, names of syntactic forms and procedures are never capitalized, even at the beginning of a sentence. The same is true for syntax variables written in italics.

In the description of a syntactic form or procedure, a pattern shows the syntactic form or the application of the procedure. The syntax keyword or procedure name is given in typewriter font, as are parentheses. The remaining pieces of the syntax or arguments are shown in italics, using a name that implies the type of expression or argument expected by the syntactic form or procedure. Ellipses are used to specify zero or more occurrences of a subexpression or argument. For example, (or exp ...) describes the or syntactic form, which has zero or more subexpressions, and (member obj list) describes the member procedure, which expects two arguments, an object and a list.