Chapter 2

Lexical conventions

This section gives an informal account of some of the lexical conventions used in writing Scheme programs. For a formal syntax of Scheme, see section 7.1.

Upper and lower case forms of a letter are never distinguished except within character and string constants. For example, Foo is the same identifier as FOO, and #x1AB is the same number as #X1ab.

2.1  Identifiers

Most identifiers allowed by other programming languages are also acceptable to Scheme. The precise rules for forming identifiers vary among implementations of Scheme, but in all implementations a sequence of letters, digits, and ``extended alphabetic characters'' that begins with a character that cannot begin a number is an identifier. In addition, +, -, and ... are identifiers. Here are some examples of identifiers:

lambda                   q
list->vector             soup
+                        V17a
<=?                      a34kTMNs

Extended alphabetic characters may be used within identifiers as if they were letters. The following are extended alphabetic characters:

! $ % & * + - . / : < = > ? @ ^ _ ~ 

See section 7.1.1 for a formal syntax of identifiers.

Identifiers have two uses within Scheme programs:

2.2  Whitespace and comments

Whitespace characters are spaces and newlines. (Implementations typically provide additional whitespace characters such as tab or page break.) Whitespace is used for improved readability and as necessary to separate tokens from each other, a token being an indivisible lexical unit such as an identifier or number, but is otherwise insignificant. Whitespace may occur between any two tokens, but not within a token. Whitespace may also occur inside a string, where it is significant.

A semicolon (;) indicates the start of a comment. The comment continues to the end of the line on which the semicolon appears. Comments are invisible to Scheme, but the end of the line is visible as whitespace. This prevents a comment from appearing in the middle of an identifier or number.

;;; The FACT procedure computes the factorial
;;; of a non-negative integer.
(define fact
  (lambda (n)
    (if (= n 0)
        1        ;Base case: return 1
        (* n (fact (- n 1))))))

2.3  Other notations

For a description of the notations used for numbers, see section 6.2.