1.1. Scheme Syntax

Scheme programs are made up of keywords, variables, structured forms, constant data (numbers, characters, strings, quoted vectors, quoted lists, quoted symbols, etc.), whitespace, and comments.

Keywords, variables, and symbols are collectively called identifiers. Identifiers may be formed from the following set of characters:

the lower-case letters a through z,
the upper-case letters A through Z,
the digits 0 through 9, and
the characters ? ! . + - * / < = > : $ % ∘ & _ ~ @.

Identifiers normally cannot start with any character that may start a number, i.e., a digit, plus sign ( + ), minus sign ( − ), or decimal point ( . ). Exceptions are +, −, and ..., which are valid identifiers. For example, hi, Hello, n, x, x3, and ?$&*!!! are all identifiers. Identifiers must be delimited by whitespace, parentheses, a string (double) quote ( " ), or the comment character ( ; ). All implementations must recognize as identifiers any sequences of characters that adhere to these rules. Other sequences of characters, such as -1234a, that do not represent numbers or other syntactic entities may be recognized as identifiers in some implementations, although it is best to avoid such identifiers in code that may need to run in more than one Scheme system.

There is no inherent limit on the length of a Scheme identifier; programmers may use as many characters as necessary. Long identifiers are no substitute for comments, however, and frequent use of long identifiers can make a program difficult to format and consequently difficult to read.

Identifiers may be written in any mix of upper-case and lower-case letters. The case is not important, in that two identifiers differing only in case are identical. For example, abcde, Abcde, AbCdE, and ABCDE all refer to the same identifier. Scheme systems typically print an identifier in either all upper-case or all lower-case letters regardless of the way it is entered.

Structured forms and list constants are enclosed within parentheses, e.g., (a b c) or (* (- x 2) y). The empty list is written (). Some implementations permit the use of brackets ( [ ] ) in place of parentheses, and brackets are sometimes used to set off particular subexpressions for readability.

The boolean values representing true and false are written as #t and #f. Scheme conditional expressions actually treat #f as false and all other objects as true, so 3, (), "false", and nil all count as true.

Vectors are written similarly to lists, except that they are preceded by #( and terminated by ), e.g., #(this is a vector of symbols). Strings are enclosed in double quotation marks, e.g., "I am a string". Characters are preceded by #\, e.g., #\a. Case is important within character and string constants, unlike within identifiers. Numbers may be written as integers, e.g., -123, as ratios, e.g., 1/2, in oating-point or scientific notation, e.g., 1.3 or 1e23, or as complex numbers in rectangular or polar notation, e.g., 1.3-2.7i or -1.2@73. Details of the syntax for each type of constant data are given in the individual sections of Chapter 6 and in the formal syntax of Scheme given in the back of the book.

Scheme expressions may span several lines, and no explicit terminator is required. Since the number of whitespace characters (spaces and newlines) between expressions is not significant, Scheme programs are normally indented to show the structure of the code in a way that is pleasing to the author of the program. Comments may appear on any line of a Scheme program, between a semicolon ( ; ) and the end of the line. Comments explaining a particular Scheme expression are normally placed at the same indentation level as the expression, on the line before the expression. Comments explaining a procedure or group of procedures are normally placed before the procedures, without indentation. Multiple comment characters are often used to set off the latter kind of comment, e.g., ;;; The following procedures ....