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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.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 ....
