Identifiers

Most identifiersidentifier 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:

$\begin{scheme} lambda q list->vector soup {+} V17a <=? a34kTMNs the-word-recursion-has-many-meanings\end{scheme}$

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

$\begin{scheme} + - . * / < = > !\ ?\ :\ \$ \% \char''5F{} \char''26{} \char''7E{} \char''5E % \end{scheme}$

See section $[*]$ for a formal syntax of identifiers.

Identifiers have several uses within Scheme programs:

Certain identifiers are reserved for use as syntactic keywords (see below).
Any identifier that is not a syntactic keyword may be used as a variable (see section $[*]$ ).
When an identifier appears as a literal or within a literal (see section $[*]$ ), it is being used to denote a symbol (see section $[*]$ ).

The following identifiers are syntactic keywords, and should not be used as variables:

$\begin{scheme} => do or and else quasiquote begin if quote case lambda set! cond let unquote define let* unquote-splicing delay letrec\end{scheme}$

Some implementations allow all identifiers, including syntactic keywords, to be used as variables. This is a compatible extension to the language, but ambiguities in the language result when the restriction is relaxed, and the ways in which these ambiguities are resolved vary between implementations.