1.2. Scheme Naming Conventions

Scheme's naming conventions are designed to provide a high degree of regularity. The following is a list of these naming conventions:

Predicate names end in a question mark ( ? ). Predicates are procedures that return a true or false answer, such as eq?, zero?, and string=?. The common numeric comparators =, <, >, <=, and >= are exceptions to this naming convention.
Type predicates, such as pair?, are created from the name of the type, in this case pair, and the question mark.
The names of most character, string, and vector procedures start with the prefix char-, string-, and vector-, e.g., string-append. (The names of some list procedures start with list-, but most do not.)
The names of procedures that convert an object of one type into an object of another type are written as type₁->type₂, e.g., vector->list.
The names of procedures and syntactic forms that cause side effects end with an exclamation point ( ! ). These include set! and vector-set!. Procedures that perform input or output technically cause side effects, but their names are exceptions to this rule.

Programmers should employ these same conventions in their own code whenever possible.