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By convention, ‘?’ is the final character of the names of procedures that always return a boolean value. Such procedures are called predicates. Predicates are generally understood to be side-effect free, except that they may raise an exception when passed the wrong type of argument.
Similarly, ‘!’ is the final character of the names of procedures that store values into previously allocated locations (Storage model). Such procedures are called mutation procedures. The value returned by a mutation procedure is unspecified.
By convention, ‘->’ appears within the names of procedures that
take an object of one type and return an analogous object of another
type. For example, list->vector takes a list and returns a
vector whose elements are the same as those of the list.
A command is a procedure that does not return useful values to its continuation.
A thunk is a procedure that does not accept arguments.