Mathematically, numbers are arranged into a tower of subtypes in which each level is a subset of the level above it:
number complex number real number rational number integer
For example, 3 is an integer. Therefore 3 is also a rational, a real, and
a complex number. The same is true of the Scheme numbers that model 3. For
Scheme numbers, these types are defined by the predicates number?,
complex?, real?, rational?, and integer?.
There is no simple relationship between a number’s type and its representation inside a computer. Although most implementations of Scheme will offer at least two different representations of 3, these different representations denote the same integer.
Scheme’s numerical operations treat numbers as abstract data, as independent of their representation as possible. Although an implementation of Scheme may use multiple internal representations of numbers, this ought not to be apparent to a casual programmer writing simple programs.