3.3.2 Exactness

It is useful to distinguish between number objects that are known to correspond to a number exactly, and those number objects whose computation involved rounding or other errors. For example, index operations into data structures may need to know the index exactly, as may some operations on polynomial coefficients in a symbolic algebra system. On the other hand, the results of measurements are inherently inexact, and irrational numbers may be approximated by rational and therefore inexact approximations. In order to catch uses of numbers known only inexactly where exact numbers are required, Scheme explicitly distinguishes exact from inexact number objects. This distinction is orthogonal to the dimension of type.

A number object is exact if it is the value of an exact numerical literal or was derived from exact number objects using only exact operations. Exact number objects correspond to mathematical numbers in the obvious way.

Conversely, a number object is inexact if it is the value of an inexact numerical literal, or was derived from inexact number objects, or was derived using inexact operations. Thus inexactness is contagious.

Exact arithmetic is reliable in the following sense: If exact number objects are passed to any of the arithmetic procedures described in Propagation of exactness and inexactness, and an exact number object is returned, then the result is mathematically correct. This is generally not true of computations involving inexact number objects because approximate methods such as floating–point arithmetics may be used, but it is the duty of each implementation to make the result as close as practical to the mathematically ideal result.