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Numeric­Tower

cowan
2010-09-07 21:20:00
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The numeric tower

This is an analysis of the R5RS provision that the full numeric tower may be subsetted. Four boolean feature settings characterize different kinds of numeric towers: exactness-preserving, ratios, inexact, and complex, which refer to the closure of exact numbers under rational operations (except /), exact non-integer values, inexact rationals, and non-real numbers, respectively. A priori, any combination of these feature groups might be implemented, except that ratios makes no sense without exactness-preserving. I also assume that complex makes no sense without either ratios or inexact: Gaussian integers by themselves are of little use.

So here are ten possible numeric towers. I use + if a feature is present and - if it is absent, and give the numeric properties of the tower is.

exactness-preserving

ratios

inexact

complex

Description

-

-

-

-

A "toy" Scheme with fixnums (bounded exact integers) only. Appropriate where no numerical work will be done.

-

-

+

-

A Scheme where all numbers are real and all arithmetic happens in essentially constant time. Most non-Lisp languages are like this, except that their fixnums wrap instead of switching to flonums. Chicken uses this.

-

-

+

+

The complex-number equivalent of the preceding. Non-real numbers are typically inexact/inexact.

+

-

-

-

Unlimited-precision integers only. Possibly usable for crypto work. Fortunately, ratios are fairly cheap given bignums.

+

-

+

-

A good all-round compromise, providing exact integers and inexact real numbers. ISLisp uses this. Many people seem to like this design.

+

-

+

+

The same as the preceding, but with inexact/inexact non-real numbers as well.

+

+

-

-

Exact numbers only. Reasonable for some purposes, but will often run very slowly without inexact support, so unsuitable for scientific-type work.

+

-

+

+

The same as the preceding, but with exact general complex numbers. Even less suitable for scientific-type work, given that in practice non-real numbers are usually inexact/inexact.

+

+

+

-

Full numeric tower for real numbers only.

+

+

+

+

Full numeric tower. R6RS requires this.