numspell.scm: Spelling Numbers as English in Scheme

by Neil Van Dyke <neil@neilvandyke.org>

Copyright © 2006 Neil Van Dyke. This program is Free Software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See <http://www.gnu.org/copyleft/lesser.html> for details. For other license options and consulting, contact the author.

Introduction

The numspell.scm library provides the ability to “spell” Scheme numbers in English. This is useful for writing numbers on banking checks and other legal documents, as well as for speech generation.

Most rational numbers in Scheme are presently supported. For example:

(number->english 123456) => "one hundred twenty-three thousand four hundred fifty-six" (number->english (/ 4 -6)) => "negative two over three" (number->english (exact->inexact (/ 4 -6))) => "negative zero point six six six six six six" ; approx.

The number names supported by numspell.scm are taken from a version of the Wikipedia “Names of large numbers” article. Both short and long scales are supported, through different procedures, with short scale being the default. For example:

(number->english (expt 10 15)) => "one quadrillion" (number->short-scale-english (expt 10 15)) => "one quadrillion" (number->long-scale-english (expt 10 15)) => "one thousand billion"

Note: Some numbers, such as very large and very small non-integers printed by some Scheme implementations in exponential notation, are not supported by the current version of numspell.scm.

numspell.scm requires R5RS, SRFI-6 (string ports), and SRFI-11 (let-values).

Interface

— Procedure: write-number-as-english num port
— Procedure: write-number-as-short-scale-english num port
— Procedure: write-number-as-long-scale-english num port

Spell number num to output port port. If num cannot be spelt, an error is signaled.

— Procedure: number->english num
— Procedure: number->short-scale-english num
— Procedure: number->long-scale-english num

Yield a string that spells number num. If num cannot be spelt, an error is signaled.

Tests

The numspell.scm test suite can be enabled by editing the source code file and loading Testeez.