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A modest proposal for operator overloading in JavaScript
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/* | |
It is a melancholy object-oriented language to those who walk through | |
this great town or travel in the country, when they see the JavaScript | |
programs crowded with method calls begging for more succinct syntax. | |
I think it is agreed by all parties, that whoever could find out a | |
fair, cheap and easy method of making these operators sound and useful | |
members of the language, would deserve so well of the publick, as | |
to have his statue set up for a preserver of the nation. I do therefore | |
humbly offer to publick consideration a pure JS library I've written | |
for overloading operators, which permits computations like these: | |
Complex numbers: | |
>> Complex()({r: 2, i: 0} / {r: 1, i: 1} + {r: -3, i: 2})) | |
<- {r: -2, i: 1} | |
Automatic differentiation: | |
Let f(x) = x^3 - 5x: | |
>> var f = x => Dual()(x * x * x - {x:5, dx:0} * x); | |
Now map it over some values: | |
>> [-2,-1,0,1,2].map(a=>({x:a,dx:1})).map(f).map(a=>a.dx) | |
<- [ 7, -2, -5, -2, 7 ] | |
i.e. f'(x) = 3x^2 - 5. | |
Polynoomials: | |
>> Poly()([1,-2,3,-4]*[5,-6]).map((c,p)=>''+c+'x^'+p).join(' + ') | |
<- "5x^0 + -16x^1 + 27x^2 + -38x^3 + 24x^4" | |
Big rational numbers (with concise syntax!): | |
// 112341324242 / 22341234124 + 334123124 / 5242342 | |
// > with(Rat){Rat()(n112341324242d22341234124 + n334123124d5242342).join(' / ')} | |
// "2013413645483934535 / 29280097495019602" | |
The implementation is only mildly worse than eating babies. | |
*/ | |
((global) => { | |
// Increase for more complicated fancier expressions | |
var numVars = 5; | |
var vars = Array.from(Array(numVars)).map((_,i)=>i); | |
var randoms = vars.map(() => Math.random()); | |
var table = {}; | |
// n is number of internal nodes | |
// f is a function to process each result | |
var trees = (n, f) => { | |
// h is the "height", thinking of 1 as a step up and 0 as a step down | |
// s is the current state | |
var enumerate = (n, h, s, f) => { | |
if (n === 0 && h === 0) { | |
f(s + '0'); | |
} else { | |
if (h > 0) { | |
enumerate(n, h - 1, s + '0', f) | |
} | |
if (n > 0) { | |
enumerate(n - 1, h + 1, s + '1', f) | |
} | |
} | |
}; | |
enumerate(n, 0, '', f); | |
}; | |
var toFunction = (s, pos, opCount, varCount) => { | |
if (s[pos] == '0') { | |
return [`x[${varCount}]`, pos + 1, opCount, varCount + 1]; | |
} | |
var left, right, op; | |
pos++; | |
op = `ops[${opCount}]`; | |
[left, pos, opCount, varCount] = toFunction(s, pos, opCount + 1, varCount); | |
[right, pos, opCount, varCount] = toFunction(s, pos, opCount, varCount); | |
return [`${op}(${left},${right})`, pos, opCount, varCount]; | |
}; | |
var add = (x,y) => x+y; add.toString = ()=>'+'; | |
var sub = (x,y) => x-y; sub.toString = ()=>'-'; | |
var mul = (x,y) => x*y; mul.toString = ()=>'*'; | |
var div = (x,y) => x/y; div.toString = ()=>'/'; | |
var round = (x) => { | |
var million = Math.pow(2,20); | |
return Math.floor(x * million) / million; | |
}; | |
var populate = (expr, ops, opCount) => { | |
if (ops.length == opCount) { | |
var result; | |
var order=[]; | |
var x = vars.map(y => ({ | |
valueOf:()=>{ | |
order.push(y); | |
return randoms[order.length - 1]; | |
} | |
})); | |
with ({ops, x}) { result = round(eval(expr)); } | |
table[result] = {ops: ops.map(x => '' + x), expr, order}; | |
} else { | |
populate(expr, ops.concat(add), opCount); | |
populate(expr, ops.concat(sub), opCount); | |
populate(expr, ops.concat(mul), opCount); | |
populate(expr, ops.concat(div), opCount); | |
} | |
}; | |
var allExprs = (s) => { | |
var [expr, , opCount, ] = toFunction(s, 0, 0, 0); | |
populate(expr, [], opCount); | |
}; | |
vars.forEach(x=>trees(x, allExprs)); | |
var makeContext = (constr, opTable) => () => { | |
// Set up values array | |
var V = []; | |
// Install temporary valueOf | |
var valueOf = constr.prototype.valueOf; | |
constr.prototype.valueOf = function () { | |
return randoms[V.push(this) - 1]; | |
}; | |
// Return function expecting key | |
return (key) => { | |
var {ops, expr, order} = table[round(+key)]; | |
constr.prototype.valueOf = valueOf; | |
var result; | |
var index = 0; | |
var W = []; | |
V.forEach((v, i) => W[order[i]] = V[i]); | |
with ({ops: ops.map(x => opTable[x]), x: W}) { result = eval(expr); } | |
V = []; | |
return result; | |
}; | |
}; | |
global.makeContext = makeContext; | |
})(this); | |
var Complex = makeContext(Object, { | |
'+': (x, y) => ({r: x.r + y.r, i: x.i + y.i}), | |
'-': (x, y) => ({r: x.r - y.r, i: x.i - y.i}), | |
'*': (x, y) => ({r: x.r * y.r - x.i * y.i, i: x.r * y.i + x.i * y.r}), | |
'/': (x, y) => { | |
const norm = y.r**2 + y.i**2; | |
return {r: (x.r * y.r + x.i * y.i) / norm, i: (x.i * y.r - x.r * y.i) / norm}; | |
} | |
}); | |
var Dual = makeContext(Object, { | |
'+': (a, b) => ({x: a.x + b.x, dx: a.dx + b.dx}), | |
'-': (a, b) => ({x: a.x - b.x, dx: a.dx - b.dx}), | |
'*': (a, b) => ({x: a.x * b.x, dx: a.x * b.dx + a.dx * b.x}), | |
'/': (a, b) => ({x: a.x / b.x, dx: (a.dx * b.x - a.x * b.dx) / (b.x ** 2)}) | |
}); | |
var Poly = makeContext(Array, { | |
'+': (a, b) => (a.length >= b.length ? a.map((x,i) => x + (b[i]?b[i]:0)) : b.map((x,i) => x + (a[i]?a[i]:0))), | |
'-': (a, b) => (a.length >= b.length ? a.map((x,i) => x - (b[i]?b[i]:0)) : b.map((x,i) => x - (a[i]?a[i]:0))), | |
'*': (a, b) => { | |
var result = []; | |
for (var i = 0; i < a.length; ++i) { | |
for (var j = 0; j < b.length; ++j) { | |
result[i+j] = result[i+j] ? result[i+j] : 0; | |
result[i+j] += a[i] * b[j]; | |
} | |
} | |
return result; | |
}, | |
'/': (a, b) => { throw new Error('Not implemented'); } | |
}); | |
var Str = new Proxy(makeContext(Array, { | |
'+':(a,b) => [a[0]+b[0]], | |
'*':(a,b) => [Array.from(Array(b[0])).map(x=>a[0]).join('')] | |
}), { | |
has (target, key) { return key.match(/^[a-z0-9A-Z_]+$/) && key !== 'Str'; }, | |
get(target, prop, receiver) { | |
if (typeof prop == 'string') { | |
return [prop]; | |
} else { | |
return target[prop]; | |
} | |
} | |
}); | |
function reduce(numerator,denominator) { | |
var gcd = function gcd(a,b){ | |
return b ? gcd(b, a%b) : a; | |
}; | |
gcd = gcd(numerator,denominator); | |
return [numerator/gcd, denominator/gcd]; | |
} | |
var numDenom = /^n([0-9]+)d([0-9]+)$/; | |
var Rat = new Proxy(makeContext(Array, { | |
'+':(a,b) => reduce(a[0]*b[1] + a[1]*b[0], a[1]*b[1]), | |
'-':(a,b) => reduce(a[0]*b[1] - a[1]*b[0], a[1]*b[1]), | |
'*':(a,b) => reduce(a[0]*b[0], a[1]*b[1]), | |
'/':(a,b) => reduce(a[0]*b[1], a[1]*b[0]) | |
}), { | |
has (target, key) { return !!key.match(numDenom); }, | |
get(target, prop, receiver) { | |
if (typeof prop == 'string') { | |
var m = prop.match(numDenom); | |
return reduce(BigInt(m[1]), BigInt(m[2])); | |
} else { | |
return target[prop]; | |
} | |
} | |
}); |
I make no claims as to the readiness of this code -- this seems to be a more recent attempt than I recall...
/*
* mathlib.js
*
*/
class Primitive extends Number {
static list = [];
static stack = [];
constructor(n) {
super(n ?? Math.random());
const parent = this.constructor;
parent.list.push(this);
}
valueOf() {
const parent = this.constructor;
parent.stack.push(this);
setTimeout(() => parent.stack.pop(), 1);
return this;
}
}
class Tuple extends Primitive {
constructor(...args) {
super();
var buffer = new ArrayBuffer(4 * args.length);
var matrix = new Float32Array(buffer);
matrix.__defineGetter__('length', () => args.length);
matrix.__defineSetter__('length', () => args.length);
}
parse(r, n, m, op) {
var primitives = Tuple.primitives = Tuple.primitives || [];
return (primitives.find(n => r == n)) ||
((n = primitives.find(n => m = primitives.find(m => op = findop(r, n, m)))) && n[op](m)) ||
((n = primitives.find(n => op = findopMatrix(r, n))) && n[op](m)) ||
((n = primitives.find(n => op = findopInt(r, n))) && n[op](m)) ||
((n = primitives.find(n => op = findopFloat(r, n))) && n[op](m)) ||
((n = r.constructor()) && (n.primitive = r) || n);
function findop(r, n, m) {
if (r == n * m) return 'times';
if (r == n / m) return 'over';
if (r == m / n) return 'into';
if (r == n + m) return 'plus';
if (r == n - m) return 'minus';
if (r == m - n) return 'from';
}
function findopMatrix(r, n) {
if (r == 0 - n) return 'neg';
if (r == 1 / n) return 'inv';
//if (r == n.adjugate) return 'adj';
//if (r == n.transpose) return 'trans';
//if (r == n.conjugate) return 'conj';
//if (r == n.transjugate) return 'transj';
}
function findopInt(r, n) {
if (r / n == (r / n) >> 0) return m = r / n, 'times';
if (n / r == (n / r) >> 0) return m = n / r, 'over';
if (n * r == (n * r) >> 0) return m = n * r, 'into';
if (r - n == (r - n) >> 0) return m = r - n, 'plus';
if (n - r == (n - r) >> 0) return m = n - r, 'minus';
if (n + r == (n + r) >> 0) return m = n + r, 'from';
}
function findopFloat(r, n) {
if (r.insignificand(7) == (r - n).insignificand(7)) return m = (r - n).significand(7), 'plus';
if (r.insignificand(7) == (n - r).insignificand(7)) return m = (n - r).significand(7), 'minus';
if (r.insignificand(7) == (n + r).insignificand(7)) return m = (n + r).significand(7), 'from';
}
}
}
class Scalar extends Tuple {
constructor(...args) {
super(...args.slice(0, 1));
this.order = 1;
Object.defineProperties(this, {
transpose: {get: () => Scalar(this[0])},
});
}
}
class Vector extends Tuple {
constructor(...args) {
super(...args.slice(0, 2));
this.order = 2;
Object.defineProperties(this, {
transpose: {get: (b) => (b = Matrix(2,1)) && b.set(a) || b},
});
}
}
class Matrix2 extends Tuple {
constructor(...args) {
super(...args.slice(0, 4));
this.order = 2;
//this.primitive = arguments[4] || a.primitive;
}
tr = () => this[0] + this[3]; //trace
det = () => Math.abs(this[0] * this[3] - this[1] * this[2]); //determinant
adj = () => Matrix2(this[3], -this[1], -this[2], this[0]); //adjugate
inv = () => Tuple.parse(this.adj / this.det); //-1 = inverse
trans = () => Matrix2(this[0], this[2], this[1], this[3]); //T = transpose
conj = () => Matrix2(this[0], -this[1], -this[2], this[3]); //conjugate
tranj = () => Matrix2(this[0], -this[2], -this[1], this[3]); //H = transjugate
neg = () => Matrix2(-this[0], -this[1], -this[2], -this[3]); //(-) = negative
exp = () => this.det - this.tr * this.tr / 4; //? = exponent
times = (b) => Matrix2(Matrix2['*'][b.constructor.name](this, b));
over = (b) => Matrix2(Matrix2['*'][b.constructor.name](this, 1 / b));
into = (b) => Matrix2(Matrix2['*'][b.constructor.name](this.inv, b));
dot = (b) => Matrix2(Matrix2['*'][b.constructor.name](this.trans, b));
plus = (b) => Matrix2(Matrix2['+'][b.constructor.name](this, b));
minus = (b) => Matrix2(Matrix2['+'][b.constructor.name](this, -b));
from = (b) => Matrix2(Matrix2['+'][b.constructor.name](this.neg, b));
['*'] = {
Number: (a, b) => [
a[0] * b,
a[1] * b,
a[2] * b,
a[3] * b
],
Scalar: (a, b) => [
a[0] * b[0],
a[1] * b[0],
a[2] * b[0],
a[3] * b[0]
],
Vector: (a, b) => [
a[0] * b[0],
a[1] * b[0],
a[2] * b[1],
a[3] * b[1]
],
Matrix2: (a, b) => [
a[0] * b[0] + a[2] * b[1],
a[1] * b[0] + a[3] * b[1],
a[2] * b[3] + a[0] * b[2],
a[3] * b[3] + a[1] * b[2]
],
};
['+'] = {
Number: (a, b) => [
a[0] + b,
a[1],
a[2],
a[3] + b
],
Scalar: (a, b) => [
a[0] + b[0],
a[1],
a[2],
a[3] + b[0]
],
Vector: (a, b) => [
a[0] + b[0],
a[1] + b[1],
a[2] + b[1],
a[3] + b[0]
],
Matrix2: (a, b) => [
a[0] + b[0],
a[1] + b[1],
a[2] + b[2],
a[3] + b[3]
],
};
_exp() {
a.delta = () => sqrt((a[0] - a[3]) * (a[0] - a[3]) + (4 * a[1] * a[2]));
a.exp = () => Matrix2(a.exp_0, a.exp_1, a.exp_2, a.exp_3) / a.delta;
a.exp[1] = () => Math.exp((a[0] + a[3]) / 2) * (2 * a[1]) * Math.sinh(a.delta / 2);
a.exp[2] = () => Math.exp((a[0] + a[3]) / 2) * (2 * a[2]) * Math.sinh(a.delta / 2);
a.exp[3] = () => Math.exp((a[0] + a[3]) / 2) * (a.delta * Math.cosh(a.delta / 2) - (a[0] - a[3]) * Math.sinh(a.delta / 2));
a.exp[0] = () => Math.exp((a[0] + a[3]) / 2) * (a.delta * Math.cosh(a.delta / 2) + (a[0] - a[3]) * Math.sinh(a.delta / 2));
a.exp_est = () => Math.exp((a[0] + a[3]) / 2) * Matrix2((1 + (a[0] - a[3]) / 2), a[1], a[2], (1 - (a[0] - a[3]) / 2));
}
}
const l = new Lookup();
class Matrix3 extends Tuple {
constructor() {
super(...args.slice(0, 9));
a.trace = () => (a[0] + a[3]);
a.determinant = () => Math.abs(0
+ a[0] * a[4] * a[8] + a[1] * a[5] * a[6] + a[2] * a[3] * a[7]
- a[2] * a[4] * a[6] - a[1] * a[3] * a[8] - a[0] * a[5] * a[7]);
a.transpose = () => Matrix2(a[0], a[1], a[2], a[3]); //TODO
a.adjoint = () => Matrix2(a[3], -a[2], -a[1], a[0]); //TODO
a.negative = () => Matrix2(-a[0], -a[2], -a[1], -a[3]); //TODO
a.inverse = () => Tuple.parse(a.determinant() * a.adjoint());
//constant.call(window);
}
}
class MatrixSquare extends Tuple {
constructor() {
super(...args.slice(0));
var a = new Tuple([].concat.apply([], arguments));
a.constructor = Matrix2;
a.trace = () => [].reduce.call(a, (t,e,i) => t += i % (a.order + 1) ? 0 : e);
a.minor = (y,z) => ((z = y / a.order | 0), (y = y % a.order), [].filter.call(a, (x,i,j) => ((j = i / a.order | 0), (i = i % a.order), i != y && j != z)));
a.cofactor = (y,z) => ((z = y / a.order | 0), (y = y % a.order), [].filter.call(a, (x,i,j) => ((j = i / a.order | 0), (i = i % a.order), i != y && j != z)));
a.determinant = () => [].reduce.call(a, (t,e,i) => i > a.order ? t : a.minor(i).times(i % 2 ? -e : e));
//constant.call(window);
}
}
function MatrixDiagonal() {
a.exp = () => MatrixDiag(Math.pow(ê, a[0]), Math.pow(ê, a[3]));
}
function NaturalNumber(n) {
var buffer = new ArrayBuffer(4);
var natural = new Int32Array(buffer);
natural.constructor = NaturalNumber;
natural.set(n);
return natural;
}
function IntegralNumber(n) {
var buffer = new ArrayBuffer(4);
var integer = new Uint32Array(buffer);
integer.constructor = IntegralNumber;
integer.set(n);
return integer;
}
function RationalNumber(n, d) {
var rational = ε.times(Number(n));
infinitesimal.constructor = InfinitesimalNumber;
return infinitesimal;
}
function ValentNumber() {
var a = new Tuple([].concat.apply([], arguments));
a.valueOf = () => ([].unshift.call(a, [].slice.call(a, -1)), a[0]);
a.constructor = ValentNumber;
return a;
}
function DuplexNumber(n, m) {
var infinitesimal = ε.times(Number(n));
infinitesimal.constructor = InfinitesimalNumber;
return infinitesimal;
}
function ContinuedFractionNumber() {
var rational = ε.times(Number(n));
infinitesimal.constructor = InfinitesimalNumber;
return infinitesimal;
}
function RealNumber(n) {
var buffer = new ArrayBuffer(4);
var real = new Float32Array(buffer);
real.constructor = RealNumber;
real.set(n);
return real;
}
function ComplexNumber(n, m) {
//nn = n; nm = mn = m; mm = -n;
var imaginary = î.times(Number(n));
imaginary.constructor = ImaginaryNumber;
return imaginary;
}
function QuaternionNumber(n, m1, m2, m3) {
var e = new Matrix2(1, 0, 0, 1);
var i = new Matrix2(0, 1,-1, 0);
var j = new Matrix2(0, 1,-1, 0);
var k = new Matrix2(0, 1,-1, 0);
var quaternion = new Tuple(n * e, m1 * i, m2 * j, m3 * k);
//nn = n; nm = mn = m; mm = -n; m1m2 = -m2m1 = m3; m1m2m3 = -n;
quaternion.constructor = QuaternionNumber;
quaternion.conj = () => e - i - j - k;
quaternion.scalar = (a) => (a + a.conj) / 2;
quaternion.vector = (a) => (a - a.conj) / 2;
quaternion.norm = (a) => sqrt(a * a.conj);
quaternion.det = (a) => sqrt(a * a.conj);
quaternion.plus = (a, b) => [
a[0] + b[0],
a[1] + b[1],
a[2] + b[2],
a[3] + b[3]
];
quaternion.times = (a, b) => [
a[0] * b[0] - a[1] * b[1] - a[2] * b[2] - a[3] * b[3],
a[0] * b[1] + a[1] * b[0] + a[2] * b[3] - a[3] * b[2],
a[0] * b[2] - a[1] * b[3] + a[2] * b[0] - a[3] * b[1],
a[0] * b[3] + a[1] * b[2] - a[2] * b[1] + a[3] * b[0]
];
quaternion.vector.dot = (a, b) => a[1] * b[1] + a[2] * b[2] + a[3] * b[3];
quaternion.vector.cross = (a, b) => (1) + (2) + (3);
return quaternion;
}
function OctonionNumber(n, m1, m2, m3, m4, m5, m6, m7) {
//nn = n; nm = mn = m; mm = -n; m3m4 = -m4m3 = m7
//conjugate = n - m1 - m2 - m3 - m4 - m5 - m6 - m7;
var octonion = î.times(Number(m1));
octonion.constructor = OctonionNumber;
return octonion;
}
function SedenionNumber(n, m1, m2, m3, m4, m5, m6, m7, m8, m9, mA, mB, mC, mD, mE, mF) {
//nn = n; nm = mn = m; mm = -n; m7m8 = -m8m7 = mF; m1(m2m3) = -(m1m2)m3;
var sedenion = î.times(Number(n));
sedenion.constructor = SedenionNumber;
return sedenion;
}
function PerplexNumber(n, m) {
var unitesimal = ĵ.times(Number(n));
unitesimal.constructor = UnitesimalNumber;
return unitesimal;
}
function CoQuaternionNumber(n, m1, m2, m3) {
//conjugate = w − xi − yj − zk
var coquaternion = î.times(Number(n));
coquaternion.constructor = CoQuaternionNumber;
return coquaternion;
}
const Ω = 1/0;
const Ʊ = 0/0;
const î = new Matrix2(0, 1,-1, 0, 0.20787957635076190854695561983497877003387784163176960807); //imaginary, sqrt(-1)
const ĵ = new Matrix2(0, 1, 1, 0, 0.99999999999999994448884876874217297881841659545898437499); //unitesimal, sqrt(1)
const ε = new Matrix2(0, 1, 0, 0, 1.57172778470262867296192256111765622564011176170918094545e-162); //infinitesimal, sqrt(0)
const ω = new Matrix2(0, 1, Ω, 0, 1.79769313486231580793728971405303415079934132710037826936e+308); //infinite, sqrt(1/0)
const I = new Matrix2(1, 0, 0, 1, 1.0); //IdentityMatrix
const Ø = new Matrix2(0, 0, 0, 0, 0.0); //ZeroMatrix
const δ = 0.00000000000000011102230246251565404236316680908203125001; //Number.EPSILON/2 ~ 2^-53
const π = 3.14159265358979323846264338327950288419716939937510582097;
const ℮ = 2.71828182845904523536028747135266249775724709369995957496;
const φ = 1.61803398874989484820458683436563811772030917980576286213;
const γ = 0.57721566490153286060651209008240243104215933593992359880;
const ℕ = NaturalNumber;
const ℤ = IntegralNumber;
const ℚ = RationalNumber;
const ℝ = RealNumber;
const ℂ = ComplexNumber;
const ℍ = QuaternionNumber;
const ℙ = PerplexNumber;
const SQRT2 = new ValentNumber(
1.41421356237309504880168872420969807856967187537694807317,
1.41421356237309481240771447119186632335186004638671875000);
const SQRT3 = new ValentNumber(
1.73205080756887729352744634150587236694280525381038062805,
1.73205080756887724868775535469467286020517349243164062501);
const SQRT5 = new ValentNumber(
2.23606797749978969640917366873127623544061835961152572427,
2.23606797749978956912908500953562906943261623382568359375);
const SQRT7 = new ValentNumber(
2.64575131106459059050161575363926042571025918308245018036,
2.64575131106459043861534041752747725695371627807617187501);
const SQRT11 = new ValentNumber(
3.31662479035539984911493273667068668392708854558935359705,
3.31662479035539984911493273667068668392708854558935359704);
const SQRT13 = new ValentNumber(
3.60555127546398929311922126747049594625129657384524621271,
3.60555127546398929311922126747049594625129657384524621270);
const SQRT17 = new ValentNumber(
4.12310562561766054982140985597407702514719922537362043439,
4.12310562561766054982140985597407702514719922537362043438);
const SQRT19 = new ValentNumber(
4.35889894354067355223698198385961565913700392523244493689,
4.35889894354067355223698198385961565913700392523244493688);
const RCPR3 = new ValentNumber(
0.33333333333333333333333333333333333333333333333333333333,
0.33333333333333333333333333333333333333333333333333333333,
0.33333333333333333333333333333333333333333333333333333334);
const RCPR7 = new ValentNumber(
0.14285714285714285714285714285714285714285714285714285715,
0.14285714285714285814285714285714285714285714285714285714,
0.14285714285714285914285714285714285714285714285714285715);
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Awesome, I had done something similar years ago, and was just thinking that modern js may make some of that easier. I will include a link to it if I can find it -- hopefully will provide some inspiration as yours has for me.
One of the uses I found was for something I was calling ValentNumbers, where a number can have multiple values. Useful to define a SQRT2, which alternates between [Math.sqrt(2), 2/Math.sqrt(2)] so that: SQRT2 * SQRT2 == 2 (instead of 2.000000004). Or INV7, which alternates between [1/7, 1/7, (1-6/7)].
A few thoughts: