A first stab at combining measure, decimal, and normalized decimal data types

Decimal128.d.mts

export declare class Decimal128 {
    constructor(value: string | number | bigint | NormalizedDecimal128);

    // predicates
    isFinite(): boolean;
    isNegative(): boolean;
    isNaN(): boolean;
    isZero(): boolean;

    // comparison
    equals(other: Decimal128): boolean; // mathematical equality
    lessThan(other: Decimal128): boolean;

    // serialization
    toJSON(): string;
    toString(): string;
    toFixed(opts?: {digits?: number}): string;
    toLocaleString(locale?: string): string;
    toPrecision(opts?: {digits?: number}): string;

    // no arithmetic

    // no rounding

    // conversion
    toNumber(): number;
    toBigInt(): bigint;
    toNormalizedDecimal(): NormalizedDecimal128; // or just "normalize"?
}

Measure.d.mts

type ExpandedNumber = string // digit strings, not arbitrary strings
    | number
    | bigint
    | Decimal128
    | NormalizedDecimal128;

export declare class Measure {
    constructor(value: ExpandedNumber, unit?: string, precision?: number);
    convertTo(unit: string, precision?: number): Measure;
    toString(): string;
    toComponents(): {value: ExpandedNumber, unit: string}[];
    getValue(): Decimal128;
}

NormalizedDecimal128.d.mts

export declare class NormalizedDecimal128 { // or just "Decimal"?
    constructor(x: string | number | bigint | Decimal128);

    // predicates
    isFinite(): boolean;
    isNegative(): boolean;
    isNaN(): boolean;
    isZero(): boolean;

    // comparison
    equals(other: NormalizedDecimal128): boolean;
    lessThan(other: NormalizedDecimal128): boolean;

    // arithmetic
    abs(): NormalizedDecimal128;
    add(x: NormalizedDecimal128): NormalizedDecimal128;
    divide(x: NormalizedDecimal128): NormalizedDecimal128;
    multiply(x: NormalizedDecimal128): NormalizedDecimal128;
    negate(): NormalizedDecimal128;
    subtract(x: NormalizedDecimal128): NormalizedDecimal128;

    // serlialization
    toJSON(): string;
    toString(): string;
    toFixed(opts?: {digits?: number}): string;
    toLocaleString(locale?: string): string;
    toPrecision(opts?: {digits?: number}): string;

    // rounding
    round(numFractionalDigits: number, mode: RoundingMode): NormalizedDecimal128;

    // conversion

    toDecimal128(): Decimal128;
    toNumber(): number;
    toBigInt(): bigint;
}

the absence of arithmetic support in the Decimal128 class reflects two discussions:

• the main driver for support for full Decimal128 comes mainly from i18n use cases, where arithmetic needs seem to be largely absent. I've kept equals and lessThan in there, but perhaps those could also be dropped (or maybe lessThan, keeping equals).
• the lack of a good theory for what the rules for quantum preservation should be for this class of values