insurance.age-banding Unreviewed

export function ageBand(birthDate: string, inceptionDate: string, bands: readonly AgeBand[]): AgeBanding

/** One band of ages, inclusive at both ends. */
export interface AgeBand {
  readonly label: string;
  readonly minAge: number;
  /** null for no upper limit, on the last band only */
  readonly maxAge: number | null;
}

/** The age and its band. */
export interface AgeBanding {
  /** completed years on the inception date */
  readonly age: number;
  readonly label: string;
}

import { ageBand } from "#fune/insurance.age-banding@^1";

import { age } from "./dates_age.ts";  ← from dates.age ^1.0.0 · built alongside by fune
import { type AgeBand, type AgeBanding } from "./insurance_age_banding_types.ts";

/**
 * The policyholder's age on the inception date and the band it falls in.
 *
 * Age is completed years by dates.age, so a 29 February birthday ages on
 * 1 March in other years. The bands are checked before use: youngest first,
 * inclusive at both ends, no overlaps, and only the last may be open-ended.
 * An age no band covers is an error, not the nearest band.
 */
export function ageBand(birthDate: string, inceptionDate: string, bands: readonly AgeBand[]): AgeBanding {
  if (bands.length === 0) throw new RangeError("bands must not be empty");
  let previousMax = -1;
  for (let i = 0; i < bands.length; i++) {
    const band = bands[i];
    if (!Number.isInteger(band.minAge) || band.minAge < 0) {
      throw new RangeError(`minAge must be a whole number of 0 or more, received ${band.minAge}`);
    }
    const open = band.maxAge === null || band.maxAge === undefined;
    if (open && i !== bands.length - 1) {
      throw new RangeError(`only the last band may have no maxAge: "${band.label}" is not last`);
    }
    if (!open && (!Number.isInteger(band.maxAge) || (band.maxAge as number) < band.minAge)) {
      throw new RangeError(`maxAge must be a whole number no less than minAge in band "${band.label}"`);
    }
    if (band.minAge <= previousMax) {
      throw new RangeError(`bands must be youngest first and not overlap: "${band.label}" starts at ${band.minAge}`);
    }
    previousMax = open ? Number.MAX_SAFE_INTEGER : (band.maxAge as number);
  }
  const years = age(birthDate, inceptionDate);
  for (const band of bands) {
    const open = band.maxAge === null || band.maxAge === undefined;
    if (years >= band.minAge && (open || years <= (band.maxAge as number))) {
      return { age: years, label: band.label };
    }
  }
  throw new RangeError(`no band for age ${years}`);
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add insurance.age-banding

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./insurance.age-banding-1.0.1-typescript.fune, or fetch it from a terminal with fune pull insurance.age-banding@1.0.1:typescript.

The whole function, every language, is one file too: insurance.age-banding-1.0.1.fune, 21,490 bytes, sha256 4105698c1939caa7e55028e28b250af68328f96d6a2bf614fe066223725563b3. It installs into a project of any language.

// fune: before insurance.age-banding

// fune: after insurance.age-banding

// fune: replace dates.age in insurance.age-banding

// fune: step insurance.age-banding after <n|label>

More from the author

Completed years on `inceptionDate`, by `dates.age`: a birthday on the inception date counts, and a 29 February birthday ages on 1 March in other years (UK law), so someone born on 29 February 2004 is still 20 on 28 February 2025. The date is always passed in: insurers rate on the age at inception or renewal, never on the day the code runs, and a mid-term change is usually rated on the age at the start of the term.

## Bands

The band table is the insurer's, so it is an argument. Each band is inclusive at both ends in whole years; the last may have no upper limit (`maxAge` null). The table is checked first: youngest first, no overlaps, and only the last band open-ended. Gaps are allowed, and an age that falls in a gap, or below the first band, is an error naming the age rather than being put in the nearest band, because a quote for someone the table does not cover should stop, not be priced.

Look the label up in a factor table with `insurance.rating-factors`.

## Before you rely on this

**Not professional advice.** This capability calculates insurance figures from published rules. It is a software component for developers, not financial advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have an actuary review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified actuary has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

1.0.1 marks it unreviewed. The code and the tests are unchanged.

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