import { premap, number } from "./comparable.ts";
import { pipe } from "./fn.ts";

const dateToNumber = (d: Date): number => d.valueOf();
const date = pipe(number, premap(dateToNumber));

const now = new Date();
const alsoNow = new Date(Date.now());
const later = new Date(Date.now() + 60 * 60 * 1000);

const result1 = date.compare(now)(alsoNow); // true
const result2 = date.compare(now)(later); // false

Another use for premap with eq is to check for equality after normalizing data. In the following we can compare strings ignoring case by normalizing to lowercase strings.

import { premap, string } from "./comparable.ts";
import { pipe } from "./fn.ts";

const lowercase = (s: string) => s.toLowerCase();
const insensitive = pipe(
  string, // exact string compare
  premap(lowercase), // makes all strings lowercase
);

const result1 = insensitive.compare("Hello")("World"); // false
const result2 = insensitive.compare("hello")("Hello"); // true