KindFn
import type { KindFn } from "https://github.gh-proxy.cn/raw.githubusercontent.com/baetheus/fun/main/fn.ts";Specifies Fn as a Higher Kinded Type, with covariant parameter A corresponding to the 0th index of any Substitutions and a contravariant parameter D corresponding to the 0th index of any Substititions. The Fn KindFn is unique in that it constrains the Fn type to taking a single argument for the purposes of type substitution while the implementations of Fn combinators such as map, flatmap, etc are mostly variadic (multiple arguments).