composableListOfFunctions.ts

type Func<A = any, B = any> = (arg: A) => B;
type OkFuncList<L> = L extends [
  Func<infer A1, infer B1>,
  Func<infer A2, infer B2>,
  ...infer Rest
]
  ? B1 extends A2
    ? [Func<A1, B1>, ...OkFuncList<[Func<A2, B2>, ...Rest]>]
    : never
  : L extends [Func<any, any>]
  ? L
  : never;

type FirstType<L> = L extends [Func<infer A1, any>, ...infer _] ? A1 : never;
type LastType<L> = L extends [...infer _, Func<any, infer B1>] ? B1 : never;

const composeFunctions = <L>(
  fns: OkFuncList<L>
): ((arg: FirstType<L>) => LastType<L>) => {
  return (arg) => fns.reduce<any>((x, fn) => fn(x), arg);
};
const fn: (arg: string) => number[] = composeFunctions([
  (arg: string) => arg.length,
  (arg: number) => [arg],
]);