Typescript ifThen pattern

ifThen.ts

import { curry } from 'lodash';

export type Predicate = () => boolean;
export type Func<a> = () => a;

export interface ThenElse<T = void, E = T> {
  t: Func<T>,
  e: Func<E>
}

export const makeThenElse = <T = void, E = T>(t: Func<T>, e: Func<E>): ThenElse<T, E> => ({ t, e });

// Some type predicates
export const isFunc = (a: Function | any): a is Function => typeof a === 'function';
export const isBool = (c: boolean | Predicate | any): c is boolean => typeof c === 'boolean';
export const isNum = (n?: number): n is number => !!n;

const _ifThen = <T, E = T>(cond: boolean, o: ThenElse<T>): T => cond ? o.t() : o.e();
export const ifThen = curry((cond: boolean | Predicate, o: ThenElse): void =>
  isBool(cond)
    ? _ifThen(cond, o)
    : _ifThen(cond(), o));

export const ifNumThen = (n: number | undefined, t: Func<number>): void =>
  ifThen(isNum(n), makeThenElse(t, () => null));