/**
 * @module lib/compose.js
 */

/**
 * Identity function
 *
 * @param   {any} arg An input
 * @returns {any}     The original input
 * @private
 */
const identity = arg => arg;

/**
 * Check if function is promise
 *
 * @param   {function} fn Function to check
 * @returns {boolean}     Whether function is promise
 * @private
 */
const isPromise = fn => {
  return fn != null
    && typeof fn === 'object'
    && typeof fn.then === 'function'
    && typeof fn.catch === 'function';
};

/**
 * Create composed function
 *
 * @param   {function}    first First function
 * @param   {...function} rest  Rest of the functions
 * @returns {function}          Composed function
 */
export default (first = identity, ...rest) => {
  return (...args) => {
    return rest.reduce(
      (computed, fn) => {
        return isPromise(computed)
          ? computed.then(fn)
          : fn(computed);
      },
      first(...args)
    );
  };
};