chainableAction.js

import { actions } from 'tarnish';

function handleResponse(form) {
  return function ({ token, errors }) {
    if (errors) {
      return actions.form.validationErrors(form, errors);
    } else {
      return actions.state.merge({ page: 'home#Index', token });
    }
  };
}

export function createSession(form) {
  return actions.chain(actions.http.post('https://api/sessions.json', form.values),
                       handleResponse(form));
}

proceeduralAction.js

import { actions } from 'tarnish';

export function createSession(form) {
  const { errors, token } = actions.http.post('https://api/sessions.json', form.values);
  
  if (errors.length) {
    return actions.form.validationErrors(form, errors);
  } else {
    return actions.state.merge({ page: 'home#Index', token });
  }
}