How to notify users from within a module I'm building a custom module for a project and would like to display a notification/warning to the user once something happens. For instance, my module would trigger a Mailchimp action. Once I get the Ok from Mailchimp, I'd like to show a message to user, something like using Devel's dvm or dpm function. 

How can I accomplish this (preferably without additional modules)?

Thanks in advance.

PS: I could use Devel, but I'd rather disable it in production env.
drupal_set_message() is what you're looking for.

Sample usage:

drupal_set_message('Mailchimp action is successfully executed.');