Slack.Lookups.lookup_direct_message_id

You're seeing just the function lookup_direct_message_id, go back to Slack.Lookups module for more information.

lookup_direct_message_id(user, slack)

Turns a string like "@USER_NAME" or a user ID ("U…") into the ID for the direct message channel of that user ("D…"). nil is returned if a direct message channel has not yet been opened.

NOTE: Referencing "@USER_NAME" is deprecated, and should not be used. For more information see https://api.slack.com/changelog/2017-09-the-one-about-usernames