Slack.Lookups.lookup_user_id

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

lookup_user_id(arg, slack)

Turns a string like "@USER_NAME" into the ID that Slack understands ("U…").

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