{{E-mail aliases (AKA, redirects)}}

extern type emailUser;
{{A valid username to appear before the "@" in an e-mail address}}

extern type email;
{{A valid e-mail address.
  It may also be a username only, in which case it is interpreted as a local
  user's mailbox.}}

extern type aliasSource;
{{An e-mail recipient whose mail you want to redirect}}
extern val userSource : emailUser -> aliasSource;
{{The part appear before the "@" in your desired source address}}
extern val defaultSource : aliasSource;
{{Matches any mail to this domain that doesn't match any other rule, with the
  exception of systemwide usernames like UNIX users.}}
extern val catchAllSource : aliasSource;
{{Matches any mail to this domain that doesn't match any other rule, even
  for systemwide usernames.}}

extern type aliasTarget;
{{A place to redirect messages}}
extern val addressTarget : email -> aliasTarget;
{{Redirect to this e-mail address.}}
extern val addressesTarget : [email] -> aliasTarget;
{{Redirect to all of these addresses.}}
extern val dropTarget : aliasTarget;
{{Silently delete all mail to the associated source.}}

extern val aliasPrim : aliasSource -> aliasTarget -> [Domain] {MailNodes: [mail_node]};
{{Request redirection of all mail from the source to the target, specifying on
  which nodes this redirection should be applied.}}

val emailAlias = \user -> \email -> aliasPrim (userSource user) (addressTarget email);
{{Redirect mail for the user at the current domain to the e-mail address.}}
val aliasMulti = \user -> \emails -> aliasPrim (userSource user) (addressesTarget emails);
{{Redirect mail for the user at the current domain to all of the e-mail
  addresses listed.}}
val aliasDrop = \user -> aliasPrim (userSource user) dropTarget;
{{Silently delete mail to the user at the current domain.}}

val defaultAlias = \email -> aliasPrim defaultSource (addressTarget email);
{{When a message to the current domain doesn't match any other alias, send it to
  this e-mail address.}}