Most of IRI classes can be declared by a sprintf format string, but sophisticated cases may require calculations, not only printing the string. create IRI class using function allows the application transform relational values to IRIs by any custom routines.

Let us extend the previous example about users and groups by a new class for grantees. Both users and groups are grantees and we have defined two IRI classes for them. Classes oplsioc:user_iri and oplsioc:group_iri work fine for quad maps of U_ID if and only if the value of U_IS_ROLE is accordingly restricted to FALSE or TRUE, otherwise one may occasionally generate, say, user IRI for a group. To create and parse IRIs that correspond to any U_IDs, two functions should be created:

create function DB.DBA.GRANTEE_URI (in id integer)
returns varchar
{
  declare isrole integer;
  isrole := coalesce ((SELECT top 1 U_IS_ROLE
      FROM DB.DBA.SYS_USERS WHERE U_ID = id ) );
  if (isrole is null)
    return NULL;
  else if (isrole)
    return sprintf ('http://%s/sys/group?id=%d', id);
  else
    return sprintf ('http://%s/sys/user?id=%d', id);
};

create function DB.DBA.GRANTEE_URI_INVERSE (in id_iri varchar)
returns integer
{
  declare parts any;
  parts := sprintf_inverse (id_iri,
      'http://myhost/sys/user?id=%d', 1 );
  if (parts is not null)
    {
      if (exists (SELECT top 1 1 FROM DB.DBA.SYS_USERS
          WHERE U_ID = parts[0] and not U_IS_ROLE ) )
        return parts[0];
    }
  parts := sprintf_inverse (id_iri,
      'http://myhost/sys/group?id=%d', 1 );
  if (parts is not null)
    {
      if (exists (SELECT top 1 1 FROM DB.DBA.SYS_USERS
          WHERE U_ID = parts[0] and U_IS_ROLE ) )
        return parts[0];
    }
  return NULL;
};

These functions may be more useful if the SPARQL web service endpoint is allowed to use them:

grant execute on DB.DBA.GRANTEE_URI to "SPARQL";
grant execute on DB.DBA.GRANTEE_URI_INVERSE to "SPARQL";

The next declaration creates an IRI class based on these two functions:

create iri class oplsioc:grantee_iri using
  function DB.DBA.GRANTEE_URI (in id integer)
    returns varchar,
  function DB.DBA.GRANTEE_URI_INVERSE (in id_iri varchar)
    returns integer .

In common case, IRI class declaration contains an N-array function that composes IRIs and N inverse functions that gets an IRI as an argument and extracts the Nth SQL value. IRI composing function should silently return NULL on incorrect arguments instead of error signal. Inverse functions should return NULL if the argument has an incorrect type or value.

It is possible to specify only composing function without any of inverse functions. However option (bijection) can not be used in that case, obviously.