Question on "current_user"

Is there something special about function current_user?
In v.7.3.4 it only works when I do "select current_user;"
and breaks when called with the empty arg. list:
 syncdb=# select current_user(); ERROR:  parser: parse error at or near "(" at character 20

This behavior is just the opposite to that of some other functions 
that take no arguments (e.g. version, timeofday)

Mike.