You need to use the .~ operator, to convert the field "s.default_year" into an AST node, try this :
update(songs)(s =>
where(s.name isNull)
set(s.year_str := s.default_year.~ || " AD")
)
If you're using a pre 0.9.6 version of Squeryl you might have to do this instead :
set(s.year_str := s.default_year.~ || Some(" AD"))