functor Make(M : sig type key con id :: Name con parent :: Name con cols :: {Type} constraint [id] ~ [parent] constraint [id, parent] ~ cols val key_inj : sql_injectable key val option_key_inj : sql_injectable (option key) table tab : ([id = key, parent = option key] ++ cols) end) = struct open M fun tree (f : $([id = key, parent = option key] ++ cols) -> xbody) (root : option M.key) = let fun recurse (root : option key) = queryX' (SELECT * FROM tab WHERE {eqNullable' (SQL tab.{parent}) root}) (fn r => children <- recurse (Some r.Tab.id); return
  • {f r.Tab}
  • ) in recurse root end end