fun sqexps [env] [fields] (fl : folder fields) (inj : $(map sql_injectable fields)) (r : $fields) =
    @map2 [sql_injectable] [id] [sql_exp env [] []]
     (fn [t] => @sql_inject)
     fl inj r

fun selector [tn :: Name] [fs] [ofs] [fs ~ ofs] (fl : folder fs) (m : $(map sql_injectable fs)) (r : $fs) =
    @foldR2 [sql_injectable] [id]
     [fn key => rest :: {Type} -> [rest ~ key] => sql_exp [tn = key ++ rest] [] [] bool]
     (fn [nm :: Name] [t :: Type] [key :: {Type}] [[nm] ~ key]
                      (inj : sql_injectable t) (v : t)
                      (exp : rest :: {Type} -> [rest ~ key] => sql_exp [tn = key ++ rest] [] [] bool)
                      [rest :: {Type}] [rest ~ [nm = t] ++ key] =>
         (WHERE {{tn}}.{nm} = {@sql_inject inj v} AND {exp [[nm = t] ++ rest] !}))
     (fn [rest :: {Type}] [rest ~ []] => (WHERE TRUE))
     fl m r [_] !