class (Applicative 𝔪, Monad 𝔪) => DefaultParam 𝔪 α where
    def_param :: (𝔪 α, α -> 𝔪 ())
    return_typ_assign :: α -> α -> 𝔪 ()
    return_typ_assign =
        error "define either def_return_typ or return_typ_assign"

instance (HzMonadBase 𝔪, ExprConstr (ExprTyp 𝔪) α) =>
    DefaultParam 𝔪 (ExprT (ExprTyp 𝔪 α)) where

    def_param = (var "param" bottom, \x -> var "arg" x >> return ())
    return_typ_assign = assign