如何为递归单例类型定义NFData实例?

我正在使用 singletons 图书馆我有此数据类型:

import Control.DeepSeq
import Data.Singletons.Prelude
import Data.Singletons.TH

data T =
      A
    | B [T]

genSingletons [''T]

我想要生成的单例类型 ST 成为…的实例 NFData . 如果键入 T 不会是递归的。 我试着这样写:

instance NFData (ST a) where
    rnf SA = ()
    rnf (SB (x `SCons` xs)) = rnf x `seq` rnf xs

但这在最后一行失败,并显示消息:

Could not deduce (NFData (Sing n1)) arising from a use of `rnf'
from the context (a ~ 'B n)
  bound by a pattern with constructor
             SB :: forall (z_azEs :: T) (n_azEt :: [T]).
                   (z_azEs ~ 'B n_azEt) =>
                   Sing n_azEt -> Sing z_azEs,
           in an equation for `rnf'
or from (n ~ (n0 : n1))
  bound by a pattern with constructor
             SCons :: forall (a0 :: BOX) (z0 :: [a0]) (n0 :: a0) (n1 :: [a0]).
                      (z0 ~ (n0 : n1)) =>
                      Sing n0 -> Sing n1 -> Sing z0,
           in an equation for `rnf'
In the second argument of `seq', namely `rnf xs'
In the expression: rnf x `seq` rnf xs
In an equation for `rnf':
    rnf (SB (x `SCons` xs)) = rnf x `seq` rnf xs

我知道GHC想要 x 和 xs 在模式中 SB (x ``SCons`` xs)) 成为…的实例 NFData公司 ,但我很难弄清楚如何确切地说出这一点。 我应该在这个实例的上下文中写些什么来使其工作?