我如何定义NFData实例的递归单例类型

Question

我使用的是singletons库。我的数据类型是:

import Control.DeepSeq
import Data.Singletons.Prelude
import Data.Singletons.TH
data T =
      A
    | B [T]
genSingletons [''T]

我希望生成的单例类型ST是NFData的实例。如果类型T不是递归的，这将是直接的。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，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

我理解GHC希望SB (x ``SCons`` xs))模式中的x和xs成为NFData的实例，但我很难弄清楚如何准确地告诉这一点。我应该在这个实例的上下文中写些什么来使它工作呢?

Answer 1

首先，需要为单例列表提供NFData实例。

instance NFData (SList '[]) where
  rnf SNil = ()
instance (NFData (Sing x), NFData (SList xs)) => NFData (SList (x ': xs)) where
  rnf (SCons x xs) = rnf x `seq` rnf xs

注意，你不能在一个实例中解决这个问题，因为那样你就不能提供递归的NFData约束:

instance NFData (SList xs) where
  rnf SNil = ()
  rnf (SCons x xs) = ? -- no way to know if NFData (Sing x)

同样，您必须为T情况编写单独的实例:

instance NFData (ST A) where
  rnf SA = ()
instance NFData (SList xs) => NFData (ST (B xs)) where
  rnf (SB xs) = rnf xs