代码之家 › 专栏 › 技术社区 › Alexey Vagarenko

如何为递归单例类型定义NFData实例?

type-level-computation type-families haskell

3

Alexey Vagarenko · 技术社区 · 9 年前

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?

1 回复 | 直到 9 年前

1

3

András Kovács 9 年前

首先,您需要提供 NFData 单例列表的实例。

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

请注意,您不能在单个实例中解决此问题,因为这样就不能提供递归 NFData公司 约束条件:

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

类似地,您必须为 T 案例:

instance NFData (ST A) where
  rnf SA = ()

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