haskell - 带有 DataKinds 的 FromJSON 实例

Question

尝试使用 TypeLits 对数据类型进行 JSON 反序列化时，我遇到了以下问题:

Couldn't match type ‘n’ with ‘2’      ‘n’ is a rigid type variable bound by        the instance declaration at test.hs:14:10      Expected type: aeson-0.11.2.1:Data.Aeson.Types.Internal.Parser                       (X n)        Actual type: aeson-0.11.2.1:Data.Aeson.Types.Internal.Parser                       (X 2)

How would be the correct syntax to allow Nat generically in the FromJSON instance in the following example:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedStrings #-}

import GHC.TypeLits
import Data.Aeson
import Control.Monad (mzero)

data X (n :: Nat) where
  A :: Integer -> X 1
  B :: Integer -> X 2

instance FromJSON (X n) where
  parseJSON (Object o) = do
      v <- o .: "val"
      t <- o .: "type"
      case t of
        "a" -> return $ A v
        "b" -> return $ B v
  parseJSON _       = mzero

Answer 1

由于您显然无法知道要在编译时反序列化的类型，因此需要将确切的类型隐藏在存在值中，然后通过模式匹配恢复。我通常使用通用的 Some 类型来隐藏幻像类型。

{-# LANGUAGE PolyKinds #-}

data Some (t :: k -> *) where
    Some :: t x -> Some t

现在您可以将实例编写为

instance FromJSON (Some X) where
    parseJSON (Object o) = do
        v <- o .: "val"
        t <- o .: "type"
        case (t :: String) of
          "a" -> return $ Some $ A v
          "b" -> return $ Some $ B v
    parseJSON _       = mzero

但是，您还需要启用 FlexibleInstances 扩展。