Haskell 在JSON实例中使用数据类型名称

我试图为

FromJSON

typeclass编写一个通用实例。其思想是在解析JSON时使用数据类型名称。我认为GHC应该能够做到这一点，但到目前为止，我的尝试失败了。下面是使用

Typeable

typeclass的最简单版本

data GetResponse a = GetResponse { getCode :: Int, getItem :: a } deriving (Show)

instance (Typeable a, FromJSON a) => FromJSON (GetResponse a) where
    parseJSON =
      withObject "GetResponse" $ \o -> do
          getCode <- o .: "code"
          getItem <- o .: toLower (pack typeName)

          return GetResponse {..}
        where
          typeName = showsTypeRep (typeRep Proxy :: Proxy a) ""

我试着用泛型做同样的事情，但还是得到了同样的错误

---

完整代码：

启用

ScopedTypeVariables

扩展，我可以编译这个示例。使用此扩展名，

Proxy a

中的

a

对应于实例声明中相同的

a

[1 of 1] Compiling Main             ( generics.hs, interpreted )

generics.hs:74:48: error:
    • Could not deduce (Typeable a0) arising from a use of ‘typeRep’
      from the context: (Typeable a, FromJSON a)
        bound by the instance declaration at generics.hs:66:10-61
      The type variable ‘a0’ is ambiguous
    • In the first argument of ‘showsTypeRep’, namely
        ‘(typeRep (Proxy :: Proxy a))’
      In the second argument of ‘($)’, namely
        ‘showsTypeRep (typeRep (Proxy :: Proxy a)) ""’
      In the second argument of ‘($)’, namely
        ‘pack $ showsTypeRep (typeRep (Proxy :: Proxy a)) ""’