在JSON实例中使用数据类型名称

Question

我正在尝试为FromJSON typeclass写一个不知何故的通用实例。这个想法是在解析JSON时使用数据类型名称。我认为这是GHC应该能够做到的事情，但到目前为止，我的尝试失败了。最简单的版本，使用Typeable typeclass如下。

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) "" 

它失败，出现以下错误编译：

[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)) ""’ 

我想这样做与仿制药相同的事情，但得到了同样的错误了。

全码：http://codepad.org/Gh3ifHkP

Answer 1

启用ScopedTypeVariables扩展，我能够编译这个例子。通过此扩展，a在Proxy a中对应于实例声明中的相同a。