默认值须在Haskell存在约束

Question

考虑以下类型类定义的类型的对：

class Constraint a b where 
    g :: a -> b 

对于约束所有的情况下，我们可以推导出一组类型a，基本上是一个隐含的类型类，姑且称之为A。对于类型类别A的每个实例，存在另一个隐式类型B，其包括Constraint A b的所有可能类型b。

所以这里是一段代码。

{-# LANGUAGE ExistentialQuantification #-} 
{-# LANGUAGE MultiParamTypeClasses #-} 
import Debug.Trace 

-- Contraining class 
class (Show a, Show b) => QandA a b where 
    g :: a -> b 

-- Some data types 
data A = A1 | A2 deriving (Show, Eq) 
data B = B1 | B2 deriving (Show, Eq) 
data C = C1 | C2 deriving (Show, Eq) 

instance QandA A B where 
    g A1 = B1 
    g A2 = B2 

instance QandA A C where 
    g A1 = C1 
    g A2 = C2 

-- We want to define a set of types that includes all the types that 
-- have Constraint a b given a. This can be done via an intermediate 
-- type. 
data DefaultAnswer q = forall a . (DefaultingQuestion q, QandA q a) => DefaultAnswer {answer :: a}; 

-- Polymorphism 
class DefaultingQuestion q where 
    def :: DefaultAnswer q 

instance DefaultingQuestion A where 
    def = DefaultAnswer C1 

其中typechecks但在ghci中

> (def :: DefaultAnswer A) 
(def :: DefaultAnswer A) :: DefaultAnswer A 

但

> answer (def :: DefaultAnswer A) 
<interactive>:574:1: 
    Cannot use record selector "answer" as a function due to escaped type variables 
    Probable fix: use pattern-matching syntax instead 
    In the expression: answer (def :: DefaultAnswer A) 
    In an equation for "it": it = answer (def :: DefaultAnswer A) 

现在我明白那就是，因为我用生存类型，GHC并没有真正寻找的类型的方式的answer，它只是确保可以有一个，即使它没有办法确定它是哪一个。那么当我真的想运行answer时，它无法弄清楚如何处理它。

所以我的问题是：有没有办法来定义默认的答案实现DefaultingQuestion

Answer 1

为什么不每种类型：

import Data.Proxy 

class DefaultingQuestion q a where 
    def :: Proxy q -> a 

instance DefaultingQuestion A where 
    def _ = C1