默认值受 haskell 中的存在约束

Question

考虑以下定义类型对的类型类：

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

对于约束的所有实例，我们可以派生出一组类型 a，本质上是一个隐式类型类，我们称它为 A。对于类型类 A 的每个实例，还有另一个隐式类型类 B，它包括 Constraint A b.

的所有可能类型 b

所以这是一段代码。

{-# 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

在 ghci 中进行类型检查

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

但是

> 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)

现在我的理解是，因为我使用存在类型，GHC 并不真正寻找 answer 的类型，它只是确保可能有一个，即使它没有办法找出它是哪一个。所以当我真正想要运行 answer 时，它无法弄清楚如何处理它。

所以我的问题是：有没有办法为实现 DefaultingQuestion

的每种类型定义默认答案

Answer 1

为什么不呢：

import Data.Proxy

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

instance DefaultingQuestion A where
  def _ = C1

默认值受 haskell 中的存在约束

Default values subject to existential constraint in haskell

haskell

typeclass

existential-type