haskell - 没有因使用 `X' 而产生的 (Show a) 实例

Question

用这段代码

{-# LANGUAGE GADTs #-}
data X a where
     X :: (Show a, Num a) => a -> X a

instance Show (X a) where
         show (X x) = "X " ++ show x

instance Num (X a) where
         fromInteger x = X $ fromInteger x

我收到这个错误:

No instance for (Show a) arising from a use of `X'
Possible fix:
  add (Show a) to the context of the instance declaration
In the expression: X
In the expression: X $ fromInteger x
In an equation for `fromInteger': fromInteger x = X $ fromInteger x

谁能解释为什么我会收到此错误消息？

Answer 1

正确的解决方案是将约束添加到实例头，如下所示:

instance (Show a, Num a) => Num (X a) where
         fromInteger x = X $ fromInteger x

例如，显然您不能将 X String 视为 Num。