Hugs 类型签名包含额外的类型约束？
在玩 Haskell 时遇到了这个，我很困惑：
Hugs> :type (\x -> x^2)
\x -> x ^ 2 :: (Integral a, Num b) => b -> b
a 在那里做什么？我该怎么读呢？如果我在 GHCi 中输入相同的内容，它会给出我期望的输出：
Prelude> :type (\x -> x^2)
(\x -> x^2) :: Num a => a -> a
这是 Hugs 中的错误吗？
Came across this while playing with Haskell and I'm stumped:
Hugs> :type (\x -> x^2)
\x -> x ^ 2 :: (Integral a, Num b) => b -> b
What is a doing in there? How am I supposed to read that? If I type the same thing into GHCi, it gives me the output I would expect:
Prelude> :type (\x -> x^2)
(\x -> x^2) :: Num a => a -> a
Is this a bug in Hugs?
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（2）
Integral约束来自2的指数。请记住，在 Haskell 中，整数文字实际上是Num a => 类型的多态值。一个。然后编译器推断，因为它被用作(^) :: (Num a, Integral b) => 的指数。一个-> b-> a，它必须是更受约束的类型Integral a =>;一个。为了使您不必消除代码中数字文字的歧义，Haskell 使用 类型默认< /a> 为任何不受约束的数字类型选择合理的具体类型。在本例中，该值为
Integer。区别似乎在于 Hugs 中的:type报告发生此情况之前推断的类型，而 GHCi 则报告应用类型默认后的类型。如果您自己为指数指定具体类型，则额外的约束就会消失。
The
Integralconstraint comes from the exponent of2. Remember that in Haskell, integer literals are actually polymorphic values of typeNum a => a. The compiler then infers that since it's being used as an exponent to(^) :: (Num a, Integral b) => a -> b -> a, it must be of the more constrained typeIntegral a => a.To save you from having to disambiguate numeric literals all over your code, Haskell uses type defaulting to pick a reasonable concrete type for any unconstrained numeric types. In this case, that will be
Integer. The difference seems to be that:typein Hugs reports the inferred type before this happens, while GHCi reports the type after type defaulting has been applied.If you specify a concrete type for the exponent yourself, the extra constraint disappears.
这是一种猜测，但这可能与
(^)的类型有关。 ghci 中的:t (^)返回(^) :: (Num a, Integral b) =>;一个-> b->一个。我猜拥抱看到^的第二个参数需要是一个Integral，即使该参数只是一个常量 2，它也包含类型中的约束签名。This is sort of speculation, but this might have to do with the type of
(^).:t (^)in ghci returns(^) :: (Num a, Integral b) => a -> b -> a. I'm guessing hugs sees that the second argument of^needs to be anIntegral, and even though that argument is just a constant 2, it includes the constraint in the type signature.