移动应用？(Mobile application ? In Asp.net /)

 
 
 令我困惑的是：我把FlexibleContexts pragma放到头部的pragma列表中，但是ghcu仍然建议我把它包含在内。 
 
 
 这仅启用模块本身的扩展。 要在GHCi中编写此代码，您需要在GHCi :set -XFlexibleContexts启用扩展。 
 
 但这只是问题的一部分。 看起来你的班级p应该确定n ：你只能向上和向下移动一个Point a ，对吗？ 但是就目前而言，没有什么可以阻止你定义更多的Moving (Point a) SomeOtherType实例，并且编译器不会假设你不会。 因此，推断类型中的a和n完全不相关，您希望它们是相同的。 这可以通过添加FunctionalDependencies扩展并更改类声明来解决 
 
class Num n => Moving p n | p -> n where
 
 这意味着不能有具有相同p和不同n实例。 
 
 我认为这足以让它发挥作用。 代码仍然是欠定的，因为它允许任何数字a ，但默认规则将选择Integer 。 

 
 
And the thing that confuses me much: I put the FlexibleContexts pragma to the pragma listing in the head, but ghcu still suggest me to get it included.
 
 
This only enables the extension in the module itself. To write this code in GHCi, you need to enable the extension in GHCi: :set -XFlexibleContexts.
 
But this is only part of the problem. It looks like for your class p should determine n: you can only move a Point a up and down by a, right? But as it stands, nothing stops you from defining more Moving (Point a) SomeOtherType instances, and the compiler doesn't assume you won't. So a and n in the inferred type are completely unrelated, where you want them to be the same. This can be fixed by adding the FunctionalDependencies extension and changing the class declaration to say
 
class Num n => Moving p n | p -> n where
 
which means exactly that there can't be instances with same p and different n.
 
I think that's enough to make it work. The code will still be underdetermined because it allows any numeric a, but defaulting rules will pick Integer.