Haskell：泛型函数中的绑定值 [英] Haskell: bound-values in generic functions

问题描述

  succ'::（有界a，方程a，枚举a）=> ; a  - > 
 succ'n 
 | n ==（maxBound :: a）= minBound :: a 
 |否则= succ n 
  

但这不起作用。如何解决这个问题？

解决方案

你不需要类型注释，它们是错误的根源，重新获得：

  succ'::（有界a，Eq a，Enum a）=> a  - > 
 succ'n 
 | n == maxBound = minBound 
 |否则= succ n 
  

（这适用于Haskell 98和Haskell 2010， ）
另外，我稍微缩进了 | ，因为它们不能与函数的开头对齐;它们是 succ'定义的一部分，而不是独立代码。

以下是一些测试数据：

  data Test = A | B | C 
派生（Bounded，Eq，Enum，Show）
 
 test = map succ'[A .. C] 
  

我得到了 [B，C，A] 。

i want to do something like:

succ' :: (Bounded a, Eq a, Enum a) => a -> a
succ' n
| n == (maxBound :: a) = minBound :: a
| otherwise = succ n

but this does not work. how to solve this?

解决方案

You don't need the type annotations, and they're the source of the errors you're getting:

succ' :: (Bounded a, Eq a, Enum a) => a -> a
succ' n
 | n == maxBound = minBound
 | otherwise = succ n

(This works in Haskell 98 and Haskell 2010, so pretty much any compiler you've got lying around .) Also, I indented the | a little, because they can't line up with the start of the function; they're part of the definition of succ', not standalone code.

Here's some test data:

data Test = A | B | C 
   deriving (Bounded, Eq, Enum, Show)

test = map succ' [A .. C]

I got [B,C,A].

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