如何解决“非法数据类型上下文” （使用-XDatatypeContexts）？ [英] How to fix "Illegal datatype context" (use -XDatatypeContexts)?

问题描述

  data数字a =>单位a =单位a（SymbolicManip a）
 
派生（Eq）
  

我不是确定如何解决它？

任何人都可以帮助我？ >数据类型中的类型类上下文现在被认为是一个不太有用的特性。问题是，以下内容不能编译：

  foo ::单位a  - > a 
 foo（Units x _）= x + x 
  

只能为 a 类型构造单元a 参数，满足 Num a 。因此，在销毁（模式匹配）时，应该可以访问 Num a 实例。但是，情况并非如此，在销毁时， Num a 必须违反直觉地被提供：

  foo :: Num a =>单位a  - > a 
 foo（Units x _）= x + x 
  

标准建议是从 Units a 数据类型声明中删除约束 Num a ，并将其添加到每个涉及<$ c $的函数中c>单元a 。

另一个选项是启用GADT并将数据类型更改为：

 数据单位a其中
单位:: Num a => a  - > SymbolicManip a  - >单位a 
  

这就是正确的事情：a Num a <需要code>实例构造值，而不是 destroy 提供。通过这种方式，上面的第一个 foo 声明将是很好的类型。

---

我几乎忘记了quick& dirty选项，即启用过时数据类型上下文功能：通过在文件的开头添加行

  { - ＃LANGUAGE DatatypeContexts＃ - } 
  

不过，我宁愿修改代码，也不愿意启用此语言扩展。

I am a new learner of Haskell, my code is as follows:

data Num a=>Units a = Units a (SymbolicManip a )

      deriving (Eq)

I am not sure how to fix it?

Anyone can help me?

解决方案

Typeclass contexts in datatypes are now regarded as a not so useful feature. The problem is that the following does not compile:

foo :: Units a -> a
foo (Units x _) = x+x

This intuitively should compile, since the Units a argument can only be constructed for a type a satisfying Num a. So, on destruction (pattern matching) one should be able to access the Num a instance. However this is not the case, and a Num a must be counterintuitively provided on destruction as well:

foo :: Num a => Units a -> a
foo (Units x _) = x+x

The standard suggestion is therefore to remove the constraint Num a from the Units a datatype declaration, and add it instead to every function involving Units a.

Another option is to enable GADTs and change the datatype to:

data Units a where
   Units :: Num a => a -> SymbolicManip a -> Units a

This does the "right" thing: a Num a instance is required to construct a value, and is instead provided on destruction. In this way, the first foo declaration above will be well-typed.

---

I almost forgot the "quick & dirty" option, which is to enable the obsolescent datatype context feature: this is done by adding at the beginning of your file the line

{-# LANGUAGE DatatypeContexts #-}

Still, I would rather modify the code than to enable this language extension.

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