是否有可能具有类型类中未提及的变量的关联类型同义词？ [英] Is it possible to have an associated type synonym with variables not mentioned in the type class?

问题描述

在 关联类型同义词

  class C a where 
 type S a（k :: *  - > *）:: * 
  

然而，我尝试运行这个我得到一个编译器错误（使用 -XTypeFamilies ）：

 不在范围内：类型变量`k'
  

我是否缺少任何东西，或者是实际的在GHC中的实现与文章中提到的不一样吗？ 正如你已经发现的那样，这在GHC中是不可能的：

与关联数据声明的情况完全相同，命名的类型参数必须是子项的排列t的类参数。示例

  class C abc where {type T ca :: *}  -  OK 
 class D a where {type T ax :: *}  - 否：x不是类参数
 class D a where {type T a :: *  - > *}  -  OK 
  

无法定义 S 之类的原因。如果你这样做：

  class C a where 
 type S a ::（*  - > ; *） - > * 
 data TupK ak = TupK（a，ka）
实例C [a]其中
类型S [a] = TupK a 
  pre> 
 
 
然而，现在你被困在新的数据类型中。使用类型同义词将不起作用（类型同义词`TupK'应该有两个参数），并且向 S 添加更多参数将无济于事（参数数量必须匹配家庭声明;预期1），如票证中所记录的。
 
In Associated Type Synonyms (Chakravarty, Keller, Jones) the paper seems to indicate that the following is valid:
class C a where
  type S a (k :: * -> *) :: *
However, when I try and run this I get a compiler error (with -XTypeFamilies):
Not in scope: type variable `k'
Am I missing anything, or is the actual implementation in GHC not the same as what is mentioned in the paper?
解决方案
As you already found out, this is not possible in GHC:

  
Exactly as in the case of an associated data declaration, the named type parameters must be a permutation of a subset of the class parameters. Examples
class C a b c where { type T c a :: * }   -- OK
class D a where { type T a x :: * }       -- No: x is not a class parameter
class D a where { type T a :: * -> * }    -- OK

The ticket you referred to actually explains the reason for not being able to define something like S. It works if you do it like this:
class C a where
    type S a :: (* -> *) -> *
data TupK a k = TupK (a, k a)
instance C [a] where
    type S [a] = TupK a
However, now you're stuck with a new data type. Using type synonyms won't work ("Type synonym `TupK' should have 2 arguments"), and adding more parameters to S won't help ("Number of parameters must match family declaration; expected 1"), as documented in the ticket.

                        
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