理解HList的这个定义 [英] Understanding this definition of HList

问题描述

我对Haskell比较新，我试图理解 HList 。

 数据实例HList'[] = HNil 
 newtype实例HList（x'：xs）= HCons1（x，HList xs）
模式HCons x xs = HCons1（x，xs）
  pre> 
 
 
我有几个具体问题： 
 
 
 
 
 
什么是我看到了'[] 和（x'：xs）语法？它几乎看起来像是在可变参数类型参数上的模式匹配，但是我从未见过这种语法，也不熟悉Haskell中的可变参数类型参数。我想这是 GHC的家庭类型的一部分，但我没有看到任何关于这个在链接的页面上，并且在Google中搜索语法相当困难。
   使用 newtype  HCons1 之外，c $ c>具有元组的声明（而不是 data 两个字段的声明）           首先，您缺少部分定义： data family 声明本身。
 数据族HList（l :: [*] ）
数据实例HList'[] = HNil 
 newtype实例HList（x'：xs）= HCons1（x，HList xs）
  
这被称为 数据族 （可在 TypeF下找到）  $ b 
 模式HCons x xs = HCons1（x，xs）
  
这是一个双向模式（可在 PatternSynonyms 扩展名下使用） 。
 
 
什么是'[] 和（ x'：xs）语法 
 
当您看到 '标记在构造函数前面，它表示它们的提升类型级别对应方。作为一种语法便利，提升的列表和元组也只需要额外的记号（我们仍然可以为空的类型级别列表写'[] >并且'：为类型级缺点，所有这些都可以通过 DataKinds 扩展名获得。
 
 
除了避免使用元组（而不是带有两个字段的数据声明）使用 newtype 声明有任何意义拳击 HCons1 ？
 
 
是的，要确保 HList 具有代表性的角色 ，这意味着你可以在> HList  s  1 之间强制执行。这只是一个太复杂的问题，不能解释一个答案，但这里是事情当我们有
 
 
 数据实例HList（x'：xs）= HCons x（HList xs ）
  
而不是 newtype实例（和没有模式）。考虑下面的 newtype  s，它们代表性地等同于 Int ， Bool 和（）分别
  newtype MyInt = MyInt Int 
 newtype MyBool = MyBool Bool 
 newtype MyUnit = MyUnit（）
  
回想一下，我们可以使用  coerce  自动换行或打开这些类型。我们希望能够做同样的事情，但是对于整个 HList ：
  ghci的> l =（HCons 3（HCons True（HCons（）HNil）））:: HList'[Int，Bool，（）] 
 ghci> l'= coerce l :: HList'[MyInt，MyBool，MyUnit] 
  
 code> newtype instance  variant，但不是数据实例，因为角色。 （更多关于此处。）
 
 
 
 
 
 
  1 从技术上讲，数据族没有作用实例的角色可以是不同的 /  newtype   - 这里我们只需要 HCons 情况为代表性，因为这是被强制执行的情况。 查看此Trac票券。
 
I'm relatively new to Haskell, and I'm trying to understand one of the definitions of HList.
data instance HList '[] = HNil
newtype instance HList (x ': xs) = HCons1 (x, HList xs)
pattern HCons x xs = HCons1 (x, xs)
I have a couple specific questions:


What is the '[] and (x ': xs) syntax I'm seeing? It almost looks like it's pattern matching on variadic type parameters, but I have never seen this syntax before, nor am I familiar with variadic type parameters in Haskell. I would guess this is part of GHC's Type Families, but I don't see anything about this on the linked page, and it's rather hard to search syntax in Google.
Is there any point in using a newtype declaration with a tuple (instead of a data declaration with two fields) besides avoiding boxing of HCons1?

解决方案
First, you are missing part of the definition: the data family declaration itself.
data family HList (l :: [*])
data instance HList '[] = HNil
newtype instance HList (x ': xs) = HCons1 (x, HList xs)
This is called a data family (available under the TypeFamilies extension).
pattern HCons x xs = HCons1 (x, xs)
This is a bidirectional pattern (available under the PatternSynonyms extension).

  
What is the '[] and (x ': xs) syntax I'm seeing?
When you see ' marks in front of constructors, it is to denote their promoted type-level counterparts. As a syntactic convenience, promoted lists and tuples also just need the extra tick (and we still get to write '[] for the empty type-level list and ': for the type level cons. All of this is available through the DataKinds extension.

  
Is there any point in using a newtype declaration with a tuple (instead of a data declaration with two fields) besides avoiding boxing of HCons1?
Yes, it is to make sure that HList has a representational role, which means you can coerce between HLists1. This is a bit too involved to explain in just an answer, but here is an example of where things don't go as we want when we have
 data instance HList (x ': xs) = HCons x (HList xs)
instead of the newtype instance (and no pattern). Consider the following newtypes which are representationally equivalent to Int, Bool, and () respectively
newtype MyInt = MyInt Int
newtype MyBool = MyBool Bool
newtype MyUnit = MyUnit ()
Recall we can use coerce to wrap or unwrap these types automatically. Well, we'd like to be able to do the same thing, but for a whole HList:
ghci> l  = (HCons 3 (HCons True (HCons () HNil))) :: HList '[Int,   Bool,   ()]
ghci> l' = coerce l                               :: HList '[MyInt, MyBool, MyUnit]
This works with the newtype instance variant, but not the data instance one because of the roles. (More on that here.)



1 technically, there is no role for a data family as a whole: the roles can be different for each instance/newtype - here we only really need the HCons case to be representational, since that's the one that gets coerced. Check out this Trac ticket.

                        
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