可扩展记录（我认为） [英] Extensible records (I think)

问题描述

 数据A = ... 
数据B = ... 
 data C = ... 
 
 class HasA t其中
 getA :: t  - > A 
 
类HasB t其中
 getB :: t  - > B 
 
类HasC t其中
 getC :: t  - > C 
  

所以我可以这样做（伪代码如下）：

  a :: A 
b :: B 
 
x = mkRecord {elemA a，elemB b} 
y = mkRecord {elemB b，elemA a} 
 
  - `x`的类型==`y`的类型
  

当然，在上面的例子中，只有适当的 get 函数可以工作， getA 和 getB 。

我还想要以下功能：

  slice ::子集ab => a  - > b 
 slice x =  - 只删除不在类型b中的x的位。 
 
 add :: e  - > a  - > a ++ e 
 add ex =  - 向记录添加一个元素（如果它已经存在，则编译错误）
  

我觉得这不是一个新问题，所以也许已经存在一个解决方案。请注意，我并不要求解决方案是可扩展的，我需要处理的类型数量是有限且已知的，但当然并且可扩展的则不会受到伤害。

我发现了几个包，它们似乎处于我正在寻找的领域，即 HList 和可扩展（也许可扩展性更好，因为我想要我的记录无序的）。我在Hackage文档中有点失落，所以我只想要一些示例代码（或者一些示例代码的链接），它们大致可以实现我正在寻找的功能。

解决方案

这正是 HList 所擅长的。但是，由于我没有正确的设置来立即测试 HList 包中的某些内容（此外，它还有 singletons 的一个最小示例 HList

$ p $ {code> { - ＃LANGUAGE DataKinds，TypeOperators，GADTs，TypeFamilies，UndecidableInstances ，
PolyKinds，FlexibleInstances，MultiParamTypeClasses
＃ - }

导入Data.Singletons
导入Data.Promotion.Prelude.List

数据HList（l :: [*]）其中
HNil :: HList'[]
HCons :: x - > HList xs - > HList（x'：xs）

add 函数是最简单的：它只是 HCons ：

  add： ：x  - > HList xs  - > HList（x'：xs）
 add = HCons 
  

更有趣的是将两条记录：

   - 注意我们使用`：++`from singletons 
 combine :: HList xs  - > HList ys  - > HList（xs：++ ys）
 combined HNil xs = xs 
 combine（x`HCons` xs）ys = x`HCons`（xs`combine` ys）
  get 函数，你需要根据类型 - 级别列表。 
  class有x xs其中
 get :: xs  - > ; x 
 
实例{ - ＃OVERLAPS＃ - }有x（HList（x'：xs））其中
 get（x`HCons` _）= x 
 
实例具有x（HList xs）=>有x（HList（y'：xs））其中
获得（_`HCons` xs）=获得xs 
  
最后，我们可以使用 Has 定义一个类似的子集类。 
  class子集ys xs其中
 slice :: xs  - > ys 
 
实例子集（HList'[]）（HList xs）其中
 slice _ = HNil 
 
实例（Get y（HList xs），Subset ys）（HList xs））=> 
子集（HList（y'：ys））（HList xs）其中
 slice xs = get xs`HCons` slice xs 
  
 
 
 
 
 
 
正如你在parens中提到的那样，简单的 HList 表单并不能保证你只有一个的任何类型的字段（所以 get 只返回第一个字段，忽略其余字段）。如果你想要唯一性，你可以给 HList 构造函数添加一个约束。
  Nil :: Record'[] 
 Cons ::（NotElem x xs〜'True）=> x  - >记录xs  - > Record（x'：xs）
  
然而，定义子集使用记录看起来像涉及一些证明。 ：） 
What I roughly want is this:
data A = ...
data B = ...
data C = ...

class HasA t where
  getA :: t -> A

class HasB t where
  getB :: t -> B

class HasC t where
  getC :: t -> C
So I can do something like this (pseudocode follows):
a :: A
b :: B

x = mkRecord { elemA a, elemB b }
y = mkRecord { elemB b, elemA a }

-- type of `x` == type of `y`
Naturally, only the appropriate get functions work, in the above case getA and getB. 

I'd also like the following functions
slice :: Subset a b => a -> b
slice x = -- just remove the bits of x that aren't in type b.

add :: e -> a -> a ++ e
add e x = -- add an element to the "record" (compile error if it's already there)
I feel like this is not a new problem so perhaps a resolution for this already exists. Note that I don't require the solution to be extensible, the amount of types I need to deal with is finite and known, but of course and extensible one wouldn't hurt.

I've found a couple of packages that seem to be in the field of what I'm looking for, namely HList and extensible (perhaps extensible is better because I want my records unordered). I got a bit lost in the Hackage docs so I'd like just some sample code (or a link to some sample code) that roughly achieves what I'm looking for.
解决方案
This is exactly what HList is good for. However, since I don't have the right setup to test something with the HList package right now (and besides, it has more confusing data definitions), here is a minimal example of HList that uses singletons for the type-level list stuff.
{-# LANGUAGE DataKinds, TypeOperators, GADTs,TypeFamilies, UndecidableInstances,
    PolyKinds,  FlexibleInstances, MultiParamTypeClasses
  #-}

import Data.Singletons
import Data.Promotion.Prelude.List

data HList (l :: [*]) where
  HNil :: HList '[]
  HCons :: x -> HList xs -> HList (x ': xs)
The add function is the simplest: it is just HCons:
add :: x -> HList xs -> HList (x ': xs)
add = HCons 
Something more interesting is combining two records:
-- Notice we are using `:++` from singletons
combine :: HList xs -> HList ys -> HList (xs :++ ys)
combine HNil xs = xs
combine (x `HCons` xs) ys = x `HCons` (xs `combine` ys)
Now, for your get function, you need to dispatch based on the type-level list. To do this, you need an overlapping type class.
class Has x xs where
  get :: xs -> x

instance {-# OVERLAPS #-} Has x (HList (x ': xs)) where
  get (x `HCons` _) = x

instance Has x (HList xs) => Has x (HList (y ': xs)) where
  get (_ `HCons` xs) = get xs
Finally, we can use Has to define a similar Subset class. Same idea as before.
class Subset ys xs where
  slice :: xs -> ys

instance Subset (HList '[]) (HList xs) where
  slice _ = HNil

instance (Get y (HList xs), Subset (HList ys) (HList xs)) =>
           Subset (HList (y ': ys)) (HList xs) where
  slice xs = get xs `HCons` slice xs




As you mention in parens, the simple HList form does not ensure you have only one of any type of field (so get just returns the first field, ignoring the rest). If you want uniqueness, you can just add a constraint to the HList constructor.
data Record (l :: [*]) where
  Nil :: Record '[]
  Cons :: (NotElem x xs ~ 'True) => x -> Record xs -> Record (x ': xs)
However, defining Subset using Record looks like it involves some proofs. :)

                        
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