“One or Both”是否有规范的haskell类型？ [英] Is there a canonical haskell type for "One or Both"?

问题描述

我发现自己需要一个包含 A ，也许是 B 的数据结构，他们。如果我要为这个东西破解一个通用数据类型，它可能看起来像：

 数据OneOrBoth ab = A a | B b | AB a b 
 
 maybeA :: OneOrBoth a b  - >也许
 maybeB :: OneOrBoth a b  - >也许b 
或者L :: OneOrBoth a b  - >或者b  - 优选
或者R :: OneOrBoth a b  - >或者b  - 偏好b 
 hasBoth，hasExactlyOne，hasA，hasB :: OneOrBoth a b  - > Bool 
  

这个数据结构是否有名字？是否有一种规范的方式来处理Haskell中的一个或两个结构？

解决方案

Data.These

这可以用来表示两个值的组合，其中
组合是在任何输入时定义的。在代数上，类型
这些AB 表示（A + B + AB），它不会轻易地将因素简单地归入
的总和和乘积 - A（B，Maybe A）不清楚，
很难使用。

I find myself needing a data structure that contains maybe an A, maybe a B, and definitely one of them. If I were to hack up a general data type for this thing, it would probably look like:

data OneOrBoth a b = A a | B b | AB a b

maybeA :: OneOrBoth a b -> Maybe a
maybeB :: OneOrBoth a b -> Maybe b
eitherL :: OneOrBoth a b -> Either a b -- Prefers a
eitherR :: OneOrBoth a b -> Either a b -- Prefers b
hasBoth, hasExactlyOne, hasA, hasB :: OneOrBoth a b -> Bool

Does this data structure have a name? Is there a canonical way to handle one-or-both structures in Haskell?

解决方案

Data.These

This can be useful to represent combinations of two values, where the combination is defined if either input is. Algebraically, the type These A B represents (A + B + AB), which doesn't factor easily into sums and products — a type like Either A (B, Maybe A) is unclear and awkward to use.

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