单一案件歧视工会的目的 [英] Purpose of a single case discriminated union

问题描述

我正在定义一个Monadic可观察/反应解析器.这与普通解析器的行为完全不同，因为它是连续查询.底层类型是:

I'm defining a monadic observable/reactive parser. This behaves quite differently to a normal parser as it is a continuous query. The underlying type is:

IObservable<'a> -> IObservable<'b>

通过查看功能语言中的各种解析器实现，看来定义事物的更合适方法是使用区分大小写的单一联合:

From looking at various parser implementations in functional languages, it seems as though the more appropriate way to define things is a single case discriminated union:

type Pattern<'a,'b> = Pattern of (IObservable<'a> -> IObservable<'b>)

这意味着我需要提取基础函数才能使用它:

Which means I then need to extract the underlying function to use it:

let find (Pattern p) = p

问题是:这仅仅是出于约定，还是出于以后扩展的目的，或者即使定义永远不变，也有这样做的理由吗?

The question is: Is this just by convention, or for purposes of later extension, or is there a reason to do this even if the definition never changes?

奖金问题:如果只是为了使用更方便的类型签名，为什么不只使用类型别名:

Bonus question: If it's just for a more convenient type signature, why not just use a type alias:

type Pattern<'a,'b> = IObservable<'a> -> IObservable<'b>

我已经通过了相当多的改进，但是还没有发现不使用DU会影响可组合性的情况.

I've advanced quite a way through this, and haven't found a case where composability is affected by not using the DU.

推荐答案

F#编译器不会保留有关类型缩写的信息，因此您根本不会从类型推断中受益.类型签名可以理解为程序规范.让类型检查器执行工作是确保程序正确性的好方法.

F# compiler doesn't preserve information about type abbreviation, so you do not benefit from type inference at all. Type signature can be understood as program specification; letting the type checker do their job is a good way to ensure correctness of your programs.

在类型别名的情况下，您需要在所有位置显式指定类型注释:

You need explicitly specify type annotation everywhere in the case of type alias:

type Vector = float * float

// val add : float * float -> float * float -> Vector
let add ((x1, y1): Vector) ((x2, y2): Vector): Vector = (x1 + y1, x2 + y2)

但它不会像使用DU那样为您提供透明度:

but it doesn't give you transparency as using DUs:

type Vector = V of float * float

// val add : Vector -> Vector -> Vector
let add (V(x1, y1)) (V(x2, y2)) = V(x1 + y1, x2 + y2)

在复杂的程序中，明确的类型签名确实使维护可组合性更加容易.

In complex programs clear type signatures indeed make it easier to maintain composability.

将更多案例添加到单案例DU不仅更简单，而且使用成员方法和静态方法扩展DU更加容易.一个示例是您经常覆盖ToString()进行漂亮的打印.

Not only it's simpler to add more cases to single-case DUs, but also it's easier to extend DUs with member and static methods. One example is you often override ToString() for pretty printing.

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