F#中的空合并运算符? [英] Null Coalescing Operator in F#?

问题描述

与C#库进行交互时，我发现自己想要C#的null合并运算符同时用于Nullable结构和引用类型.

When interacting with C# libraries, I find myself wanting C#'s null coalescing operator both for Nullable structs and reference types.

是否可以用一个内联适当的if情况的单个重载运算符在F#中对此进行近似?

Is it possible to approximate this in F# with a single overloaded operator that inlines the appropriate if case?

推荐答案

是的，使用此 SO 答案中的一些小骇客"F#中的重载运算符".

Yes, using some minor hackery found in this SO answer "Overload operator in F#".

在编译时，可以内嵌对于单个运算符使用('a Nullable, 'a) ->'a或('a when 'a:null, 'a) -> 'a的正确重载.甚至可以抛出('a option, 'a) -> 'a以获得更大的灵活性.

At compiled time the correct overload for an usage of either ('a Nullable, 'a) ->'a or ('a when 'a:null, 'a) -> 'a for a single operator can be inlined. Even ('a option, 'a) -> 'a can be thrown in for more flexibility.

为了提供更接近于c#运算符的行为，我设置了默认参数'a Lazy，以便除非原始值为null，否则不会调用其源.

To provide closer behavior to c# operator, I've made default parameter 'a Lazy so that it's source isn't called unless the original value is null.

示例:

let value = Something.PossiblyNullReturned()
            |?? lazy new SameType()

实施方式:

NullCoalesce.fs [要点]:

NullCoalesce.fs [Gist]:

//https://gist.github.com/jbtule/8477768#file-nullcoalesce-fs
type NullCoalesce =  

    static member Coalesce(a: 'a option, b: 'a Lazy) = 
        match a with 
        | Some a -> a 
        | _ -> b.Value

    static member Coalesce(a: 'a Nullable, b: 'a Lazy) = 
        if a.HasValue then a.Value
        else b.Value

    static member Coalesce(a: 'a when 'a:null, b: 'a Lazy) = 
        match a with 
        | null -> b.Value 
        | _ -> a

let inline nullCoalesceHelper< ^t, ^a, ^b, ^c when (^t or ^a) : (static member Coalesce : ^a * ^b -> ^c)> a b = 
        // calling the statically inferred member
        ((^t or ^a) : (static member Coalesce : ^a * ^b -> ^c) (a, b))

let inline (|??) a b = nullCoalesceHelper<NullCoalesce, _, _, _> a b

或者，我制作了一个库，该库利用此技术以及用于处理Null/Option/Nullables的计算表达式，称为

Alternatively I made a library that utilizes this technique as well as computation expression for dealing with Null/Option/Nullables, called FSharp.Interop.NullOptAble

它改用运算符|?->.

这篇关于F#中的空合并运算符?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！