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

问题描述

在与 C# 库交互时，我发现自己需要 C# 的空合并运算符，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.

是否可以在 F# 中使用单个重载运算符来近似此操作，该运算符内联适当的 if 情况?

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， 使用在此SO 答案中发现的一些小技巧"F# 中的重载运算符".

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

在编译时使用 ('a Nullable, 'a) ->'a 或 ('a when 'a:null, 'a) 的正确重载->可以内联单个运算符的 'a.甚至 ('a option, 'a) ->'a 可以加入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 c# 操作符的行为，我使用了默认参数 'a Lazy，这样除非原始值为 null，否则不会调用它的源.

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:

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

或者我制作了一个利用这种技术以及计算表达式来处理 Null/Option/Nullables 的库，称为 FSharp.Interop.NullOptAble

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

它使用运算符 |?-> 代替.

It uses the operator |?-> instead.

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