Question

let eval a b = 
    let r = a + b
    printf "calculate.."
    r
type Foo() =
    member this.Eval = eval 5 10
[<EntryPoint>]
let main argv = 
    let f = Foo()
    let a = f.Eval
    let b = f.Eval
    0

这会调用eval 2次而不是一次。看来this.Eval是一个函数指针，每次调用.Eval时都会调用eval。

我真正想要的是变量。

我这样解决了......

let eval a b = 
    let r = a + b
    printf "calculate.."
    r
type Foo() =
    let e = eval 5 10
    member this.Eval = e
[<EntryPoint>]
let main argv = 
    let f = Foo()
    let a = f.Eval
    let b = f.Eval
    0

如果我只想要一个成员变量而不是方法，这是否正确？

Answer 1

您描述的行为是.NET属性始终有效的方式，无论是在C＃还是F＃中。每次读取属性时都会评估属性的主体。

除了你的解决方案，这是正确的，我还要补充说，它会在Foo构造时计算出来。您可能只希望在第一次读取属性时计算值，如果是，您可以使用Lazy来执行此操作：

let eval a b = 
    let r = a + b
    printf "calculate.."
    r
type Foo() =
    // a lazy computation that will be evaluated the first time .Value is called
    let e = lazy eval 5 10
    member this.Eval = e.Value
[<EntryPoint>]
let main argv = 
    let f = Foo()
    let a = f.Eval
    let b = f.Eval
    0

Answer 2

我认为更具风格的F＃风格是：

type Foo() =
    let eval a b = 
        let r = a + b
        printf "calculate.."
        r
    let e = eval 5 10
    member this.Eval = e
[<EntryPoint>]
let main argv = 
    let f = Foo()
    let a = f.Eval
    let b = f.Eval
    0

即。你将eval放在类中，使用let绑定来隐藏实现

F＃中的成员变量

2 个答案: