反正有使用F＃中的C＃隐式运算符吗？

Question

If I have a C# class with implicit conversion to double, like so:

public class Parameter
{
    private double _value;
    public Parameter(double value) { _value = value }
    public static implicit operator double(Parameter p) { return _value; }
}

F# doesn't like me trying to use it as if it were a float :

let a = Parameter(4.0)
let b = Parameter(2.0)
let c = a * Math.Sin(b) <-- 'expected float, here Parameter'

Is there any way to do this (I'm guessing there isn't, based on this question/answer ), and if not, what would be a decent workaround?

Answer 1

F# does not perform implicit conversions, but it allows you to define an explicit operator to run them. See the kvb's answer to a previous question :

let inline (!>) (x:^a) : ^b = ((^a or ^b) : (static member op_Implicit : ^a -> ^b) x) 

This is using statically resolved type parameters to say that either the input or the result needs to provide implicit conversion operator - these are compiled to methods named op_Implicit , so the F# compiler checks for a static method with this special name.

Using the !> operator, you can now explicitly say where you want to convert Parameter to a float (two times) in your code sample like this:

let a = Parameter(4.0) 
let b = Parameter(2.0) 
let c = !> a * Math.Sin(!> b)

I think the main reason for not allowing implicit conversions in F# is that it would make the type inference a bit more difficult and it would be hard for the compiler to give good error messages.

Answer 2

It won't let you do implicit conversions. Make your conversions explicit where you need to.

See here for various ways to do it explicitly: http://natehoellein.blogspot.com/2008/02/basic-type-conversions-with-f.html

Answer 3

FSharp.Interop.Dynamic uses the DLR, so for most people probably overkill, but has a function Dyn.implicitConvert for dynamically using the C# implicit operator.

   [<Test>] member basic.``Test Implicit Conversion`` ()=
                    let ele = 50
                    ele |> Dyn.implicitConvert |> should equal (decimal 50)