Define F# '**' operator in C#

Question

I see that F# uses the ** operator for powers, so 2 ** 5 = 32. This is different from C#, where you have the option to use the '^' operator in your custom types, but for some reason isn't used by the built in number types.

But how do you implement the ** operator in C# for use in an F# project?

If I do this in C#:

public static Integer operator ^(Integer left, Integer right)
{
    if (Integer.IsNaN(left) || Integer.IsNaN(right)) return NaN;
    return left.RaiseToPower(right);
}

It compiles fine, and I can use it the same way as the '+' operator, but neither of these work in F#:

let intgr3 = intgr1 ** intgr2

let intgr3 = intgr1 ^ intgr2

And in C#, this doesn't work:

public static Integer operator **(Integer left, Integer right)
{
    if (Integer.IsNaN(left) || Integer.IsNaN(right)) return NaN;
    return left.RaiseToPower(right);
}

So how do I define the F# equivalent of the ** operator in C#?
Thanks.

Answer 1

As I said in my comments, C# does not let you define new operators and ** is not an operator in C#. ^ is an operator, however it is the logical XOR operator, NOT the exponentiation operator. This gave me a hint that F# might be translating your C# operator into native F# (^^^ for logical XOR).

So, I created a couple test projects and using your definition of ^, here's what I found in F#:

open CSLib // CSLib is the C# library

let ( ** ) (x : Integer) (y : Integer) = x.RaiseToPower y

let x = new Integer()
let y = new Integer()

let a = x ^ y // error
let b = x ^^^ y // compiles, but looks like XOR
let c = x ** y // compiles

You can define new global operators in F#, however if you want this to be a general library that may not be acceptable.

You can define the Exponentiation operator for use in F# by defining a public static method Pow in the Integer type in your C# library:

public static Integer Pow(Integer left, Integer right)
{
    if (Integer.IsNaN(left) || Integer.IsNaN(right)) return NaN;
        return left.RaiseToPower(right);
}

Then, you are able to use it directly in F# as **. I will note that overloaded operators in C# are not idiomatic so having a Pow method will seem quite natural to C# users.

Answer 2

I believe the reason why you can't define a ** operator for your object is that this is not one of the operators supported by the language. It's not a recognized operator.. Why not just use the Math library http://msdn.microsoft.com/en-us/library/system.math.pow.aspx