hubFS: THE place for F#

Hi,

I have a module "MyModule.fs" with:

let f1 a b = a * b
let f2 a b = a + b

let f3 f a b = f a b

Now I can use the function (inside F#-Code) like this:

let a = MyModule.f3 MyModule.f2 10 20
a |> printfn "%A"

When I try to get this into C# I get a compile-error:
int a = MyModule.f3(MyModule.f2, 10, 20));

Error 1 The type arguments for method 'MyModule.f3(Microsoft.FSharp.Core.FastFunc>, T, U)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
When I use Reflector on the F#-Code then the get:


[CompilationMapping(SourceConstructFlags.Module)]
public static class Program
{
    // Properties
    [CompilationMapping(SourceConstructFlags.Value)]
    public static ConsoleKeyInfo a
    {
        get
        {
            return $Program.a@28;
        }
    }

    [CompilationMapping(SourceConstructFlags.Value)]
    public static int t
    {
        get
        {
            return $Program.t@24;
        }
    }

    // Nested Types
    [Serializable]
    internal class clo@24 : OptimizedClosures.FastFunc2<int, int, int>
    {
        // Methods
        public override int Invoke(int a@595, int b@595)
        {
            return Euler.f2(a@595, b@595);
        }
    }
}

public static void _main() {
  t@24 = MyModule.f3(new Program.clo@24(), 10, 20);
  int num = Program.get_t();
  FastFunc func = Pervasives.printfn>(
     new Format, TextWriter, Unit, Unit, int>("%A"));
  int num2 = num;
  func.Invoke(num2);
  a@28 = Console.ReadKey();
}

What can I do?


It's because FastFunc isn't a delegate type, you'll need to convert your delegate using the FuncConvert.ToFastFunc.

Cheers,
Rob

---

Robert Pickering, MVP
http://strangelights.com

Thanks Rob. Can you write a piece of code how you would convert the function MyModule.f2 in this sample?

It's actually harder than I thought:

int a = Module1.f3(FuncConvert.FuncFromTupled(FuncConvert.ToFastFunc(delegate(Tuple ab) { return Module1.f2(ab.Item1, ab.Item2); })), 10, 20);


You need to supply an anonymous delegate that takes a tuple of arguments then wrap this in FuncConvert.ToFastFunc and then FuncConvert.FuncFromTupled.

Cheers,
Rob

---

Robert Pickering, MVP
http://strangelights.com

Hi,
just a short addition - if your primary goal is to use the library from C# then it may be worth to design it in a more "C#-friendly" way. Instead of using F# functions in the arguments, you can declare delegate type and using the functions from C# will then be straightforward:

> type FCallback = delegate of int*int -> int;;
type FCallback =
  delegate of int * int -> int

> let f3 (f:FCallback) a b = f.Invoke(a,b);;
val f3 : FCallback -> int -> int -> int

The C# code could then use the usual style:

int a = Module1.f3(Module1.f2, 10, 20); // method gets converted to the delegate automatically in C#

Hope this helps!
T.

---

Tomas Petricek (Blog), C# MVP
My book: Real-world Functional Programming in .NET

Thanks a lot.

@tomasp:
- but with your method I can't write my call in F#:
  let a = MyModule.f3 MyModule.f2 10 20

Ok my MyModule is now:

let f3 f a b = f a b

let f1 a b = a * b
let f2 a b = a + b 

let t1 = f3 f1 10 20
let t2 = f3 f2 10 20

and in C#:

            var f2 =
                FuncConvert.FuncFromTupled(
                  FuncConvert.ToFastFunc((Tuple<int, int> ab) => MyModule1.f2(ab.Item1, ab.Item2)));

            var f1 =
                FuncConvert.FuncFromTupled(
                  FuncConvert.ToFastFunc((Tuple<int, int> ab) => MyModule1.f1(ab.Item1, ab.Item2)));

            int a = MyModule1.f3(f2, 10, 20);
            Console.WriteLine(a);

            a = MyModule1.f3(f1, 10, 20);
            Console.WriteLine(a);
            Console.ReadKey();

This works correct, but maybe there is a better solution....

I think Tomas' solution is going in the right direction, you can call f3 from F# but it's a little more complex:
[code langauge="F#"]f3 (new FCallback(fun a b -> f2 a b)) 10 20;;[/code]

So you'd probably just create a version to be used from F#:
[code langauge="F#"]let f3' f = f3 (new FCallback(f));;[/code]

---

Robert Pickering, MVP
http://strangelights.com

Thank you very much.