Function Type not System.Func stuffs ???

Question

Hi All,
So back to the porting and getting stuff rolling.  If you like, I have posted the in-progress code on
github here.  Feel free to contribute or pull for your own use.
So on to the question...
I've built a function in F#, which is a port of another function in C#, see figures below
Figure 1: F#
inherit UserManager<ApplicationUser>(store)
static member Create(options : IdentityFactoryOptions<ApplicationUserManager>, context : IOwinContext) =
//stuff
//manager is of type ApplicationUserManager
manager
Figure 2: C#
public static ApplicationUserManager Create(IdentityFactoryOptions<ApplicationUserManager> options, IOwinContext context)
//stuff
manager
I am attempting to pass said function from Figure 1 into another function call as below. 
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create) |> ignore
This function (app.CreatePerOwinContext) has a C# heritage, and I keep getting the following error...
Error 2 Possible overload: '(extension) IAppBuilder.CreatePerOwinContext<'T when 'T : not struct and 'T :> System.IDisposable>(createCallback: System.Func<Microsoft.AspNet.Identity.Owin.IdentityFactoryOptions<'T>,IOwinContext,'T>)
: IAppBuilder'. Type constraint mismatch. The type
    'b * 'c -> ApplicationUserManager   
is not compatible with type
    System.Func<Microsoft.AspNet.Identity.Owin.IdentityFactoryOptions<'a>,IOwinContext,'a>   
The type ''b * 'c -> ApplicationUserManager' is not compatible with the type 'System.Func<Microsoft.AspNet.Identity.Owin.IdentityFactoryOptions<'a>,IOwinContext,'a>'. C:\Projects\FSharpWebKit\FSharpWeb2\FSharpWeb2\Startup.fs 11 9 FSharpWeb2
I'm really just not sure why.  My static member seems exactly like it should.  Is there some sort of conversion I need to do to make this work?

Answer 1

F# functions are of type Microsoft.FSharp.Core.FSharpFunc<'T,'U>, where in many cases 'U is another FSharpFunc type -- this is how currying and partial application work.
However there's a different incompatibility in this code as well
System.Func<Microsoft.AspNet.Identity.Owin.IdentityFactoryOptions<'T>,IOwinContext,'T>)
is a delegate that takes an IdentityFactoryOptions and returns a IOwinContext -- one input argument; whereas your Create member takes both of those types as input, returning an ApplicationUserManager.
What C# is after is a delegate type, so when you have the actual argument types sorted out, the
MSDN page for delegates in F# is here.