Statically Resolved Type Parameters

Question

The following (simplified) snippet is taken from an application I'm implementing which consistently uses Type Parameters resolved statically.

type A< ^B when ^B : (static member MyMember : Unit -> Unit)> = {
  Field : unit
}
type TestA = {
  AField : A< BTy >
}
and BTy = {
  BField : Unit
} with
  static member MyMember () = ()

IntelliSense gives me the following error when I define the type of field AField (AField : A< BTy >) which is: The type 'BTy' does not support any operators named 'MyMember'.

EDITED. Declaring them separately works, but if I have a mutual reference and I cannot declare a third type to put in the top which contains the common Infos of the two types. What should I do to avoid this problem? Anyway, if I define below the definitions let pluto = ("" :> obj) :?> A< BTy > it works, I imagine because both types are visible from the let binding.

Answer 1

To be honest, I'm a bit surprised that you are even allowed to use static member constraints in a type declaration, but as mentioned by @pad, when you put the declarations in the right order and remove the recursion, it works (although I'm not sure that there won't be other limitations when you move to more complex examples):

type A< ^B when ^B : (static member MyMember : Unit -> Unit)> = 
  { Field : unit } 

type BTy = 
  { BField : Unit } 
  static member MyMember () = () 

type TestA = { AField : A<BTy> }

Anyway, I think using static member constraints in a type declaration is a bit complicated. A cleaner way to do this would be to define an interface that clearly describes (and documents) the members you need:

type IMyMember =
  abstract MyMember : unit -> unit

Now, static member constraints can still be used to create an implementation of the interface from a type that has the required member, but does not implement the interface. Using this technique, you should be able to implement exactly the same functionality as with static member constraints on types (but in a clearer way):

/// Captures 'IMyMember' implementation from another type using static constraints
let inline captureMyMember< ^B when ^B : (static member MyMember : Unit -> Unit)> =
  { new IMyMember with
      member x.MyMember () = 
        (^B : (static member MyMember : Unit -> Unit) ()) }

The function will, for example, create IMyMember from your BTy type:

/// A type that contains field and a captured implementation of 'IMyMember'
type A = 
  { Field : unit 
    Operations : IMyMember } 

let it = { Field = ()
           Operations = captureMyMember<BTy> }

Aside, I used the same technique in an article that shows how to write generic numeric code and I think that it worked there really nicely.

Answer 2

Your implementation @Tomas satisfies the question but there is some hesitation over the style of your solution because it does not respected the functional programming paradigm. I thought to a solution arising from the Type Classes implementation of Haskell. Interfaces, abstract classes, etc. have been implemented in order to allow the F# environment to “interact” with the .Net environment, for a reason of uniformity of style the use of code which implements interfaces, abstract classes etc. in a context where no need of interact with the .Net libraries, it is a “Overhead” in my opinion (although F# is a multi-paradigm language). This is the reason why I found the implementation of the Type Classes of Haskell very elegant. Below I implemented through F# the “Haskell Type Class” code to solve my problem.

type Operations< ^a> = 
  {  
    MyMember : unit -> unit
  }

type A< ^a> = {
  Operations : Operations< ^a>
}

and TestA = { 
  AField : A< BTy > 
}

and BTy = { 
  BField : Unit 
}

let it = 
  let BTy_operations : Operations< BTy > = { MyMember = fun () -> () }
  let A_of_BTy = { Operations = BTy_operations }
  { AField = A_of_BTy }