In C#, I can implement a generic interface twice on one class, using two different type-parameters:
interface IFoo<T> { void Foo(T x); }
class Bar : IFoo<int>, IFoo<float>
{
public void Foo(int x) { }
public void Foo(float y) { }
}
I would like to do the same thing in F#:
type IFoo<'a> = abstract member Foo : 'a -> unit
type Bar() =
interface IFoo<int> with
[<OverloadID("int")>]
member this.Foo x = ()
interface IFoo<float> with
[<OverloadID("float")>]
member this.Foo x = ()
But it gives a compiler error:
This type implements or inherits the same interface at different generic instantiations
'IFoo<float>'
and'IFoo<int>'
. This is not permitted in this version of F#.
I can't find any discussion of this issue on the web. Is such use frowned upon for some reason? Are there plans to allow this in an upcoming release of F#?
It is prohibited that a type implements or extends two different instantiations of the same interface. This is because the bridge method generation process cannot handle this situation.
A class can implement more than one interface at a time. A class can extend only one class, but implement many interfaces. An interface can extend another interface, in a similar way as a class can extend another class.
Only generic classes can implement generic interfaces. Normal classes can't implement generic interfaces.
Right now I don't know of plans to allow this.. The feature has been planned and is, at least partially (see comments) implemented in F# 4.0.
I think the only reasons its currently disallowed are that it's non-trivial to implement (especially with F# type inference), and it rarely arises in practice (I only recall one customer ever asking about this).
Given an infinite amount of time and resources, I think this would be allowed (I can imagine this being added to a future version of the language), but right now it does not seem like this is a feature worth the effort of supporting. (If you know a strong motivating case, please mail [email protected].)
EDIT
As an experiment for the curious, I wrote this C#:
public interface IG<T>
{
void F(T x);
}
public class CIG : IG<int>, IG<string>
{
public void F(int x) { Console.WriteLine("int"); }
public void F(string x) { Console.WriteLine("str"); }
}
and referenced it from F# (with comments suggesting the results)
let cig = new CIG()
let idunno = cig :> IG<_> // type IG<int>, guess just picks 'first' interface?
let ii = cig :> IG<int> // works
ii.F(42) // prints "int"
let is = cig :> IG<string> // works
is.F("foo") // prints "str"
so this is what typically happens on this 'boundary' stuff with F# - F# can consume this stuff ok, even if you can't author the same stuff from within the language.
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