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Need help on simple type inference error
f#

Hi,

I've bumped into a type inference error that I'm unable to fix. This is the code:

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type 'a MyInterface =
    abstract map : ('a -> 'b) -> 'b MyInterface
          
type 'a MyClass() =
    let triggerAdd, addEvent = Event.create()
    member self.OnAdd = addEvent.Add
    member self.Add = triggerAdd
    
    interface 'a MyInterface with
        member self.map(projector) =
            let result = MyClass()
            self.OnAdd(fun t -> result.Add(projector t))
            result :> 'b MyInterface

Visual Studio highlights "self.OnAdd" and returns the following error: "Type mismatch. Expecting a 'b MyClass but given a 'a MyClass. The type ''b' does not match the type ''a'".

Any hints?

Thanks,

pureza

.

I tried adding annotations until it worked and then removing them one by one. The key one appears to be

member self.Add : 'a -> unit = triggerAdd

I did not quite grok what was happening, though (maybe the Event.create() call was creating a result with a free type variable not constrained to be the same 'a as the suurounding class?)

By on 2/12/2009 7:33 PM ()

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