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Explicit Interfaces and the Dot Notation
f#

Hi,

The topic of explicit interfaces in F# has come up a few times e.g. here, here, here and here. Many people find having to explicitly cast to the interface opaque and annoying. A pattern I've started using is to publish not the members of the interface, but the interface itself. This skirts round the issue of name collisions when implementing more than one interface, allows for easy discovery (through the dot notation) and seems quite tidy. Here's an example:

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type ICar =
    abstract Make : string
    abstract Bonnet : string


type IGirl =
    abstract Name : string
    abstract Bonnet : string


type Life() =
    interface ICar with
        member x.Make = "Saab"
        member x.Bonnet = "Hinged"

    member x.ICar = (x :> ICar)
        
    interface IGirl with
        member x.Name   = "Ella"
        member x.Bonnet = "Silken"

    member x.IGirl = (x :> IGirl)
    

let my = Life()

printfn "%A" my.ICar.Make
printfn "%A" my.IGirl.Bonnet

Perhaps using the syntax public interface to declare auto-generation of this member would be convenient. Can anyone see potential problems with this pattern?
regards,

Danny

.

You could put this is a module to have pervasive access to the interfaces:

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type ICar with member x.ICar = x

type IGirl with member x.IGirl = x
By on 1/29/2009 1:32 AM ()

Brian - that's something I overlooked.

brilsmurf - nice suggestion.

The only problem is that the property ICar appears recursively e.g. 

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my.ICar.ICar.ICar.Make

is defined which could be confusing. Applying the type extension to the class gets round this:

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type Life with member x.ICar = x :> ICar

I believe this would be hidden when used from other .NET languages, so is probably the best solution here.

By on 1/29/2009 10:54 AM ()

This will look a little weird if you try to consume the type from other .Net languages like C#.

By on 1/28/2009 7:39 PM ()

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