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Why does F# require an explicit cast to access interface members?
f#

I'm working my way through "Expert F#" and I've made it to the OO section. On page 142, there is an example of a MutableCircle concrete type that implements an IShape interface in addition to some standard methods.

To invoke an IShape interface member, it is necessary to explictly upcast to the interface, e.g.,

(circle :> IShape).BoundingBox instead of the straightforard C# syntax circle.BoundingBox.

Why is this necessary? IMO, this is unfortunate because it makes using such types from F# quite clumsy. C# requires the explicit casts only in cases where it is truly necessary, e.g., name clashes amongst members from multiple interfaces.

Is there any chance that this restriction could be removed in a future version?

Thanks,
Mike

.

All interface implmented in F# currently behave like C# "explicit interface implementation". I believe this was because the language designers disliked implicitly implemented interfaces and didn't want to provide a syntax for it. However, I believe this is one of the issues under review in the productization of the language. For now its quite common to put methods on your object that just forward the call to the interface, this way an object works alot like the interface was implicitly implemented.

Cheers,
Rob

P.S. Are you a Steve Bell fan? I seem to remember laughing boy was the grey hound in his cartoons.

By on 4/17/2008 1:46 AM ()

Thanks for the insight. F# is my first real foray into a statically typed language with type inferencing, and it's really fun to see how little I have to tell the compiler about my types. That's why this little bit stuck out like a sore thumb to me -- why am I flicking my fingers here telling the compiler something it already knows? Isn't that exactly the opposite of what this language is about?

The fact that this was a deliberate aesthetic choice by the designers tells me there are some subtleties here that have flown over my head. I guess that's why they're the designers and I'm the newbie [:)]

P.S. My username does indeed originate from the world of cartoons, but in this case it's Bugs Bunny and not Steve Bell...

[link:www.phattie.net]

Mike

By on 4/18/2008 12:46 AM ()

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