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Structural Subtyping?
f#

Does F# support structural subtyping like Ocaml? I'm new to both of these languages (and languages that separate subtyping and subclassing in general), so apologies if I get some terminology wrong. The Ocaml manual implies that something closer to the duck typing present in dynamic languages is possible, but I can't seem to figure out if F# can do this.

For example, I'd like to be able to write something similar to:

let say_hi x = x.say_hi();;

and then call it with any parameter whose type has a say_hi method. The Ocaml manual makes it look like Ocaml would allow this and x would have a type with a constraint. F#, however, complains.

Is this possible with a different syntax (perhaps somehow explicitly noting the type of x as "something with a say_hi method") or not at all?

EDIT: somehow I missed this thread [link:cs.hubfs.net] , which says that the answer is "no." unfortunate.

.

Nominal subtyping is used. But that's not the entire story

  • If you're really keen to access arbitrary properties and methods in a dynamic "duck typing" style then you can do it via reflection (this can be made reasonably efficient if you work hard).
  • Operator overloading is resolved via a statically-resolved form of structural subtyping

Cheers!

don

By on 11/21/2006 4:12 PM ()

Nominal subtyping is used. But that's not the entire story

  • If you're really keen to access arbitrary properties and methods in a dynamic "duck typing" style then you can do it via reflection (this can be made reasonably efficient if you work hard).
  • Operator overloading is resolved via a statically-resolved form of structural subtyping

Cheers!

don

Are there any plans to bring structural subtyping into f# or will this remain a divergence from ocaml? I understand that for externally visible methods, the clr kind of forces you into nominal typing, but for internal-only methods this would be a welcome addition. It seems like the reflection approach is how you'd have to implement it anyway, so even if that was exposed as "object" to the outside world and documented somehow it would be nice.

I don't know enough about the generics implemention to know if it would even be possible to lobby the clr group for more finely grained constraints at that level either. :)

By on 11/22/2006 6:28 AM ()

Hi again,

We'd probably not add OCaml-style structural subtyping, instead we'd be more inclined to augment what we have with type-classes of some kind, where type-class constraints could include object members. We talk about it often but haven't made any decisions yet, if we'll do anything at all

cheers!

don

By on 11/23/2006 11:25 PM ()

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