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[Caml-list] [Q]: Co(ntra)variance and subtyping?
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Date: -- (:)
From: Pixel <pixel@m...>
Subject: Re: [Caml-list] [Q]: Co(ntra)variance and subtyping?
"Andreas Rossberg" <AndreasRossberg@web.de> writes:

> > - What does subtyping exactly mean in OCaml resp. functional
> >   programming?
> 
> OCaml has two separate notions of subtyping:
> 
> - in the core language, to support objects
> - in the module language, for signature matching
> 
> While subtyping for module types is rather standard from a superficial point
> of view, OCaml's treatment of subtyping in the core language is somewhat
> special because it does not provide so-called subsumption.

well, you forgot polymorphic variants, which do use subsumption:

# let switch = function `On -> `Off | x -> x;;
val switch : ([> `Off | `On] as 'a) -> 'a = <fun>
# (switch `On, switch `Whatever);;
- : _[> `Off | `On] * _[> `Off | `On | `Whatever] = `Off, `Whatever

> Subsumption means
> that wherever a value of type t is expected you may freely provide a value
> of some subtype t'. This is not the case in OCaml: you have to explicitly
> coerce the value to the required supertype first. This is necessary to make
> type inference feasible.

necessary is a small overstatement ;p

[...]

> The most important type constructor is "->", which has the following
> subtyping rule:
> 
>     t' < t and u' < u    =>    (t -> u') < (t' -> u)
> 
> Note the order of t and t' here: it goes in the opposite direction!
> Functions are covariant in their result type, but contravariant in their
> argument type. This may seem a bit funny at first (and in fact there are
> major programming languages that still insist on getting it plainly wrong -
> you already know one ;-)

if you want more about it, read the nice paper from Giuseppe Castagna:

Covariance And Contravariance: conflict Without A Cause 
  http://citeseer.nj.nec.com/castagna95covariance.html
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