From: Olivier Andrieu <andrieu@ijm.jussieu.fr>
To: matthieu.dubuget@laposte.net
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Coercion of arrays of objects (and some other containers)
Date: Wed, 13 Apr 2005 16:11:19 +0200 (CEST) [thread overview]
Message-ID: <20050413.161119.115918255.andrieu@ijm.jussieu.fr> (raw)
In-Reply-To: <425CBE46.5000306@laposte.net>
Matthieu Dubuget [Wed, 13 Apr 2005]:
> I'm sorry, but I feel stupid.
>
> The variance and covariance terms/notions appear often in this
> list, and seems to be very basic notions.
>
> I tried to find explanation on internet but did not succeed.
>
> Could somebody point me some links to understand, please?
I do not have a link but the idea (as I understand it) is simple :
This concerns parameterized types ('a t) and how a subtyping relation
on the parameter can yield a subtyping relation on the type.
When the parameter is covariant :
if a is a subtype of b then a t is a subtype of b t
When the parameter is contravariant :
if a is a subtype of b then b t is a subtype of a t
For instance lists are covariant. [`A|`B|`C] is a supertype of [`A|`B]
so a [`A|`B|`C] list is a supertype of [`A|`B] list.
Function arguments are contravariant : ([`A|`B] -> unit) is a
supertype of ([`A|`B|`C] -> unit). When you have a function that
accepts A B or C, you can also view it as a function that accepts just
A or B (qui peut le plus peut le moins).
Records with mutable fields (or arrays) are neither covariant nor
contravariant.
Well maybe my terminology isn't quite correct but I hope you get the
idea.
--
Olivier
prev parent reply other threads:[~2005-04-13 14:11 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-04-13 0:14 Martin Jambon
2005-04-13 0:49 ` [Caml-list] " Olivier Andrieu
2005-04-13 0:59 ` Jacques Garrigue
2005-04-13 6:37 ` Matthieu Dubuget
2005-04-13 14:11 ` Olivier Andrieu [this message]
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