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From: rick@tetrauk.UUCP (Rick Jones)
Newsgroups: comp.lang.eiffel
Subject: Re: Covariance vs Contravariance.
Message-ID: <1113@tetrauk.UUCP>
Date: 13 Mar 91 13:26:10 GMT
References: <860@puck.mrcu>
Reply-To: rick@tetrauk.UUCP (Rick Jones)
Organization: Tetra Ltd., Maidenhead, UK
Lines: 55

In article <860@puck.mrcu> paj@uk.co.gec-mrc (Paul Johnson) writes:
>
>Another thought to add to the co/contra variance debate.  Why not have
>both?
>
> [detailed definition of the co/contra variant principles]
>
>Why not remove the restrictions on the argument types.  Contravariant
>        ^^^^^^^^^^^^^^^^^^^^^^^
>typing may not be very useful, but there is no reason to disallow it
>since it is always type safe.  If the type checker can catch misuses
>of covariant typing then the compiler can allow both.

I'm not sure if you meant to say that, but you can't actually _remove_ the
restrictions.  What you can do is allow arguments to be redefined as either
ancestors or descendants of the inherited definitions.  I.e given a portion of
an inheritance tree:

		A
	       / \
	      /   \
	     B     D
	    /
	   /
	  C

then a function "foo (arg: B)" in some class may be redefined in a descendant
as either "foo (arg: C)" (covariance), or "foo (arg: A)" (contravariance).  It
_cannot_ be redefined as "foo (arg: D)".

I would agree in principle that since contravariance is guaranteed safe, there
seems no reason to disallow it because covariance is supported.  However, this
leads to a logical anomaly.  To take an extreme case, consider the following:

	class	X	declares: foo (arg: STRING)
		|
		|
	class	Y	redefines: foo (arg: ANY)  -- legal by contravariance
		|
		|
	class	Z	redefines: foo (arg: TREE)  -- legal by covariance

If class Y does nothing else apart from redefining foo, then you could arguably
say that class Z should be able to inherit directly from class X.  This _would_
require removal of all type checks on arguments, which seems from a practical
point of view to be a nonsense.

It does suggest that allowing co- and contra- variance to coexist would lead in
practice to more confusion than it resolved.

-- 
Rick Jones, Tetra Ltd.  Maidenhead, Berks, UK
rick@tetrauk.uucp

The dynamics of a freeway approximates to a shoal of fish in 1.5 dimensions