[m-rev.] for discussion: subtypes documentation

[Ralph Becket]

Fergus Henderson, Tuesday, 26 November 2002:
> On 25-Nov-2002, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > Can we include a definition of covariance and contravariance here?  I
> > can't remember what they mean and ten minutes of web trawling has left
> > me none the wiser.
> 
> Covariance means that means that they vary together; that is, the
> argument type for a subtype can be a subtype of the base type's argument type
> (and it cannot be a supertype of the base type's argument type).
> 
> Contravariance means that they vary oppositely: that is, the the argument
> type for a subtype must be a supertype of the base type's argument type.
> 
> Invariance (in this context) means that the argument type for a subtype
> must be exactly the same as the argument type for the base type.

Thought it was something like that.  We really should include a version
of the above in the reference manual.

- Ralph
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