Re: [Caml-list] Is a Cow an Animal?

[james woodyatt <jhw@wetware.com>]

On Thursday, April 18, 2002, at 05:21 PM, Remi VANICAT wrote:
> james woodyatt <jhw@wetware.com> writes:
>>
>> I borrowed a technique I learned here from Brian Rogoff that uses an
>> abstract type with a contravariant type parameter for passing the type
>> of energy a food object contains to an animal object that can eat it.
>> (I wish I were smart enough to know the name for this technique.)
>
> the use of an abstract type with a covariant or contravariant type (or
> even not variant) parameter, and where the actual implementation of
> the type doesn't use this parameter (as in type 'a eater = int) is
> often call phantom type.
>
> one can read the very interesting mail about this in the archive of
> this mailing list
> http://caml.inria.fr/archives/200109/msg00097.html

That's precisely where I learned the technique.  It's true I forgot the 
"phantom type" terminology, but the contribution I was trying to make to 
the list is that such types are frequently useful in representing 
complicated associations between related class hierarchies.

I don't think Pixel's exercise is really covered well by the 
"subject-observer" pattern, and I wonder if the pattern I used in my 
solution is one that is already identified by a better name than the 
"association-by-phantom-type" pattern.

Surely somebody has already published a paper on this by now, right?


--
j h woodyatt <jhw@wetware.com>

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