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Smells like duck-typing
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Date: 2007-10-18 (17:58)
From: Dario Teixeira <darioteixeira@y...>
Subject: Re: [Caml-list] Smells like duck-typing

> So all full stories are also blurbs, but blurbs are not full stories.  
> So full stories are a more specific type than blurbs are, because while 
> we can always use a full story as a blurb, we can't use a blurb as a 
> full story.

Nope, nothing like that.  Read carefully what I wrote: I simply said
that any method that works on a blurb should also work on a full story.
I didn't say anything about reversing the IS-A relation (and yes, I know
that OCaml's inheritance has different semantics, but I am talking here
about OOP in general).

In fact, a blurb is a kind of story.  Just one that has been restricted
such that some of story's fields/methods cannot be used on blurb.  Now
the OOP purists will jump and say that this violates inheritance.  Which
is precisely my point and why I don't call it inheritance!

> It's much easier, rather than detecting when things have to be removed, 
> to simply detect when things can be added.  Every "remove members" 
> relationship can be expressed as an "add members" relationship going the 
> other direction.  Start with the simpler (fewer members) type and then 
> create the derived type by adding members to make the more complex (more 
> members) type.  Anything you can do with one direction of deriving types 
> you can do with the other direction of deriving.

That is true for most problems.  However, I just described one particular
class of problem where it is more natural (and correct if you will) to
derive by deconstruction rather than by construction.


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