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Re: [Chicken-users] Peculiar TinyCLOS specialization bit

From: Tony Garnock-Jones
Subject: Re: [Chicken-users] Peculiar TinyCLOS specialization bit
Date: Fri, 31 Dec 2004 14:11:32 +0000
User-agent: Mozilla Thunderbird 0.8 (Macintosh/20040913)

Joel Reymont wrote:
My goal is to be able to pass an object or an absence of one.

You might be interested in how languages like ML and Haskell solve this - Haskell defines a "Maybe" type, which has values of either "Just <somevalue>" or "Nothing", and ML defines an "opt" type with "Some <somevalue>" and "None".

So Schemeish pseudocode for expressing the presence of a string would be (make-just "hello"), and the absence of a string would be (make-nothing).

Hmm, of course this doesn't type (for the purposes of TinyCLOS) quite as nicely as the Haskell/ML constructs, so an alternative might be to rearrange your inheritance hierarchy:

<widget> is the base class
<null-widget> inherits from <widget>
<nonnull-widget> inherits from <widget>

Then you can maintain a single instance of <null-widget> (or as you see fit), and specialise on both subclasses, which causes TinyCLOS to do some of the pattern-matching you need for you.


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