Re: [Chicken-users] Peculiar TinyCLOS specialization bit

[Tony Garnock-Jones]

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.

Tony