[plt-scheme] union notation in helpdesk

On Sep 15, 2007, at 4:36 AM, Stephen De Gabrielle wrote:

> Hi,
>
> I've been looking at the documentation for the graph.ss library (in  
> MrLib 7.5) and came across some notation that I am not familiar with;
>
> (add-links
>  graph-snip<%>
>  graph-snip<%>
>   (union false? (is-a?/c pen%))
>   (union false? (is-a?/c pen%))
>   (union false? (is-a?/c brush%))
>   (union false? (is-a?/c brush%))))       PROCEDURE


Try either #f or an instance of a brush%. -- Matthias



> -- http://download.plt-scheme.org/doc/371/html/mrlib/mrlib-Z- 
> H-55.html#node_sec_7.5
>
> I can tell it's asking for things that implement either a class or  
> matching interface,
>
> but the use of the 'union false?'
>
> Is union the one from the parser tools collection?
>
> I'm guessing that it returns #f if the interfce isn't implemented -  
> or do I have this all wrong.
>
>
> Cheers,
>
> Stephen
>
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