[plt-scheme] define-union

From: Anton van Straaten (anton at appsolutions.com)
Date: Fri Apr 18 21:29:59 EDT 2003

Stephen Bloch wrote:
> Example:
> (define-struct foo (a b))
> (define-struct bar (c d e))
> (define-union snark (foo bar number))
> Semantics: checks that foo, bar, and number are in fact known types, 
> with one-place discriminator predicates foo?, bar?, and number? 
> respectively, and does the equivalent of
> (define (snark? x)
>     (if (or (foo? x) (bar? x) (number? x)))

How about modifying your spec slightly to allow this:

	(define-union snark? (foo? bar? number?))

...which would allow you to use this:

	(define-syntax define-union
	  (syntax-rules ()
	    ((_ name (pred ...))
	     (define (name x) (or (pred x) ...)))))

...which has the virtue of being very simple to implement and understand.


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