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RE: Will Fitzgerald <fitzgerald@xxxxxxxxxxxx>

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Here's another use of list-difference/ldiff, which is very similar to the
parse-signature example of Darius Bacon.

I'm working on a revision of the RAP System, a language for task execution, and
doing big parts of this in Scheme. A RAP index has the form:

 (name in-var* [=> out-var*])

It's useful to have a list of just the name and in-vars, and:

 (LIST-DIFFERENCE index (memq '=> index))

does just this.

Other examples I've seen of using LDIFF/LIST-DIFFERENCE tend to be for parsing
key word-like lists.


Will Fitzgerald

-----Original Message-----
From: Darius Bacon [mailto:darius@xxxxxxxxxxxxx]
Sent: Monday, January 11, 1999 2:27 PM
To: srfi-1@xxxxxxxxxxxxxxxxx
Subject: Re: Will Fitzgerald <fitzgerald@xxxxxxxxxxxx>

> Michael Sperber <sperber@xxxxxxxxxxxxxxxxxxxxxxxxxxx> writes:
> >>>>>> "Will" == Will Fitzgerald <fitzgerald@xxxxxxxxxxxx> writes:
> >
> >Will> May I suggest the following addition to the SRFI-1 list library,
> >Will> Common Lisp's LDIFF:
> >
> >Will> I think a reasonable definition is:
> >Will> [...]
> >[...]
> >Could you suggest possible uses for this procedure?

Here's a fairly trivial use taken from a real program:

(define parse-signature
  (lambda (spec)
    (let* ((mid (memq '- spec))
	   (in  (list-difference spec mid))
	   (out (cdr mid)))
      (make-signature in out))))

And here's another:

(define (butlast L)
  (list-difference L (last-pair L)))

But groveling through my source directories reveals fewer instances of
list-difference than I expected.