[iterate-devel] thereis vs. collect ?

31 May 2006

      I'm getting an error when using thereis and collect in the same iterate:

  (iter (for x :in '(3 1 4 1 5 9))
        (thereis (evenp x))
        (collect x)))

gives

   Attempt to do COLLECT accumulation into a variable already being
   used for IF-EXISTS accumulation.

I don't really understand how these variables in collect, thereis, etc
are chosen, maybe soemone could enlighten me.  

I can rewrite it to bypass thereis:

  (iter (for x :in '(3 1 4 1 5 9))
        (let ((result (evenp x)))
          (when result
            (leave result)))
        (collect x)))

or use my own accumulation variable:

  (iter (for x :in '(3 1 4 1 5 9))
        (thereis x)
        (collect x :into y)))

but I prefer not to do either of those.  Far too much typing!

Cheers,
Chris Dean