[alexandria-devel] reductions

[Cyrus Harmon]

Hi Alexandrians,

Clojure has a function that I occasionally find useful, reductions. It's like reduce, only it returns not the last value of calling the two-arg function, but a list consisting of the first element of the input sequence, followed by the result of calling the two-arg with the first and second elements of the input sequence, then the  result of calling the two-arg function with the previous result and the third element, and so on... e.g.:

CL-USER> (reductions #'+ '(1 2 3 4))
(1 3 6 10)

Here's the code. There are probably better ways to do this, but this is at least portable and simple. I think this would be a useful addition to alexandria.

(defun reductions (function sequence &rest args)
 (let ((l (list (first sequence))))
   (apply #'reduce
          (lambda (a b)
            (let ((val (funcall function a b)))
              (push val l)
              val))
          sequence
          args)
   (nreverse l)))

thanks,

Cyrus

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