[philly-lambda] Functional Reduce

["Andrew Gwozdziewycz" <apgwoz@gmail.com>]

I don't remember who said it, but someone last night mentioned that
reduce was inherently not functional due to the accumulation (and
mutation of that accumulation) of values.

When reduce (or fold if you will) "iterates" through the values of
list it applies the passed in function to the spot in the list it's at
and the accumulation so far. This fact makes it unparallelizable (I
think), unlike map, or filter, which do not accumulate any values, but
instead return "results" of subsequent calls.

Anyway reduce can actually be implemented functionally in Scheme with
recursion. Best yet, it's actually a tail call and so it can be
optimized (and in Scheme of course has to be).

(define (reduce func accum lst)
  (cond
   ((null? lst) accum)
   (else
    (let ((accum-new (func (car lst) accum)))
      (reduce func accum-new (cdr lst))))))

;(reduce cons '() '(1 2 3)) => (3 2 1)
;(reduce + 0 '(1 2 3)) => 6

As with most functional functions, there exists a non recursive
version which would
require a mutated variable, but reduce is a tail call in scheme.

Hope this clears up some confusion...

-- 
Andrew Gwozdziewycz
apgwoz@gmail.com
http://www.apgwoz.com  | http://www.photub.com

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