Best way to append an element to a list?

[David Storrs]

If I want to add an element to the end of an immutable list, is it better to do:

(append current-list (list new-element))

or

(reverse (cons new-element  (reverse current-list)))

or

something else? 

I would imagine it's the first one because that should be a single traversal and link add instead of two traversals and rebuilds, but maybe I'm wrong.

[Sage Gerard]

I get these timings on x86_64 GNU/Linux using the following profile program.

; A: cpu time: 7444 real time: 7445 gc time: 716                                    
; B: cpu time: 9227 real time: 9228 gc time: 884

(module profile racket/base
  (define target-length #e5e4)

  (define (A current-list new-element)
    (append current-list (list new-element)))

  (define (B current-list new-element)
    (reverse (cons new-element  (reverse current-list))))

  (define (profile function)
    (printf "~a: " (object-name function))
    (time
     (void (for/fold ([l null])
                     ([n (in-range target-length)])
             (function l n)))))

  (profile A)
  (profile B))

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[David Storrs]

Cool, thank you.

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