[racket] Tail recursive module cons

From: Sam Tobin-Hochstadt (samth at cs.indiana.edu)
Date: Sun Mar 16 15:38:35 EDT 2014

On Sun, Mar 16, 2014 at 2:55 PM, Patrick Useldinger
<uselpa.list at gmail.com> wrote:
> My questions are:
> (1) Is Racket "tail-recursive modulo cons"? If not, is there a reason for
> this?

Racket is not "tail-recursive modulo cons".

> (2) Is the last example "reverse free" or does it use reverse under the
> hood?

`for/list` uses `reverse` internally -- you can see for yourself how
this works by using the Macro Stepper.

> (3) finally, what is the recommended way to write this procedure in terms of
> performance?

I took your program, and made all the functions run on much longer
lists. Here are the timings:

$ r /tmp/even-list.rkt
cpu time: 1288 real time: 1294 gc time: 936
cpu time: 440 real time: 441 gc time: 320
cpu time: 484 real time: 485 gc time: 328
cpu time: 204 real time: 204 gc time: 148
cpu time: 632 real time: 631 gc time: 416
cpu time: 460 real time: 460 gc time: 332

'mcons-convert is one that uses `mcons`, but converts back to plain
`cons` at the end.


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