[racket] Tail recursive module cons

What does tail-recursion modulo cons mean? 


On Mar 16, 2014, at 3:38 PM, Sam Tobin-Hochstadt wrote:

> 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
> 'plain
> cpu time: 1288 real time: 1294 gc time: 936
> 't
> cpu time: 440 real time: 441 gc time: 320
> 'for/fold
> cpu time: 484 real time: 485 gc time: 328
> 'mcons
> cpu time: 204 real time: 204 gc time: 148
> 'mcons-convert
> cpu time: 632 real time: 631 gc time: 416
> 'for/list
> 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.
> 
> Sam
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