[erlang-patches] Add lists:init/1 - got tired of implementing it

[Hans Svensson]

You are absolutely right about the 'badarg', it is better to have a 
consistent failure!

However on performance, the suggested implementation is somewhat 
surprisingly the best option (I admit, I did measure before writing the 
patch ;-) ) For lists up to 3000 elements, it is on average ~x1.8 faster 
than a (tail-)recursive implementation!

So a better version would probably be:
init([]) -> error(badarg);
init(List = [_ | _]) ->
   lists:reverse(tl(lists:reverse(List)));
init(_) -> error(badarg).

/Hans

On 2013-04-25 18:18, Fred Hebert wrote:
> I don't think there's any benefit in tail-recursion when you're
> rebuilding an entirely new list. Given the limited number of arguments,
> the list you're creating by not being tail-recursive is in all points
> similar to the one you would have to maintain on the stack through tail
> recursions. Maybe a bit larger, but given you don't have to trash it,
> you're likely to avoid GC costs by doing it that way.
>
> See http://erlang.2086793.n4.nabble.com/Efficiency-of-a-list-construction-td3538122.html#a3538379
> and http://ferd.ca/erlang-s-tail-recursion-is-not-a-silver-bullet.html
> for more information.
>
> What I would worry about in my implementation is adding a clause
> of 'init([]) -> error(badarg)' to be consequent in failures with the rest
> of the lists module, and not get a 'function_clause' error there when
> someone passes in an empty list.
>
> On 04/25, Yurii Rashkovskii wrote:
>> What about non-tail-recursiveness, though?
>>
>>
>> On Thu, Apr 25, 2013 at 8:54 AM, Fred Hebert <mononcqc@REDACTED> wrote:
>>
>>> Sounds to me like it would have been better implemented as something like:
>>>
>>> init([_]) -> [];
>>> init([H|T]) -> [H|init(T)].
>>>
>>> As this would avoid re-building the list twice, once on each reversal.
>>>
>>> Regards,
>>> Fred.
>>>
>>>
>>> On Thu, Apr 25, 2013 at 11:12 AM, Hans Svensson <hanssv@REDACTED> wrote:
>>>
>>>> Hi,
>>>>
>>>> Yesterday I implemented, for the umpteenth time, the init-function. (I
>>>> guess being taught Haskell at one point could be blamed partly for the
>>>> coding style requiring this function...) It is a trivial function that I
>>>> think should be part of the standard lists module. This patch adds the
>>>> function, tests in lists_SUITE, and documentation of the function.
>>>>
>>>> The implementation is trivial: reverse(tl(reverse(List))).
>>>>
>>>> If I've missed some religious reason for not having this function feel
>>>> free to drop the patch ;-)
>>>>
>>>> Cheers,
>>>> Hans
>>>>
>>>> git fetch git://github.com/hanssv/otp.**git<http://github.com/hanssv/otp.git>add_init_to_lists
>>>>
>>>> https://github.com/hanssv/otp/**compare/erlang:maint...add_**
>>>> init_to_lists<https://github.com/hanssv/otp/compare/erlang:maint...add_init_to_lists>
>>>> https://github.com/hanssv/otp/**compare/erlang:maint...add_**
>>>> init_to_lists.patch<https://github.com/hanssv/otp/compare/erlang:maint...add_init_to_lists.patch>
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>>>> erlang-patches mailing list
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>>>>
>>>
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