[erlang-questions] Baffling lists

[Joe Armstrong]

Re performance:

The 0 should have not been at the end of the list in the first place.

It is the program that generated the list that is wrong and should be changed,
not the program that consumes the list ...

Do this and *both* programs might be faster.

It's not really a good idea to have "improper" lists - so I'd check
the code base to
see this doesn't happen - there are rare "I'll save a word" cases where you
might want a non proper list - but the pain of not being able to use standard
libraries will outweigh the pleasure.

/Joe

On Sun, Sep 21, 2014 at 10:52 AM, Stu Bailey <stu.bailey@REDACTED> wrote:
> Thanks to another comment, I see this is "cons" at work making lists where
> the last element is not a list, as Dmytro said.   I got it now.
>
> 130> [[hello] | hello].
>
> [[hello]|hello]
>
> 131> [world,[hello] | hello].
>
> [world,[hello]|hello]
>
> 132> List5 = [world,[hello] | hello].
>
> [world,[hello]|hello]
>
> 133> lists:reverse(List5).
>
> ** exception error: bad argument
>
>      in function  lists:reverse/2
>
>         called as lists:reverse(hello,[[hello],world])
>
>      in call from lists:reverse/1 (lists.erl, line 152)
>
> 134> List5.
>
> [world,[hello]|hello]
>
> 135> is_list(List5).
>
> true
>
> 136>
>
>
> That is a bit quirky, but hey it's Erlang!  The quick hack for my code was
> to replace "lists:reverse(L4)" with my little strip_lists_rev(L4).
>
>
> strip_lists_rev(0,Acc)->
>     Acc;
> strip_lists_rev([H|Rest],Acc)->
>     strip_lists_rev(Rest,[H|Acc]).
>
>
> Hopefully, this won't impact performance and reliability too much.
>
> Thanks again.
>
> On Sun, Sep 21, 2014 at 8:07 AM, Stu Bailey <stu.bailey@REDACTED> wrote:
>>
>> Thanks.  I actually wanted to reverse the list of lists lists:reverse(L4).
>> I did not know about "irregular lists".  Now I do.
>>
>> Thanks again!
>>
>>
>> On Sun, Sep 21, 2014 at 8:00 AM, Dmytro Lytovchenko
>> <dmytro.lytovchenko@REDACTED> wrote:
>>>
>>> Oh and also its list of another list and 0 as tail element. You probably
>>> want to reverse first element of L4, like lists:reverse(hd(L4)).
>>>
>>> On Sun, Sep 21, 2014 at 4:51 PM, Dmytro Lytovchenko
>>> <dmytro.lytovchenko@REDACTED> wrote:
>>>>
>>>> It is irregular list, that is list which doesn't have [] as tail
>>>> element, and has 0 there instead. While its legal to have such a value, some
>>>> list handling functions will be waiting for [] as tail element thus they
>>>> will be confused.
>>>> Solution: don't have 0 there, or cut it away temporarily and append
>>>> later as X = [CutList | 0].
>>>>
>>>> On Sun, Sep 21, 2014 at 4:46 PM, Stu Bailey <stu.bailey@REDACTED>
>>>> wrote:
>>>>>
>>>>> I'm sure I'm missing something obvious, but here goes:
>>>>>
>>>>> What is going on here?  Is L4 a list?  If so, why can't I reverse it?
>>>>>
>>>>> 70> L4.
>>>>>
>>>>> [[<<"8">>,<<"7">>,<<"6">>,<<"5">>],
>>>>>
>>>>>  [<<"4">>,<<"3">>,<<"2">>,<<"1">>]|
>>>>>
>>>>>  0]
>>>>>
>>>>> 71> lists:reverse(L4).
>>>>>
>>>>> ** exception error: bad argument
>>>>>
>>>>>      in function  lists:reverse/2
>>>>>
>>>>>         called as lists:reverse(0,
>>>>>
>>>>>                                 [[<<"4">>,<<"3">>,<<"2">>,<<"1">>],
>>>>>
>>>>>                                  [<<"8">>,<<"7">>,<<"6">>,<<"5">>]])
>>>>>
>>>>>      in call from lists:reverse/1 (lists.erl, line 152)
>>>>>
>>>>> 72> [Head1|Rest1] = L4.
>>>>>
>>>>> [[<<"8">>,<<"7">>,<<"6">>,<<"5">>],
>>>>>
>>>>>  [<<"4">>,<<"3">>,<<"2">>,<<"1">>]|
>>>>>
>>>>>  0]
>>>>>
>>>>> 73> Rest1.
>>>>>
>>>>> [[<<"4">>,<<"3">>,<<"2">>,<<"1">>]|0]
>>>>>
>>>>> 74> lists:reverse(Rest1).
>>>>>
>>>>> ** exception error: no function clause matching
>>>>> lists:reverse([[<<"4">>,<<"3">>,<<"2">>,<<"1">>]|0]) (lists.erl, line 145)
>>>>>
>>>>> 75> [Head2|Rest2] = Rest1.
>>>>>
>>>>> [[<<"4">>,<<"3">>,<<"2">>,<<"1">>]|0]
>>>>>
>>>>> 76> Head2.
>>>>>
>>>>> [<<"4">>,<<"3">>,<<"2">>,<<"1">>]
>>>>>
>>>>> 77> Rest2.
>>>>>
>>>>> 0
>>>>>
>>>>> 78>
>>>>>
>>>>>
>>>>> In practice, the kind of structure demonstrated in L4 is being returned
>>>>> by a function from another library I'm trying to use, but I'm trying to
>>>>> understand what's going on in general.
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> erlang-questions mailing list
>>>>> erlang-questions@REDACTED
>>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>>>
>>>>
>>>
>>
>
>
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