[Haskell-beginners] another type problem

[akash g]

Why not just?

==================
init' [] = Nothing
init' xs = Just xs
==================

Meets your type sig and is also has a time complexity of O(1) instead of
O(n) which will be the time complexity in the solution involving
fromMaybe.  Maybe I'm missing something.

Perhaps it'd help us help you if you'd be a bit more clear on what you want
to achieve.

On Tue, May 12, 2015 at 9:01 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> On 12 May 2015 at 20:14, Roelof Wobben <r.wobben at home.nl> wrote:
>
>>  Thanks,
>>
>> The right solution was this :
>>
>> init' (x:xs) = Just (x:fromMaybe xs (init' xs))
>>
>> if I understand it right x:fromMaybe xs takes care that from xs the just
>> or Nothing is removed ?
>>
>>
> Trying it out in ghci,
>
> fromMaybe [1,2] Nothing == [1,2]
> fromMaybe [1,2] Just [3] == [3]
>
> It seems like that should indeed work.
>
> Roelof
>>
>>
>>
>> Alexey Shmalko schreef op 12-5-2015 om 16:33:
>>
>> Try fromMaybe [1]. Examples
>>
>> Prelude Data.Maybe> fromMaybe [] (Just [1,2,3])
>> [1,2,3]
>> Prelude Data.Maybe> fromMaybe [1,2,3] Nothing
>> [1,2,3]
>>
>>  [1]:
>> https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Maybe.html#v:fromMaybe
>>
>> On Tue, May 12, 2015 at 5:28 PM Roelof Wobben <r.wobben at home.nl> wrote:
>>
>>>  Oke,
>>>
>>> And how do I do this.  Haskell is a difficult one to learn,
>>>
>>> Roelof
>>>
>>>
>>>
>>> Alexey Shmalko schreef op 12-5-2015 om 16:24:
>>>
>>> Before cons'ing the result of init', you should check whether it's Just
>>> or Nothing. What you're doing now is something along the line with 5 : Just
>>> 3 -- this won't typecheck.
>>>
>>> On Tue, May 12, 2015 at 5:22 PM Roelof Wobben <r.wobben at home.nl> wrote:
>>>
>>>>  Brandon Allbery schreef op 12-5-2015 om 16:16:
>>>>
>>>>  On Tue, May 12, 2015 at 10:11 AM, Roelof Wobben <r.wobben at home.nl>
>>>> wrote:
>>>>
>>>>>  I do not understand what you are saying to me.
>>>>>
>>>>> I know that init produces a Maybe [a] . That is why I did put a Just
>>>>> before it.
>>>>>
>>>>
>>>>  You are invoking it again though, and using its result as if it
>>>> produces [a] instead of Maybe [a].
>>>>
>>>>  --
>>>>  brandon s allbery kf8nh                               sine nomine
>>>> associates
>>>> allbery.b at gmail.com
>>>> ballbery at sinenomine.net
>>>> unix, openafs, kerberos, infrastructure, xmonad
>>>> http://sinenomine.net
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>>  What is then the right way to do. I still do not understand what you
>>>> are trying to make clear to me.
>>>>
>>>>
>>>> Roelof
>>>>
>>>>
>>>>
>>>>
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>>>
>>>
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>>
>>
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>>
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>
>
> --
> Regards
>
> Sumit Sahrawat
>
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