[Haskell-beginners] another type problem

akash g akaberto at gmail.com
Wed May 13 09:53:01 UTC 2015 

*wrap, not rap. In my previous mail.

Also, there is already a library that does list operations in a safe way,
though they have used monads.  Give a look at the source code and see if it
helps.

https://hackage.haskell.org/package/listsafe-0.1.0.0/docs/Data-List-Safe.html


On Wed, May 13, 2015 at 3:19 PM, akash g <akaberto at gmail.com> wrote:

> Ah, but it doesn't.  Since this is a functional language, we have a
> pointer to it.  In fact, all it does is rap it around in the Maybe type.
>
> Also, my version doesn't do what you want.  One way to do that would be
> what you have.  Or something like this.
>
> The following turns the partial function in the Prelude to one that is safe
>
> ==========
> import Data.List
>
> init' :: [a] -> Maybe [a]
> init' [] = Nothing          -- Base case
> init' xs = Just $ tail xs   -- Inductive case
> ==========
>
>
>
>
>
>
>
> On Wed, May 13, 2015 at 2:47 PM, Roelof Wobben <r.wobben at home.nl> wrote:
>
>>  Hello,
>>
>> What my intention was to make a safe version of the init function found
>> in Data.list so it procudes a list without the last item.
>> At first look your function procudes the whole list again.
>>
>> Roelof
>>
>>
>>
>> akash g schreef op 13-5-2015 om 10:57:
>>
>>   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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>>>
>>>
>>>  --
>>>    Regards
>>>
>>>  Sumit Sahrawat
>>>
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