[Haskell-beginners] another type problem
akash g
akaberto at gmail.com
Wed May 13 09:49:27 UTC 2015
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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>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
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