[Haskell-beginners] another type problem

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

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
>>>
>>>
>>>
>>>
>>> ------------------------------
>>>    [image: Avast logo] <http://www.avast.com/>
>>>
>>> Dit e-mailbericht is gecontroleerd op virussen met Avast
>>> antivirussoftware.
>>> www.avast.com
>>>
>>>  _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>
>>
>> _______________________________________________
>> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>>
>>
>> ------------------------------
>>    [image: Avast logo] <http://www.avast.com/>
>>
>> Dit e-mailbericht is gecontroleerd op virussen met Avast
>> antivirussoftware.
>> www.avast.com
>>
>>  _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
>
> _______________________________________________
> Beginners mailing listBeginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> ------------------------------
>   [image: Avast logo] <http://www.avast.com/>
>
> Dit e-mailbericht is gecontroleerd op virussen met Avast
> antivirussoftware.
> www.avast.com
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>


-- 
Regards

Sumit Sahrawat
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20150512/4c075235/attachment-0001.html>