Send Beginners mailing list submissions to
[email protected]
To subscribe or unsubscribe via the World Wide Web, visit
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
[email protected]
You can reach the person managing the list at
[email protected]
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: another type problem (akash g)
----------------------------------------------------------------------
Message: 1
Date: Wed, 13 May 2015 15:23:01 +0530
From: akash g <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] another type problem
Message-ID:
<CALiga_fxH1+s+cHohFtvMCmO+Fg7=n+05fS=0BUNbcwfcd5f=w...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
*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 <[email protected]> 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 <[email protected]> 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) <[email protected]> wrote:
>>
>>> On 12 May 2015 at 20:14, Roelof Wobben <[email protected]> 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 <[email protected]> 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 <[email protected]>
>>>>> wrote:
>>>>>
>>>>>> Brandon Allbery schreef op 12-5-2015 om 16:16:
>>>>>>
>>>>>> On Tue, May 12, 2015 at 10:11 AM, Roelof Wobben <[email protected]>
>>>>>> 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
>>>>>> [email protected]
>>>>>> [email protected]
>>>>>> unix, openafs, kerberos, infrastructure, xmonad
>>>>>> http://sinenomine.net
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Beginners mailing
>>>>>> [email protected]http://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
>>>>>> [email protected]
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing
>>>>> [email protected]http://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
>>>>> [email protected]
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing
>>>> [email protected]http://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
>>>> [email protected]
>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>>
>>> --
>>> Regards
>>>
>>> Sumit Sahrawat
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> [email protected]
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>>
>> _______________________________________________
>> Beginners mailing
>> [email protected]http://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
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20150513/011d0430/attachment.html>
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
[email protected]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 83, Issue 31
*****************************************
