[Haskell-beginners] another type problem
Sumit Sahrawat, Maths & Computing, IIT (BHU)
sumit.sahrawat.apm13 at iitbhu.ac.in
Wed May 13 11:32:54 UTC 2015
Init returns all but the last element of a list. The idea behind the
recursive solution is to keep accepting elements till there is only one
left, and replace it with the empty list.
For example,
init [1, 2, 3]
== init (1 : 2 : 3 : []) { desugar list syntax }
== 1 : init (2 : 3 : []) { apply init }
== 1 : 2 : init (3 : []) { apply init }
== 1 : 2 : [] { init [x] = [] }
On 13 May 2015 at 15:32, akash g <akaberto at gmail.com> wrote:
> Perhaps a good place to start with functional programming is SICP :
> http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-4.html#%_toc_start
>
> While it doesn't teach you via the Haskell language, it teaches you how to
> construct functional programs and how the compiler/interpreter views it.
> Also, it teaches higher order programming quite well, At least, it is much
> better than every intro haskell book I've read.
>
> On Wed, May 13, 2015 at 3:23 PM, akash g <akaberto at gmail.com> wrote:
>
>> *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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>>>>>>
>>>>>>
>>>>>>
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>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Regards
>>>>>
>>>>> Sumit Sahrawat
>>>>>
>>>>> _______________________________________________
>>>>> Beginners mailing list
>>>>> Beginners at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>
>>>>>
>>>>
>>>>
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>
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--
Regards
Sumit Sahrawat
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