# [Haskell-beginners] cleanest way to unwrap a list?

Tim Perry tim.v2.0 at gmail.com
Tue Aug 14 20:35:32 CEST 2012

```There is a way. Please try to figure it out and if you fail post back with
your code and we can help you from there.

On Tue, Aug 14, 2012 at 11:05 AM, Carlos J. G. Duarte <
carlos.j.g.duarte at gmail.com> wrote:

> Ok, you all have been showing examples of running functions over elements.
> Add one, append value, and so on.
> This works well if there's one or more operations to apply indistinctly to
> a number of elements.
>
> Now, what if we just want to make a single operation to a single element?
> For example, let's say I have this square matrix
> [[1,2,3],
>  [4,5,6],
>  [7,8,9]]
>
> how can we increment the value 5 (position 2,2) *and* decrement the value
> 7 (position 3,1)?
>
> This is a made up example of course, I just want to see / learn if there's
> a way to apply a function to a specific subset of elements.
>
>
> On 08/14/12 00:06, Jack Henahan wrote:
>
>> Equally,
>>
>>      let map' = map . map
>>      map' (+1) . map (++[3]) \$ [[1,2],[3,4]]
>>      -- [[2,3,4],[4,5,4]]
>>
>> And you can really keep stacking those up. I think this approach will be
>> cleaner in the long run.
>>
>> For instance, let's start naming our parts.
>>         let list = [[1,2],[3,4]]
>>     let map' = map . map
>>     let addOne = map' (+1)
>>     let appendThree = map (++[3])
>>     let reverseInner = map reverse
>>
>> So, from here we can do the following:
>>         list
>>     -- [[1,2],[3,4]]
>>
>>     -- the first example
>>     addOne list
>>     -- [[2,3],[4,5]]
>>         -- now the second example
>>     addOne . appendThree \$ list
>>     -- [[2,3,4],[4,5,4]]
>>
>>     -- now add one to all members of the list, append three to the list,
>> reverse the inner lists,
>>     -- then add one to all members of the new list
>>
>>     addOne . reverseInner . appendThree . addOne \$ list
>>     -- [[4,4,3],[4,6,5]]
>>
>> Now how would you construct that as a list comprehension? With the method
>> I've proposed, you need
>> only use map to operate on the nested lists themselves and map' to
>> operate on the elements of those
>> lists.
>>
>> ====
>> Jack Henahan
>> jhenahan at uvm.edu
>>
>> On Aug 13, 2012, at 6:41 PM, Christopher Howard <christopher.howard@**
>> frigidcode.com <christopher.howard at frigidcode.com>> wrote:
>>
>>  On 08/12/2012 09:37 PM, Shakthi Kannan wrote:
>>>
>>>> Hi,
>>>>
>>>> --- On Mon, Aug 13, 2012 at 10:51 AM, Christopher Howard
>>>> <christopher.howard@**frigidcode.com<christopher.howard at frigidcode.com>>
>>>> wrote:
>>>> | Say, for example, I have the list
>>>> | [[1,2],[3,4]] and want to add 1 to each inner element, resulting in
>>>> | [[2,3],[4,5]].
>>>> \--
>>>>
>>>> Like this?
>>>>
>>>> ghci> let xxs = [[1,2], [3,4]]
>>>>
>>>> ghci> [ [ x+1 | x <- xs] | xs <- xxs ]
>>>> [[2,3],[4,5]]
>>>>
>>>> SK
>>>>
>>>>  Thanks everyone for the responses. I found the list comprehension
>>> approach satisfactory, as it allows me to cleanly modify each layer of
>>> the nested array as I unwrap it:
>>>
>>> code:
>>> --------
>>> b = [[ x+1
>>>     | x <- xs ++ [3] ]
>>>     | xs <- [[1,2],[3,4]] ]
>>>
>>> *Main> b
>>> [[2,3,4],[4,5,4]]
>>> --------
>>>
>>> The only downside is that I have to write the layers out in reverse of
>>> the way I would normally think of them, but that isn't too big of a
>>> challenge.
>>>
>>> I'm not sure how that would be done with map in a way that would be neat
>>> and readable and wouldn't require declaring extra identifiers. I can't
>>> give a fair evaluation of the Lens approach because I don't understand
>>> enough of the theory yet.
>>>
>>> --
>>> frigidcode.com
>>> indicium.us
>>>
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>>>
>>
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>>
>
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