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

Carlos J. G. Duarte carlos.j.g.duarte at gmail.com
Tue Aug 14 20:05:56 CEST 2012

```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
>     -- [[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
>
>     -- [[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 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 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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