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

Ben Kolera ben.kolera at gmail.com
Mon Aug 13 07:48:30 CEST 2012

Maybe this is overkill, but if you want a more generic way for accessing and changing nested immutable data in general, you could check this package out as it is pretty exciting (to me at least): 


For more information on exactly what a lens is, read here:



On Monday, 13 August 2012 at 3:21 PM, Christopher Howard wrote:

> Hi. Is the some generic, clean syntax to unwrap a nested list, modify
> the value, and put it back together? 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]].
> After reading about the list monad, I was rather excited, because I
> (mistakenly) thought something like this would work:
> code:
> --------
> a = do b <- [[1,2],[3,4]]
> c <- b
> return (c + 1)
> --------
> That would be awesome, because I would be able to modify the list at
> each level of unwrapping, while leaving the code very neat and readable.
> However, what the above example actually does is produce a /single/ list
> from the values:
> code:
> --------
> *Main> a
> [2,3,4,5]
> --------
> Obviously wishing won't change how the list monad works, but I thought
> it might be worth asking if there is some other monad or syntactic trick
> that does something along the lines of what I am looking for.
> -- 
> frigidcode.com (http://frigidcode.com)
> indicium.us (http://indicium.us)
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