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    <p>Some comments on your problem:</p> <p><strong>Answer the Question</strong>: There may be a way to do what you want to do. The Lens library is amazingly generic. What there is not is a simple or obvious way to make it happen. I think the it will involve the <a href="http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Lens-Traversal.html#v:partsOf" rel="nofollow noreferrer"><code>partsOf</code> combinator</a> but I'm not sure.</p> <p><strong>Comments on Lenses</strong>: The lens library is really cool and can apply to an amazing number of problems. My initial temptation as I am learning the library was to try to fit everything into a Lens access or mutation. What I discovered was that it was better to use the lens library to dig into my complex data structures, but once I had a simple element it was better to use the more traditional functional techniques I already knew rather then stretching the Lens library past it's useful limit.</p> <p><strong>Advice you didn't ask for</strong>: Inserting an element into the middle of a list is a bad idea. Not that it cannot be done but it can end up being an O(n^2) operation. (<a href="https://stackoverflow.com/questions/9611904/haskell-lists-arrays-vectors-sequences/9613203#9613203">See also this StackOverflow answer.</a>)Zip lists or some other <a href="http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf" rel="nofollow noreferrer">functional data structure</a> may be a better idea. As a side benefit, some of these structures could be made instance of the <code>At</code> class allowing for insertion and deletion using the partial lens combinators.</p>
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    1. USJohn F. Miller
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