# Talk:Tying the Knot

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[[User:WarDaft|WarDaft]] 17:25, 11 April 2012 (UTC) | [[User:WarDaft|WarDaft]] 17:25, 11 April 2012 (UTC) | ||

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+ | == takeF and takeR in the DList example do not compile for me == | ||

+ | |||

+ | I had to modify them as so: | ||

+ | |||

+ | <haskell> | ||

+ | takeF :: Integer -> DList a -> [a] | ||

+ | takeF 0 _ = [] | ||

+ | takeF n (DLNode _ x next) = x : (takeF (n-1) next) | ||

+ | |||

+ | takeR :: Show a => Integer -> DList a -> [a] | ||

+ | takeR 0 _ = [] | ||

+ | takeR n (DLNode prev x _) = x : (takeR (n-1) prev) | ||

+ | </haskell> | ||

+ | |||

+ | [[User:Psybur|Psybur]] 15:10, 12 October 2017 (UTC) |

## Latest revision as of 14:54, 12 October 2017

There's a conceptually much simpler way build a circular structure, though it has a substantial performance overhead (n^2) the first time you run through the nodes:

mkDLList list = head result where (result, n) = (zipWith mknode list [0..], length list) mknode x i = DLList (result !! ((i - 1) `mod` n) ) x (result !! (i + 1 `mod` n) )

Since we already have the result - the list of all the relevant nodes - we just simply point to the items at the right points on the list. When we do it this way, it's obvious what is going on from just a basic understanding of laziness, then we see a huge waste of operations in the repeat list traversing, and look for some way to make it O(n). The trick, of course, being tying the knot.

With a slight tweak, this also serves as a simple method for defining arbitrary graphs, which is best given a different sort of optimization.

WarDaft 17:25, 11 April 2012 (UTC)

## [edit] takeF and takeR in the DList example do not compile for me

I had to modify them as so:

takeF :: Integer -> DList a -> [a] takeF 0 _ = [] takeF n (DLNode _ x next) = x : (takeF (n-1) next) takeR :: Show a => Integer -> DList a -> [a] takeR 0 _ = [] takeR n (DLNode prev x _) = x : (takeR (n-1) prev)

Psybur 15:10, 12 October 2017 (UTC)