haskell - Generate a list of unique combinations from a list -


i want generate list of unique ways choose 2 list of numbers in haskell. list [1,2,3] [[1,2],[2,3],[1,3]]. order not important want avoid producing both [1,2] , [2,1] example.

my current solution is:

pairs :: ord => [a] -> [[a]] pairs x = nub $ map sort $ map (take 2) (permutations x)

this isn't particularly nice solution , has serious performance issues. there simple , efficient solution problem?

pairs xs = [[x1, x2] | (x1:xs1) <- tails xs, x2 <- xs1]

...assuming list starts out unique, or compose nub otherwise.


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