Map with unordered tuple as key

Question

I have a mapping between a tuple with both parts of the same type and a Int.

Map (a,a) Int

Independent of the ordering of the as in the tuple I want later to be able to get it out of the map.

lookup (2,1) map == lookup (1,2) map

Is this possible without inserting the tuple twice?

Ørjan Johansen · Accepted Answer · 2014-08-13 11:06:53Z

8

You can make the key (max a b, min a b).

answered Aug 13, 2014 at 11:06

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Comments

ThreeFx · Accepted Answer · 2014-08-13 11:07:25Z

You can just sort the tuple and then use this to inseet the tuples (and search too):

sortTup :: (Ord a) => (a, a) -> (a, a) sortTup (a, b) = (min a b, max a b)

Using it you would look something like this:

Prelude Data.Map> let a = Map.fromList [(sortTup (2,1), 5] Prelude Data.Map> lookup (sortTup (1,2)) 5

shree.pat18 · Accepted Answer · 2014-08-13 13:32:16Z

You could try this:

import Control.Applicative mylookup :: (Ord a) => (a,a) -> Map (a,a) Int -> Maybe Int mylookup (x,y) m = lookup (x,y) m <|> lookup (y,x) m