[Haskell-cafe] Bi-directional Maps

Andrew Wagner wagner.andrew at gmail.com
Sun Aug 19 18:55:45 EDT 2007

Hi folks!

So, in writing my chess engine, I've been trying to maintain 2 Map
objects. One maps squares on the board to Ints, the other maps Ints to
actual Pieces. It occurred to me that it would be useful to explicitly
have a Bi-directional Map, which does the maintenance of keeping the
Maps synchronized behind the scenes. Thus, Bimap was born! I've taken
the API for Data.Map (which you can find at ), and cannibalized it for
Bimap. The new API is at http://hpaste.org/2344 . The idea is that if
you have a Bimap k a, and you want to treat the k's as keys, and use a
function like Data.Map.foo, it will be called Data.Map.left_foo in
Bimap. And if they want to do the same thing, but using the a's as
keys, then they simply use right_foo. The metaphor is that we can view
it as a Map in 2 directions, manipulating it from the left (on the
k's), or from the right (on the a's).

Is this useful? Is there a better way? Is the API too big, and if so,
how can it be pared down?

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