Implementation of Donald Knuth's Algorithm X. This is NOT the dancing links algorithm. Instead of matrices, I just use maps and sets. If I remember correctly, the performance got WORSE when I tried to switch over to IntMap and IntSet (wtf haskell?). I don't remember if I tried using IOArrays.
$ ghci
GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :load Inst.Queens
[1 of 3] Compiling ExactCover ( ExactCover.hs, interpreted )
[2 of 3] Compiling Lib.GeneralizedExactCover ( Lib/GeneralizedExactCover.hs, interpreted )
[3 of 3] Compiling Inst.Queens ( Inst/Queens.hs, interpreted )
Ok, modules loaded: Inst.Queens, Lib.GeneralizedExactCover, ExactCover.
*Inst.Queens> solveQueens 4
Loading package array-0.4.0.0 ... linking ... done.
Loading package deepseq-1.3.0.0 ... linking ... done.
Loading package containers-0.4.2.1 ... linking ... done.
[[Move 0 1,Move 1 3,Move 2 0,Move 3 2],[Move 0 2,Move 1 0,Move 2 3,Move 3 1]]
There are also matrix, sudoku, and pentominos examples.
They might have bitrotted, though.