[Haskell-beginners] suggestions for optimizing sudoku solver

[Peter Hall]

I set myself a learning task of writing a sudoku solver.
(https://github.com/peterjoel/sudoku/blob/master/src/Sudoku.hs)
It works pretty well, but struggles with some of the harder grids.
e.g. data/hard4.txt and data/hard5.txt take around 18 seconds to
solve. Obviously there's a limit, but I feel like I should be able to
make this faster.

I think the main issue is reading/writing to the cells in the grid,
since (!!) is O(n). Even though it never has to go beyond index 8, it
will add up over the millions of times it has to do it. I imagine it
could be a lot faster if I use a static, non-list data structure, but
I was hoping to keep it a bit more flexible.

Also, I'm struggling to get started with performance profiling. Can
someone point me to some good resources?

Peter