Thank you Stephen,
Quite probably the algorithm is working correctly just that for boards
5 it takes a pathologically long time (for boards <= 5 I haven't checked the answer is correct just that it is computed).
I checked it out with 5 and looks like the answer is correct also :)
In the first message I was looking for places where the input data grew or stayed constant size rather than got pruned. If you have runaway recursive a first debugging step is often to look for errors in pruning. Unfortunately I read the code wrongly, apologies again.
no problem :)
To get it to work better, an improved search strategy would help, also better data structure that allows efficient indexing would be nice. That said, a clever algorithm can do a lot without indexing - you might want to search for Richard Bird's Sudoku slide presentation which is expanded in his new book.
Did you mean this http://www.cs.tufts.edu/~nr/comp150fp/archive/richard-bird/sudoku.pdf? Also, is this the book - http://www.flipkart.com/pearls-functional-algorithm-design-richard-book-0521... I dont have this book ... but looks like its a must have.
This paper has search strategies in a lazy setting and a version of n-Queens:
Thomas Nordin and Andrew Tolmach - Modular Lazy Search for Constraint Satisfaction Problems http://web.cecs.pdx.edu/~apt/jfp01.ps
This is probably what I like (and dislike - in a nice way) - my quest to master Haskell has been like traversing through a ever expanding tree :) Very different from a bunch of "learn over a weekend" language!!! -- Regards, Kashyap