To measure how long it takes GHCi to evaluate something, use :set +s
The main function in Haskell should alway be of type IO ().
Instead of duplicating a complete file and then making changes or additions, you should use import and then put additional new functions in the second file. See here and here for examples and here for a whole chapter on modules.
Malvin made publicity for the Atom editor. Works on any of OS X, Windows, Linux, and has great support for Haskell software development.
The average number of hints in a Sudoku is decreasing in this order: Classic, Crossed, NRC, NRCX.
Counting Sudokus: We discussed different ideas from Stefania and Eric but it does not seem straightforward.
A good source on the mathematics of sudoku is this wikipedia entry.
The example looked wrong in some browsers because a strikedthrough four did look exactly the same as a normal four. The numbers are grayed out now as well: https://en.wikipedia.org/wiki/Stable_roommates_problem#Example