Scala Options are a nice alternative to null values. They force us to consider the None case explicitly, rather than hiding in documentation, for example, the fact that a method can return null in some circumstances. They also tie in nicely to chained behavior, so that a sequence of operations that produce a None along the way can collapse to just the None, similar to constructs such as Groovy’s elvis operator ?:.

I have been working through a sudoku solver as a Scala learning project. This solution is based on constraint propagation – eliminating possibilities and placing values in a mutually recursive relationship – and search, as described in Peter Norvig’s article. I wanted to distinguish between valid puzzle states (some or all values placed in the grid, but no conflicts) and contradictory states (trying to place a value in a position where it is not possible), using a construct in the type system, rather than using null as might be done in Java.

I thought an Option would be a good choice for this:

def placeConjecture(row:Int, col:Int, conjecture:Int):Option[SudokuGrid]

Placing a conjecture returns a new SudokuGrid with updated values, wrapped in an Option. If the conjecture is possible, Some(newgrid) is retured, but if the conjecture leads to a contradiction, None is returned.

To build up a puzzle, for example establishing the starting state or just setting up a scenario for testing, it is necessary to place multiple conjectures. Without Option, we’d use something like:

emptyGrid.placeConjecture(0,0,5).placeConjecture(3,5,7).placeConjecture(2,6,1)

With option, we have to account for the possibility of one of these placeConjecture calls returning None. The Option object of course doesn’t have the placeConjecture method, so we need to extract the contained SudokuGrid in the case of Some(grid), or short-circuit and return None for the whole chain.

Having no idea how to go about this, I started with a simple case for tests:

emptyGrid.placeConjecture(0,0,5).get.placeConjecture(3,5,7).get.placeConjecture(2,6,1)

This obviously doesn’t handle propagation of the None value. For setting up some tests when I knew that no contradictions would arise, this sufficed, but was far from ideal.

One nice feature of Option, is that as a monad (not that I completely understand the concept; have no fear that a monad post is coming), it can be used in a for comprehension. In this case, the chain of Options get passed through, evaluating to the final result when all the Options are Some() and short-circuiting to None if any of them are None. So my attempt to use a for comprehension with this sequence looked something like this:

for { a <- empty.placeConjecture(0,0,5) b <- a.placeConjecture(3,5,7) c <- b.placeConjecture(2,6,1) } yield c

This works fairly well for small examples like above, but it gets unwieldy quickly. It also doesn’t allow (at least, not that I could figure out) use of a generator to generate some of the values. I wanted to be able to place conjectures (or equivalently, eliminate possibilities) for a number of cells at once, and needed to determine how to make the results from one iteration carry into the next iteration, ideally without using a var.

After banging my head against this for a bit, I realized I was using the wrong approach. Rather than for comprehension, what I was really doing was combining results across a list – folding!

List((0,0,5),(3,5,7),(2,6,1)).foldLeft(Some(empty))( (gridopt,(row,col,conjecture)) => gridopt match { case None => None case Some(grid) => grid.placeConjecture(row,col,conjecture) })

The starting list contains the conjectures I want to place. Starting with an empty SudokuGrid, the combination function calls the placeConjecture method on Somes, passing the result on to the next iteration. When None is encountered, this gets passed the rest of the way along.

This approach then combines well with generating the list programmatically:

val thelist = for (i <- 1 to 8) yield i thelist.foldLeft(Some(Empty))( (gridopt,poss) => gridopt match { case None => None case Some(grid) => grid.eliminatePossibility(0,0,poss) } )

In the end, I decided that layering my semantics on top of Option wasn’t quite as expressive as I would like. Specifically, this approach means that the calling code needs to know that None means a contradictory puzzle state. I instead created an algebraic type with LiveSudokuGrid subclasses for non-contradictory states and a ContradictorySudokuGrid object subclass for contradictory states. By implementing placeConjecture and eliminatePossibility appropriately for the subtypes I can propagate the contradictory result across further calls. At some point I’ll try to make these monadic as an exercise, but that’s for another post!

Kero