Creating a compound iterator in F#

Question

I'm implementing a checkers-like game, and I need a sequence that enumerates all legal moves for a given configuration.

I've got the following function, directly translated from C#:

seq {
    for y1 = 0 to BOARDSIZE-1 do
        for x1 = 0 to BOARDSIZE-1 do
             for dy = -2 to 2 do
                 for dx = -2 to 2 do
                     let x2 = x1 + dx;
                     let y2 = y1 + dy;
                     let currentMove = new MoveStruct(x1, y1, x2, y2);
                     if (currentMove.SomeCondition = true) then
                             yield currentMove;
   }

It works, but it's awkward, and not quite the "F# way", let alone I have a sneaking suspicion that what I'm doing here is not performance optimal.

What I would like is to "flatten this out" into something that uses a combination of "iterate over all cells", "iterate over all valid moves from this cell".

And here are the functions I'm hoping to combine:

let AllCells =
    seq {
        for y=0 to BOARDSIZE-1 do
            for x=0 to BOARDSIZE-1 do
                yield (x,y);
    };

AND

let LegalMovesAround(x1,y1) = 
    seq {
      if board.[x1, y1] = WHITE then
        for dy = -2 to 2 do
          for dx = -2 to 2 do
                let x2 = x1 + dx;
                let y2 = y1 + dy;
                let currentMove = new MoveStruct(x1, y1, x2, y2);
                if (currentMove.DetermineMoveType <> MoveType.ILLEGAL 
                    && board.[x2, y2] = NONE) then
                        yield currentMove;
     }

I'm going to spare you the details of my various attempts to make it work, because none of them were successful. But to make the long story short, the best I could come up with is an iterator that returns a seq with each yield, instead of the flattened version I'm looking for, which would return a simple MoveStruct.

Anyone have a good idea how to combine AllCells, and LegalMovesAround(x,y)?

Regards, Aleks

Answer 1

You could use yield! in a new sequence expression:

let allLegalMoves = seq {
  for cell in AllCells do
    yield! LegalMovesAround cell
}