[paipa]

Your constraint should be that if the board is the special case of regular chess, the game should play exactly the same. Allowing the pawn to move towards any side is too much because it would violate this principle.

Perhaps define forward as the direction(s) which, when extrapolated as a rook, would reach the opponent's backrank. If it doesn't exist, the direction(s) which, when extrapolated, terminate on the opponent's half. This would allow extra freedom to pawns in some funky topologies that can occur in your game, but generally follows the principle of least surprisal.

[frading]

yes, GP reached the same conclusion as you. https://news.ycombinator.com/item?id=40550176

I'm still unsure, as I describe in my reply there, an issue I see is that you can't know the path a pawn would take, unless you know where it comes from.

But since both of you reached the same solution, I'll keep thinking about it.

[paipa]

I get the dillemma, but I don't think it's a problem that the string of squares used to define "foward" isn't always the same as path the pawn will actually take if it chugs along.

This only happens when there's no straight path to the opponent's backrank, so you are allowed to go "sideways-foward" until you hit a square from which there is a clean forward path again.

Agreed that the past should not matter, only the current position.