[to3m]

If you really insist on having an enum for your directions, what you could do is something like this: (assuming 2s complement and TRUE being ~0)

    : DIREQ DIRECTION @ = ;
    : DELTA DIREQ SWAP DIREQ NEGATE + ;
    : MOVE-SNAKE-HEAD
      LEFT RIGHT DELTA SNAKE-X-HEAD +!
      UP DOWN DELTA SNAKE-Y-HEAD +! ;

It's an open question whether this approach is any better but I think it looks a bit more Forthlike, insofar as I have any grasp of what that is.

I think having offsets would make more sense - e.g., suppose your playfield is 80 units wide, then you'd have left and right as -1 and +1, and up and down as -80 and +80 (which I assume is what you mean).