G'day all.
Quoting Bertram Felgenhauer
successors n b = sortWith (length . succs) . succs [...] successors n b = sortWith (length . (succs =<<) . succs) . succs [...] successors n b = sortWith (length . (succs =<<) . (succs =<<) . succs) . succs [...] These improved heuristics don't come cheap though.
The heuristic is cheaper and more useful when the ply of the tree is low. It's more expensive and less useful when the ply is high. Moreover, deeper neighbour searches may only be useful in cases where the shallower searchers fail to settle on the best course of action. So something like the following might be better. Note that "d" here is an example only; I don't promise it's good. successors n b = sortWith heuristic . succs where heuristic p = let ps = succs p d = 5 - length ps `div` 2 in map length . take d . iterate (succs =<<) $ ps One more thing: Deeper neighbour searches are also unnecessarily expensive because they recompute "succs" a lot. It seems to me that if you memoed this, what you'd actually have is an explicit lazy search tree data structure. Hint hint. Cheers, Andrew Bromage