I am looking for a good (preferably lazy) way to implement some kind of best-first search. The problem is, the expansion of the 'best' node in the search space forces other 'inferior' nodes to expand too. So my function expand :: Node -> ([Node],(Node -> [Node])) does not only return some successor nodes, but also a function to expand other Nodes with certain characteristics. So in fact, after one expansion, I need to fold over my complete search space, maybe updating other nodes and recompute the value of the cost function for the affected nodes. At the moment I am using StateT monad transformers, and retrieving the affected Nodes from a map (to avoid going through whole space), but this is not very elegant and I doubt whether it is efficient, too. I have already read through Spivey's paper "Algebras for combinatorial search". Unfortunately, he only proposes the use of heuristics and best-first search in combination with his framework for future work. Does anybody now some further papers on this topic? Thanks a lot in advance, Martin