heaps VS fgl

You can implement all this in Haskell (and Kmett has), but the double-layer functor presentation is beautiful in its expression of recursion and memoization through the composition of first-class modules. Yes, you could do this with typeclasses in Haskell, but typeclasses but I think there's just an element of elegance to first-class modules that can't be found with typeclasses. (Not the mention Haskell's horridly bare-bones module system.)

Your names are good, I reckon it is Martin Erwig's fgl stuff and Andrey Mokhov's algebraic-graphs that you have in mind.

I am about to start a new project in Haskell, model checking with (new) tree-like data structures. I think it is best to start building on a library such that i can already have elegant base functions, yet i am wondering what library is currently the standard? I read about fgl ( https://hackage.haskell.org/package/fgl ), yet it is a very old library.

Couple of approaches to graphs that are state-free: functional graphs and algebraic graphs

Using fgl but only as a data structure this time, with edge labels denoting whether the target is a big room. Not using any of its algorithms as it doesn't have anything built-in for "traversal with re-visiting".

For part 2, instead of trying to union-merge from the lowest points, I simply found all connected regions of <9. I say "simply" because I just threw things at fgl, but setting the graph up first took a bit of work. buildGr is fast but picky about the exact order things come in with.