fgl VS containers

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.

"Make invalid laziness unrepresentable" means you should use strict versions of container types instead of lazy ones. However, for better or for worse, sometimes the "strict version" of a data type is not actually a strict data type, it's just a strict API to the lazy type. Examples include Data.Map.Strict (not Data.Map or Data.Map.Lazy) or Data.HashMap.Strict (not Data.HashMap.Lazy) (sadly there is no Data.Sequence.Strict but perhaps there will be one day).

Indeed it's rather subtle. See https://github.com/haskell/containers/issues/752 for more discussion on the matter. Nonetheless, I believe it is "spine strict" in the sense that thunks for all values always exist, even if the spine that holds them can be rearranged due to laziness.

Note that the list index operator has O(n) complexity. Ideally you'd want to use something like Seq from the containers package

Edison and containers both have sequence types that support efficient, cons, snoc, viewL, viewR, append, map, and length.

containers

Calling linked lists Haskell's "primary data structure" seems off-base to me. Yes, there's String, yes, there's built-in syntax for List... but there's also everything in containers, and vector is pretty easy to use in practice, though it would probably be good for more learning material to mention it more prominently.

Haskell has much more sophisticated immutable data structures, you can find them in the "containers" package: https://hackage.haskell.org/package/containers

I asked this on the libraries mailing list but thought posting here would bring in potentially more responses. I made a recent change to the behaviour of lookup and find (see here for more details: https://github.com/haskell/containers/pull/800).

Rust has nothing on Haskell. containers, which might as well be considered part of the standard library, has been out for almost 14 years and is still 0.x