[Haskell-beginners] Haskell implementation of longest path algorithm

[Jeremy]

Compared to the Nim code
[https://github.com/logicchains/LPATHBench/blob/master/nim.nim] for a
longest path algorithm, Haskell
[https://github.com/logicchains/LPATHBench/blob/master/hs.hs] looks
horrendously verbose and ugly, even if you ignore the pragmas and imports.

Is this idiomatic Haskell style? Could it be clearer, but has to be written
that way for performance - although it still takes 3.7x as long as the Nim
version [https://github.com/logicchains/LPATHBench/blob/master/writeup.md]?