Reusing Throwaway Code

I've just had my first go at writing a monad, and it's harder than it would seem. As a result of my moderately dismal failure, I've decided to go back and actually read the monads sections of the various Haskell tutorials (the Gentle Introduction, Yet Another Haskell Tutorial, and anything else I find laying around on my hard disc). I've just been looking at Yet Another Haskell Tutorial and the section on computations (Section 8.4.2 in the PDF) defines a small graph library which might be a useful starting point when I need to start dealing with transitive, Euclidean and even more bizarre frames and rules with conditions on them.

In one section, I get some [hopefully] usable code, and a better understanding of monads.