- [Inductive graphs and Functional Graph Algorithms](http://web.engr.oregonstate.edu/~erwig/papers/abstracts.html#JFP01).
- [FGL/Haskell - A Functional Graph Library](http://web.engr.oregonstate.edu/~erwig/fgl/haskell/old/fgl0103.pdf).
- [Data.Graph source from Containers package](http://hackage.haskell.org/package/containers-0.5.5.1/docs/Data-Graph.html).
- The [graphs package](https://hackage.haskell.org/package/graphs).
- [SO question about PHOAS](http://stackoverflow.com/questions/24369954/separate-positive-and-negative-occurrences-of-phoas-variables-in-presence-of-rec)
- [PHOAS for free](https://www.fpcomplete.com/user/edwardk/phoas).
- [Tying the Knot](http://www.haskell.org/haskellwiki/Tying_the_Knot).