[Haskell-cafe] What is your favourite graph library?

[Ignat Insarov]

Hello!

I really like graphs. People seem to like thinking in terms of dots
and arrows, and graph theory provides a formalization for this.
(Homogeneous binary) relations, finite state automata, networks and
categories are sorts of graphs too.

But unfortunately it is not possible to represent a graph as an
algebraic type. For example, a list is a fixed point of a maybe tuple,
and every usual linear container _(an array, a vector, a sequence…)_
is isomorphous to a list, so we have an easy time prototyping and
specifying the behaviour of any such thing. With graphs, we have a
hard time!

The most popular library for graphs in Haskell is `fgl`.
Unfortunately, it is really easy to get weird behaviour with it,
because folds tear away the limbs from contexts so that your
observation of said contexts depends on the order of folding. I have
been poking at it, trying to make graphs resemble a comonad… but of
course you can have an empty graph, and a comonad cannot be empty.

There is also `alga` — it seemed promising, but now it is deprecated
for some reason… The usual `containers` also provide something they
call a graph, but it does not offer much in terms of instances and
algorithms — a graph is merely a type alias to an array.

So, is there any cool library that I missed? Or some promising
research? Do you know of any packages that use graphs heavily and in
interesting ways? Are you curious to see progress in this area?