Given a node N and an edge label L, I want to do things like "find N, and all of N's L-children, and all of their L-children ... and add up their values". Ideally the fold would not traverse the whole graph. (Note that in general, the subgraph induced by N and L-succession is not a tree.)

Writing the fold myself, I can only imagine a very inefficient solution -- I would find each of N's L-children first, collect them, then find each of theirs, etc.

In Data.Graph.Inductive.Basic, FGL provides a function called "gfold". This is its type signature:

-- | Directed graph fold.

gfold :: (Graph gr) =>   (Context a b -> [Node])    -- ^ direction of fold

        -> (Context a b -> c -> d)    -- ^ depth aggregation

        -> (Maybe d -> c -> c, c)      -- ^ breadth\/level aggregation

        -> [Node]

        -> gr a b

        -> c

Is it what I'm looking for? If so, how does it work?