[Haskell-cafe] Folding part of a graph, and FGL's gfold function.

[Jeffrey Brown]

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?
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