On Wed, Nov 25, 2009 at 11:02 AM, Neil Brown
David Menendez wrote:
From what I can tell, insEdge inserts an edge between two nodes which are already in the graph. The code is calling insEdge on arbitrarily-labeled nodes, which may not exist in the graph.
That's what I thought initially, but in fact what it is doing is exactly what you suggest:
Instead of picking arbitrary node labels, try selecting arbitrary elements from the list of node labels.
That "nGen = elements ns" line assigns into nGen a random generator that will pick from ns, the list of nodes.
You're right. I've tried this in ghci, and I'm not able to reproduce
the error. I did get an exception from QuickCheck when it tried to
call elements on an empty list, though.
This code works fine for me:
a :: Gen ([LNode Char], [LEdge Char], Gr Char Char)
a = do
NonEmpty ns' <- arbitrary
let ns = nub ns'
let nGen = elements ns
lns <- mapM (\n -> liftM ((,) n) arbitrary) ns
les <- listOf $ liftM3 (,,) nGen nGen arbitrary
return (lns, les, mkGraph lns les)
I suspect that there's no value to generating an arbitrary list of
node IDs, as opposed to something like:
ns <- liftM (\(Positive n) -> [0..n]) arbitrary
--
Dave Menendez