Edges from nodes not in the vertex list reappear unexpectedly about fgl HOT 4 CLOSED

Big thanks to nick8325 for helping me understand this behavior in nick8325/quickcheck#53!

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By the way - I'm using fgl 5.5.1.0.

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Hmmmm.... this is a bug in that:

let g = Graph.mkGraph [(1,())] [(1,2,True), (2,1,False)] :: Gr () Bool

Should return an error: you're referencing nodes that don't exist in the graph!

(And that's what I get when I try this:

mkGraph *** Exception: Data/Graph/Inductive/Graph.hs:229:5-38: Irrefutable pattern failed for pattern (Data.Maybe.Just (pr,
                                                         _,
                                                         la,
                                                         su),
                                        g')

but that might be from changes I've made since the release.)

This occurs, as there is no validation that occurs in mkGraph; there are two possible ways of handling this:

• Add validation to mkGraph; this will make it slower and less efficient for the (more common I think) cases of correct usage
• Keep doing what it does (in HEAD) of throwing an error

Unless there's a compelling reason, I don't think I'll change the behaviour.

To be able to use QuickCheck with FGL, see #12.

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Ah, I see. Good to know that the behavior has been fixed in master. Thanks!

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