On Mon, Nov 26, 2012 at 5:21 PM, Chris Dollin <chris.dol...@epimorphics.com>wrote:
> On Monday, November 26, 2012 04:46:32 PM Reto Bachmann-Gmür wrote: > > On Sun, Nov 25, 2012 at 3:19 PM, Andy Seaborne <a...@apache.org> wrote: > > > > > Slight confusion between identity and equivalence. Two graph are > > > equivalent by bNode-isomorphism; they are still different. > > > > > > > Object.equals should implement an equivalence relation. As two graphs are > > equivalent by rdf semantics iff they are isomorphic equals should return > > true for isomorphic graphs. > > .equals() is tossed around fairly casually in Java programs (perhaps that's > just me); it's not clear to me that having a modelorgraph's .equals() being > graph isomorphism -- potentially muchly expensive if bnodes are in play -- > is the most effective choice. > For jena 1 Model.equals used to be isomorphism . This was clearly bad as Models are mutable objects and so you could end up with broken HashSets or -Maps if they have Models as keys. In Clerezza the equivalent to a jena Model or Graph is an MGraph (where the M stands for mutable) for this there is no isomorphism based equality. But if you take a time-slice of it then equals is based on isomorphism. This is because Garph represents a medium level of abstraction between the serialized graph and the expressed content. Might not be cheap to compute yet the problem is easy compared to comparing the expressed semantic content. This level of abstraction is handy when doing things like synchronization and versioning based on decomposed graphs. Or in situation were you don't want to assume logical omniscience, e.g. we might have a: HashMap<Graph, Set<Citizen>> petitionSigner which maps from a petition to the citizens that signed it, if a citizen signs a petition we look up the key and add them to the value. Would two petitions be equals just because they logically entail the same people would be surprised to see what they subscribed to. On the other hand if two petitions would not be considered equals if they came on two different sheets of paper (or memory segments or data connections) it would be very hard to find some allies. > > ["equals should implement equivalence" doesn't entail "equals is > isomorphism". Identity is an equivalence relation too.] > Leibniz introduced the principle that "Two entities that do not have any properties allowing to distinguish them should be seen as a single entity" now immutable graphs have no properties distinguishing so they should be considered identical. Btw, for bnodes the Identity of Indiscernibles doesn't hold (similar to [2]), as the graph ____ _____ | | ------ foaf: knows ------>| | | | | | |___| <---- foaf: knows ------ |_____| contains two bnodes which even though they are not distinguishable they are not the same as the graph is true for more possible worlds than ____ | | ------ foaf: knows ---| | | | |___| <----------------------| the graph with only one bnode. (I hope my poor ascii arts survives transport) Cheers, Reto 1. http://plato.stanford.edu/entries/identity/ 2. http://plato.stanford.edu/entries/identity-indiscernible/ > -- > "The wizard seemed quite willing when I talked to him." /Howl's Moving > Castle/ > > Epimorphics Ltd, http://www.epimorphics.com > Registered address: Court Lodge, 105 High Street, Portishead, Bristol BS20 > 6PT > Epimorphics Ltd. is a limited company registered in England (number > 7016688) > >
