Hi Marko,
While I am not claiming that a traverser in Scala would be as powerful as
Gremlin, the combination of for comprehensions and traversals allows for a
concise formulation of working with graphs in Neo4J.
In my previous post I already mentioned the use of for comprehensions which
allows constructs like:
val startNode = //some means to obtain the startnode
for(
rel1 <- startNode.getRelationships(OUTGOING)
if(rel1.getProperty("bla0") == "bla1");
node1 <- rel.getOtherNode(startNode)
if(node1.getProperty("bla2") == "bla3");
rel2 <- node1.getRelationships(OUTGOING)
if(rel2.getProperty("bla4") == "bla5");
node2 <- rel.getOtherNode(node1)
if(node2.getProperty("bla6") == "bla7");
){
//do things with rel1, node1, rel2, node2
}
This is nothing more than a concise way of writing 4 nested loops, so not a
genuine traversal, although a traversal can be part of a for comprehension.
Being an Iterable, a traversal is a monadic operation and can be used in the
composition of a for comprehension.
The traverer I am working on at the moment, has the following signature:
class Traverser(startNode: Node // startnode
, relationshipTypes: Set[(RelationshipType, Direction)]
//Set of relationshiptype, direction pairs
, relationshipFilters: Set[Relationship => Boolean] // Set
of functions from relationship to boolean
, nodeFilters: Set[Node => Boolean] //Set of functions from
node to boolean
, depthExpression: Int => Boolean //Expression of traversal
depth to boolean
, options: TraversalOption // ReturnStopnode(n) or ReturnAll
or ReturnAllExceptStartNode
, mode: DepthFirst or BreadthFirst
)
Am thinking about adding subtraversals, but need to further work out the
details on that. Another thing I am considering is a traverser using the
metamodel. This doesn't necessarily increase the expressiveness of the
traversal, but could make writing fine grained traversals easier. Anything a
metamodel based traverser can do, can be expressed in relationshipfilters and
nodefilters, but allows for nicer organization of the code.
Kind regards,
Niels Hoogeveen
> From: okramma...@gmail.com
> To: user@lists.neo4j.org
> Date: Wed, 17 Mar 2010 21:59:27 -0600
> Subject: Re: [Neo] basic questions
>
> Hi,
>
> > The example I provided is in effect nothing but a couple of nested
> > iterations, so nothing like a real traversal, but certainly
> > comparable to the the java pipes or the Gremlin path expressions.
> >
> > I am working on a Scala based traverser, having higher order
> > functions makes it possible to supply filter expressions to the
> > traverser. The result of a traversal can then be used in a for
> > comprehension to further navigate the results or to compose
> > different traversals.
>
> Can you talk about what the Scala-based traverser can express beyond
> Gremlin? I ask this cause Gremlin provides Turing completeness in
> function and path definitions. Thus it allows you to move through a
> graph in an arbitrary manner----explicit adjacency, probability
> distribution control, teleportations, path/function composition, etc.?
> --- though Gremlin lacks i/o manipulations of a traverser (e.g. data
> from a socket connection altering the traversal, file data, keyboard,
> etc).
>
> See http://markorodriguez.com/Lectures_files/cnls-gremlin-2010.pdf
>
> Thanks,
> Marko.
>
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