Random walk using petgraph

Question

I am trying to implement a random walk on a directed graph using the petgraph crate.

So far, I have defined a RandomWalk struct which implements the Walker trait:

extern crate petgraph; // 0.4.13
use petgraph::visit::{GraphBase, Walker};
use petgraph::Direction;

pub struct RandomWalk<G> 
    where G: GraphBase
{
   next: G::NodeId, 
}

impl<G> Walker<G> for RandomWalk<G>
    where G: GraphBase
{
   type Item = G::NodeId; 

   fn walk_next(&mut self, graph: G) -> Option<Self::Item> {
       // Even this deterministic walk does not work:
       graph.neighbors_directed(self.next, Direction::Incoming).next()
   }
}

However, I get the error:

error[E0599]: no method named `neighbors_directed` found for type `G` in the current scope
  --> src/lib.rs:50:11
   |
50 |     graph.neighbors_directed(self.next, Direction::Incoming).next()
   |           ^^^^^^^^^^^^^^^^^^
   |
   = note: the method `neighbors_directed` exists but the following trait bounds were not satisfied:
           `&G : petgraph::visit::IntoNeighborsDirected`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `neighbors_directed`, perhaps you need to implement it:
           candidate #1: `petgraph::visit::IntoNeighborsDirected`

I don't really understand how the petgraph API works, is GraphBase not the correct type?

As the error message states, the correct trait is called `IntoNeighborsDirected`, if you really want to be generic over `G`. If you just want to get something working, it may be quicker to implement this for a concrete type first (e.g. `Graph`). — Sven Marnach, Oct 18 '18 at 12:19
Always try to give us an [MCVE](https://stackoverflow.com/help/mcve). I edited it, so you can see, what I mean. Now I can copy&paste the code into the playground (better you provide us a playground ;) ) and start solving your problem. — hellow, Oct 18 '18 at 12:57
Thanks! I didn't know that the Rust playground supports crates as well - that's a nifty feature. — fuji, Oct 18 '18 at 16:52

score 1 · Accepted Answer · answered Oct 18 '18 at 12:54

The solution is quiet clear, if you understand the compiler.
Rust does not assume anything about the type, unless you specify it, e.g. you can't add two types T together, unless the implement the Add trait. Then you can write T + T.
Your problem is very similar.

You are trying to use the function neighbors_directed which is not implemented for G (which is bound to GraphBase in your example). Instead you have to specify that G also must implement the trait IntoNeighborsDirected by adding that to your impl block.

impl<G> Walker<G> for RandomWalk<G> where G: GraphBase + IntoNeighborsDirected

This will tell the compiler, that G has the method neighbors_directed implemented and you can use it (playground)

Random walk using petgraph

1 Answers1