Concepts

[boia01]

@cdunn2001 I'm not sure why you think it doesn't work. I typed up your example 
and the following works (I'm using Nim 0.18.1):
    
    
    type GraphConcept* = concept g
      type NodeType = type(g.node())
    
    type Node* = object
      n*: int
    
    type Graph* = object
      nodes*: seq[Node]
    
    proc node*(g: Graph): Node =
      Node()
    
    proc shortest_path*(g: GraphConcept): seq[g.NodeType] =
      g.nodes
    
    let nodes = @[Node(n:1), Node(n: 2)]
    let g = Graph(nodes: nodes)
    echo shortest_path(g)
    
    
    Run