module Relax(relax, Comparer, Combiner) where

  import Graph
  import PropertyMap

  type Combiner a = a -> a -> a
  type Comparer a = a -> a -> Bool

  relax :: (Graph g e v, ReadWritePropertyMap d v n,
	    ReadWritePropertyMap p v v, ReadPropertyMap w e n) =>
    e -> g -> w -> d -> p -> (Combiner n) -> (Comparer n)
      -> (d,p,Bool)
  relax e g w d p plus less =
    let { u = src e g; v = tgt e g;
          du = get d u; dv = get d v; 
          we = get w e }
    in if (less (plus du we) dv) then
         (put d v (plus du we), put p v u, True)
       else
         (d, p, False)