import "StandardLibrary"

topologicalSort :: Show a => [(a,a)] -> <Proc> [a]
topologicalSort dependencies = MList.freeze answer
  where
    answer = MList.create ()
    
    (?x,?y) <- dependencies           =>  print "RULE 1, x=\(?x), y=\(?y)", Dep ?x ?y, InDegree ?x 0, InDegree ?y 1
    InDegree ?x ?a                    =>  print "pre InDegree \(?x) \(?a)"    
    -InDegree ?x ?a, -InDegree ?x ?b  =>  print "RULE 2, x=\(?x), a=\(?a), b=\(?b)", InDegree ?x (?a + ?b)
    InDegree ?x ?a                    =>  print "InDegree \(?x) \(?a)"
    InDegree ?x 0                     =>  print "RULE 3, x=\(?x)", AdjustInDegrees ?x, MList.add answer ?x
    AdjustInDegrees ?x, Dep ?x ?y     =>  print "RULE 4, x=\(?x), y=\(?y)", InDegree ?y (-1)
    
main = topologicalSort [(2,4),(3,7),(7,2),(1,3)]