diff --git a/src/Data/Graph/DirectedAcyclic/View/Tree.hs b/src/Data/Graph/DirectedAcyclic/View/Tree.hs
index c7b6c70..3bce816 100644
--- a/src/Data/Graph/DirectedAcyclic/View/Tree.hs
+++ b/src/Data/Graph/DirectedAcyclic/View/Tree.hs
@@ -24,6 +24,7 @@ import Prelude
 
 import Control.Arrow ((***))
 import Data.Function (on)
+import Data.Hashable (Hashable)
 import Data.HashMap.Lazy (HashMap)
 import Data.HashSet (HashSet)
 import Data.List (groupBy, sortOn)
@@ -32,17 +33,18 @@ import Data.Monoid (Endo (..))
 import qualified Data.HashMap.Lazy as M
 import qualified Data.HashSet as S
 
-type Graph a b = HashMap Int (a, [(Int, b)])
+type Graph n a b = HashMap n (a, [(n, b)])
 
 data DagViewTree a b = FullNode a [DagViewTree a b] | LinkNode b
 
 -- | Update the map according to a choice of a full parent for a given child.
 -- Also specifies whether the choice was sucessfully applied.
 chooseParent
-    :: Int
-    -> Int
-    -> HashMap Int [(Int, Bool)]
-    -> Maybe (HashMap Int [(Int, Bool)])
+    :: (Eq n, Hashable n)
+    => n
+    -> n
+    -> HashMap n [(n, Bool)]
+    -> Maybe (HashMap n [(n, Bool)])
 chooseParent c p h =
     case M.lookup c h of
         Nothing -> Nothing
@@ -70,28 +72,30 @@ markFst :: [a] -> [(a, Bool)]
 markFst []     = []
 markFst (x:xs) = (x, True) : map (, False) xs
 
-labeledDeps :: HashMap Int [(Int, b)] -> [(Int, Int, b)]
+labeledDeps :: Hashable n => HashMap n [(n, b)] -> [(n, n, b)]
 labeledDeps =
     let mk c (p, full) = (c, p, full)
     in  concatMap (\ (c, ps) -> map (mk c) ps) . M.toList
 
 edgeView
-    :: HashMap Int Int
+    :: (Eq n, Hashable n)
+    => HashMap n n
     -- ^ Full parent user choices
-    -> (Int, Int, Bool)
+    -> (n, n, Bool)
     -- ^ Child, parent, and whether the parent is full
-    -> Maybe (HashMap Int Int)
+    -> Maybe (HashMap n n)
     -- ^ New edge label. For a full edge, 'Nothing'. For a link edge, 'Just' an
     -- updated choice map that chooses this edge as the new full edge for the
     -- child.
 edgeView _       (_,     _,      False) = Nothing
 edgeView choices (child, parent, True)  = Just $ M.insert child parent choices
 
-reverseEdge :: (Int, Int, a) -> (Int, Int, a)
+reverseEdge :: (n, n, a) -> (n, n, a)
 reverseEdge (x, y, l) = (y, x, l)
 
 -- | Given labeled nodes and labeled edges, prepare a hashmap.
-mkGraph :: HashMap Int a -> [(Int, Int, b)] -> Graph a b
+mkGraph
+    :: (Eq n, Ord n, Hashable n) => HashMap n a -> [(n, n, b)] -> Graph n a b
 mkGraph nodeMap edges =
     let pair23 (x, y, z) = (x, (y, z))
         edgeMap = M.fromList $ groupSnd $ sortOn fst $ map pair23 edges
@@ -102,7 +106,11 @@ mkGraph nodeMap edges =
 keySet :: HashMap k v -> HashSet k
 keySet = S.fromMap . M.map (const ())
 
-buildTree :: [(Int, Maybe b)] -> Graph a (Maybe b) -> [DagViewTree a (a, b)]
+buildTree
+    :: (Eq n, Hashable n)
+    => [(n, Maybe b)]
+    -> Graph n a (Maybe b)
+    -> [DagViewTree a (a, b)]
 buildTree nodes graph = go nodes
     where
     go []               = []
@@ -120,15 +128,16 @@ buildTree nodes graph = go nodes
                         in  LinkNode (fst c, info) : ts
 
 dagViewTree
-    :: [(Int, a)]
+    :: (Eq n, Ord n, Hashable n)
+    => [(n, a)]
     -- ^ Nodes: Numbers and details
-    -> [(Int, Int)]
+    -> [(n, n)]
     -- ^ Edges: Child-parent pairs
-    -> [(Int, Int)]
+    -> [(n, n)]
     -- ^ Full parent choices as child-parent pairs. This is whatever user input
     -- has been received, even if it includes duplicates or nonexistent node
     -- numbers. So just pass the user input directly here.
-    -> [DagViewTree a (a, HashMap Int Int)]
+    -> [DagViewTree a (a, HashMap n n)]
 dagViewTree nodes deps choices =
     let choose ns@(c, p) acc@(h, l) =
             case chooseParent c p h of