Clean up a few warnings

Turning these into explicit errors, all of which should be impossible.

Author: travitch

src/Data/Graph/Haggle/Classes.hs

@@ -219,6 +219,7 @@ class (Graph g, HasEdgeLabel g, HasVertexLabel g) => InductiveGraph g where
     return g'
 
 addEdgeLabel :: (HasEdgeLabel g) => g -> Edge -> (Edge, EdgeLabel g)
-addEdgeLabel g e = (e, el)
-  where
-   Just el = edgeLabel g e
+addEdgeLabel g e =
+  case edgeLabel g e of
+    Just el -> (e, el)
+    Nothing -> error "Expected an edge label for a graph implementation satisfying the HasEdgeLabel constraint"

src/Data/Graph/Haggle/Internal/Adapter.hs

@@ -324,17 +324,19 @@ labeledVertices :: (I.Graph g) => LabeledGraph g nl el -> [(I.Vertex, nl)]
 labeledVertices g = map toLabVert $ I.vertices (rawGraph g)
   where
     toLabVert v =
-      let Just lab = vertexLabel g v
-      in (v, lab)
+      case vertexLabel g v of
+        Just lab -> (v, lab)
+        Nothing -> error "Impossible: LabeledGraphs always have vertex labels"
 
 -- | Likewise, we use 'edges' here instead of directly reading from the edge
 -- label storage array.
 labeledEdges :: (I.Graph g) => LabeledGraph g nl el -> [(I.Edge, el)]
 labeledEdges g = map toLabEdge $ I.edges (rawGraph g)
   where
     toLabEdge e =
-      let Just lab = edgeLabel g e
-      in (e, lab)
+      case edgeLabel g e of
+        Just lab -> (e, lab)
+        Nothing -> error "Impossible: LabeledGraphs always have edge labels"
 
 mapEdgeLabel :: LabeledGraph g nl el -> (el -> el') -> LabeledGraph g nl el'
 mapEdgeLabel g f = g { edgeLabelStore = V.map f (edgeLabelStore g) }

src/Data/Graph/Haggle/PatriciaTree.hs

@@ -79,8 +79,10 @@ instance I.HasEdgeLabel (PatriciaTree nl el) where
   labeledEdges gr = map toLabEdge (I.edges gr)
     where
       toLabEdge e =
-        let Just lab = I.edgeLabel gr e
-        in (e, lab)
+        case I.edgeLabel gr e of
+          Just lab -> (e, lab)
+          Nothing -> error "Impossible: PatriciaTree instances always have edge labels"
+
   labeledOutEdges (Gr g) (I.V s) = fromMaybe [] $ do
     Ctx _ _ _ ss <- IM.lookup s g
     return $ IM.foldrWithKey toOut [] ss
@@ -95,8 +97,9 @@ instance I.HasVertexLabel (PatriciaTree nl el) where
   labeledVertices gr = map toLabVert (I.vertices gr)
     where
       toLabVert v =
-        let Just l = I.vertexLabel gr v
-        in (v, l)
+        case I.vertexLabel gr v of
+          Just l -> (v, l)
+          Nothing -> error "Impossible: PatriciaTree instances always have vertex labels"
 
 instance I.Bidirectional (PatriciaTree nl el) where
   predecessors (Gr g) (I.V v) = fromMaybe [] $ do