Hello I'm trying to write a simple graph library for a project of mine (actually, I'm porting it from OCaml) but I've got a design question right in the beginning. My Graph type is the following. data Graph a b = Graph { adjacencies :: Map Int (a, (Map Int b)) , numVertices :: Int , numEdges :: Int } Types "a" and "b" refer to vertex and edge labels (I know it's kind of weird to use a Map of Maps to represent a graph, but it helps me removing vertices later, which is something I'll need to do.) Creating an empty graph is trivial: empty :: Graph a b empty = Graph Map.empty 0 0 The issue I hit was when writing the function to add a vertex to the graph. Since I need to update the vertex counter, I've used the state monad: addVertex :: Int -> a -> State (Graph a b) () addVertex vertex label = do g <- get let adj = Map.insert vertex (label, Map.empty) (adjacencies g) put $ g { adjacencies = adj, numVertices = numVertices g + 1 } That works fine, but from the point of view of a user of the library, the addVertex function should take a "Graph a b" as its first argument, as one would expect to be able to say which graph is going to be modified. So I'm confused right now about how I should proceed. Any hints regarding that? Best regards, Andre