I use FGL, which (roughly) defines type Gr a b as a graph on nodes of type a and edges of type b.

Suppose you wanted a graph that described which people own which hamsters, knowing only their name. You would have to make node and edge types like this:

where the strings represent their names.

Suppose then you wanted to write a function that, given a person, returns the names of all their hamsters. To make sure the call makes sense, the function would have to first check that the input is in fact a person. Since persons and hamsters are both constructors of the same type, you can't let Haskell's robust, beautiful type-checking system distinguish them for you; you've got to write something like "case n of Person _ -> True; _ -> False".

Is there some way around writing such manual checks?