[Haskell-cafe] Manual type-checking in graphs: Avoidable?

Jeffrey Brown jeffbrown.the at gmail.com
Fri Feb 19 02:50:26 UTC 2016

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
    data GraphNode = Person String | Hamster String
    data GraphEdge = Has
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?

Jeffrey Benjamin Brown
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