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POHaskell can't match type, claims rigid variable
 

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    <p>I am new to Haskell, and I am playing around with creating a typeclass for graphs and the nodes in them. Since I want both directed and undirected graphs, I have</p> <pre><code>data Node = Node { label :: Char , index :: Int } deriving (Ord, Eq) type Graph edgeType = ([Node], [edgeType]) data Edge = DirectedEdge {h :: Node, t :: Node} | UndirectedEdge {a :: Node, b :: Node} instance Show Node where show n = ['(', label n, ')'] instance Show Edge where show (DirectedEdge h t) = show h ++ "-&gt;" ++ show t show (UndirectedEdge a b) = show a ++ "-" ++ show b </code></pre> <p>So I am distinguishing between directed and undirected edges. A graph must only have edges of either type. I also have the following:</p> <pre><code>nodes :: [Node] nodes = zipWith Node ['a'..] [0..] emptyGraph :: [Node] -&gt; Graph edgeType emptyGraph ns = (ns, []) </code></pre> <p>So far so good, however I am writing a function <code>connect</code>, with connects a node to an existing graph. Ideally, I only want it to apply to undirected graphs, but that doesn't seem to be an option. Instead, I have something like this:</p> <pre><code>connect :: Graph edgeType -&gt; Node -&gt; Graph edgeType connect (ns, es) n = (n:ns, e:es) where e = UndirectedEdge n (head ns) </code></pre> <p>But this gives the following error:</p> <pre><code>Couldn't match type `edgeType' with `Edge' `edgeType' is a rigid type variable bound by the type signature for connect :: Graph edgeType -&gt; Node -&gt; Graph edgeType </code></pre> <p>What is the best way to accomplish what I am trying to achieve?</p>
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