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I have two records that both have a field I want to extract for display. How do I arrange things so they can be manipulated with the same functions? Since they have different fields (in this case firstName and buildingName ) that are their name fields, they each need some "adapter" code to map firstName to name . Here is what I have so far: class Nameable a where name :: a -> String data Human = Human { firstName :: String } data Building = Building { buildingName :: String } instance Nameable Human where name x = firstName x instance Nameable Building where -- I think the x is redundant here, i.e the following should work: -- name = buildingName name x = buildingName x main :: IO () main = do putStr $ show (map name items) where items :: (Nameable a) => [a] items = [ Human{firstName = "Don"} -- Ideally I want the next line in the array too, but that gives an -- obvious type error at the moment. --, Building{buildingName = "Empire State"} ] This does not compile: TypeTest.hs:23:14: Couldn't match expected type `a' against inferred type `Human' `a' is a rigid type variable bound by the type signature for `items' at TypeTest.hs:22:23 In the expression: Human {firstName = "Don"} In the expression: [Human {firstName = "Don"}] In the definition of `items': items = [Human {firstName = "Don"}] I would have expected the instance Nameable Human section would make this work. Can someone explain what I am doing wrong, and for bonus points what "concept" I am trying to get working, since I'm having trouble knowing what to search for. This question feels similar, but I couldn't figure out the connection with my problem.

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