Hello (first post), I have spent so many time learning typeclasses and thinking it was part of Haskell essence... and suddenly I read the discussion "[Haskell-beginners] Can fields in a record be optional?". So typeclasses are not always recomended? I'm implementing a Machine Learning framework and I am in a sort of related dilemma. I found three ways of implementing the same distance function between "examples" (aka "attribute vectors" or simply "Float vectors" for mere mortals :) ): [obs: "Example" datatype will be added more fields later] --------------first------------------------------------ module ML where data Example = Example [Float] deriving (Show) class ExampleClass a where (distance) :: a → a → Float instance ExampleClass Example where (Example atts1) distance (Example atts2) = sqrt $ sum $ map (λ(x, y) → (x-y)↑2) $ zip atts1 atts2 ================================= --------------second------------------------------------ module ML where data Example = Example {attributes :: [Float]} deriving (Show) distance :: Example → Example → Float distance ex1 ex2 = sqrt $ sum $ map (λ(x, y) → (x-y)↑2) $ zip (attributes ex1) (attributes ex2) ================================= --------------third------------------------------------ module ML where data Example = Example [Float] deriving (Show) distance :: Example → Example → Float distance (Example att1) (Example att2) = sqrt $ sum $ map (λ(x, y) → (x-y)↑2) $ zip (att1) (att2) ================================= All three reserves the word "distance" for itself and the second reserves also the word "attributes". How could I implement the module ML and which would be the best way to set "attributes" outside the module? Thanks Davi