There seems to be the need for (at least) two classes for showing values in Haskell: One class that outputs values in a way that can be copied and pasted into Haskell programs. This is useful in GHCi. Another class (say Pretty) that shows values in a prettily formatted way. Current libraries use the Show class for both applications depending on the taste of the author. A third class would be useful for outputting values with markup, say HTML or LaTeX. The Show class converts to String, the Pretty class could convert to a pretty printer data type like Doc. The Show class should output all data that is necessary to reconstruct the value, but not necessarily in the form of the underlying data structure, but maybe using appropriate generating functions. Thus e.g. 'show' for Doubles does not need a parameter for precision, it just emits all available digits. On the other hand a Pretty class should have such parameters (maybe then requiring a multi-parameter type class with functional dependency from value type to formatting style type). A nice infix operator like (//) like in the GSL wrapper could flatten the Pretty data to String, and thus is easy to use in GHCi. Matrix> fiboMatrix matrix [[1,0], [1,1]] Matrix> fiboMatrix // 3 /1.000 0.000\ \1.000 1.000/ where (//) :: Pretty style value => value -> style -> IO String x // p = putStr (Doc.toString (Pretty.toDoc p x)) class Pretty style value | value -> style where toDoc :: style -> value -> Doc If such a separation would become consensus then libraries could better separate the concerns of pretty output and re-usable output.