I'm writing a small unit library to excercise my barely existing Haskell skills. However I can't figure out how to make Haskell accept any type of a specific class.
[snip] What you want is a technique called "existential types". The wiki page is here: http://haskell.org/hawiki/ExistentialTypes The problem with existentials is that they are pretty picky about how you use them; I'm pretty sure you can't create a labeled record containing existentials (because the accessor functions aren't allowed to exist). You have to pattern match to get at the encapsulated value. Also, once you have done the pattern match on the "UnitValue," you can only use polymorphic functions on the pattern variables. The following does what I think you want. {-# OPTIONS -fglasgow-exts #-} class Unit u where shortName :: u -> String data Meter = Meter instance Unit Meter where shortName u = "m" data UnitValue = forall u n. (Unit u,Num n) => UnitValue u n main = let x = UnitValue Meter 10 in case x of UnitValue unit magnitude -> putStrLn $ (show magnitude)++(shortName unit)