> type classes are different because type classes are solved at compile time data Nested a = Nest a (Nested [a]) | Done deriving Show build :: Int ->

> type classes are different because type classes are solved at compile time data Nested a = Nest a (Nested [a]) | Done deriving Show build :: Int -> a -> Nested a build 0 a = Done build i a = Nest a $ build (i- 1) [a] main = do i <- readLn print (build i i) if you build a value of Nested at runtime then it is impossible to do static dispatch and the dictionary has to be constructed at runtime as well. Sorry that I mistyped the example at first. If you run this version: > main 23 Nest 23 (Nest [23] (Nest [[23]] (Nest [[[23]]] (Nest [[[[23]]]] (Nest [[[[[23]]]]] (Nest [[[[[[23]]]]]] (Nest [[[[[[[23]]]]]]] (Nest [[[[[[[[23]]]]]]]] (Nest [[[[[[[[[23]]]]]]]]] (Nest [[[[[[[[[[23]]]]]]]]]] (Nest [[[[[[[[[[[23]]]]]]]]]]] (Nest [[[[[[[[[[[[23]]]]]]]]]]]] (Nest [[[[[[[[[[[[[23]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[23]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[23]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]]]]]] (Nest [[[[[[[[[[[[[[[[[[[[[[23]]]]]]]]]]]]]]]]]]]]]] Done)))))))))))))))))))))) The innermost Nested has type `Nest [[[[[[[[[[[[[[[[[[[[[[Int]]]]]]]]]]]]]]]]]]]]]]` which wasn't known at compile time so the show instance has to be constructed at runtime. This is called polymorphic recursion.