derived-instances

Suppose I have the following code: {-# LANGUAGE GADTs, DeriveDataTypeable, StandaloneDeriving #-} import Data.Typeable class Eq t => OnlyEq t class (Eq t, Typeable t) => BothEqAndTypeable t data Wrapper a where Wrap :: BothEqAndTypeable a => a -> Wrapper a deriving instance Eq (Wrapper a) deriving instance Typeable1 Wrapper Then, the following instance declaration works, without a constraint on t : instance OnlyEq (Wrapper t) and does what I expect it to do. But the following instance declaration doesn't work: instance BothEqAndTypeable (Wrapper t) since GHC - I'm using 7.6.1 - complains that:

Algebraic Data Types (ADTs) in Haskell can automatically become instances of some typeclasse s (like Show , Eq ) by deriving from them. data Maybe a = Nothing | Just a deriving (Eq, Ord) My question is, how does this deriving work, i.e. how does Haskell know how to implement the functions of the derived typeclass for the deriving ADT? Also, why is deriving restricted to certain typeclasses only? Why can't I write my own typeclass which can be derived? sclv The short answer is, magic :-). This is to say that automatic deriving is baked into the Haskell spec, and every compiler can choose to