Daryoush Mehrtash wrote:
What happens when a type adds driving such as:newtype SupplyT s m a = SupplyT (StateT [s] m a) deriving (Functor, Monad, MonadTrans, MonadIO) Two questions: How does the deriving implement the instance?
With GeneralizedNewtypeDeriving, since newtypes are just a reinterpretation of the underlying type, the deriving clause just uses the instance for the underlying type (massaging it with the wrapping/unwrapping functions for the newtype). The primary use for this is for defining a newtype on monad transformer stacks, for which it succeeds excellently. However, if the semantics of your newtype are different than the underlying type w.r.t. the type class, then it's not so good.
For the more basic kind of deriving clause, the compiler knows about how to create generic instances for obvious things, e.g. the compiler can derive the obvious Show instance by doing introspection on the source code. However, these derivations are limited to classes with an obvious implementation based solely on the structure of the type definition.
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