The newtype deriving extension is one of the most powerful tools for writing robust, typesafe code. However it suffers from a couple limitations. * you can only newtype derive the last argument to a MPTC. * you cannot co-derive an instance for multiple newtype renamings. it seems that both these can be solved when combined with the other proposed extension, allowing deriving clauses to be separate from data definitions. basically, we would allow deriving anywhere.
deriving (Show Foo)
means the same thing as
data Foo = ... deriving (Show)
will derive an instance of Show for Foo. now we are no longer bound by either of the above constraints.. imagine we have a class
class MapLike k v | v -> k where insert :: .. delete :: ... ...
instance MapLike Int (IM.IntMap a) where insert = IM.insert ....
now, we want a newtype that describes Id's.
newtype Id = Id Int deriving(Eq,Ord)
now, we want to create a type of substitutions from Ids -> Terms
newtype Subst = Subst (IM.IntMap Term)
ideally, we'd like an MapLike instance, but we'd have to tediously write it ourselves. if we allow the supergeneralized newtype deriving, we can do:
deriving(MapLike Id Subst)
and be done with it. this would be worlds of useful. John -- John Meacham - ⑆repetae.net⑆john⑈