[Haskell-cafe] generalized newtype deriving allows the definition
of otherwise undefinable functions
Wolfgang Jeltsch
g9ks157k at acme.softbase.org
Mon Mar 8 16:45:19 EST 2010
Hello,
some time ago, it was pointed out that generalized newtype deriving could be
used to circumvent module borders. Now, I found out that generalized newtype
deriving can even be used to define functions that would be impossible to define
otherwise. To me, this is surprising since I thought that generalized newtype
deriving was only intended to save the programmer from writing boilerplate
code, not to extend expressiveness.
Have a look at the following code:
> {-# LANGUAGE
> GeneralizedNewtypeDeriving,
> MultiParamTypeClasses,
> FlexibleInstances
> #-}
>
> class Iso a b where
>
> conv :: item a -> item b
>
> instance Iso a a where
>
> conv = id
>
> newtype Wrapped a = Wrap a deriving (Iso a, Show)
Now any value whose type contains some type t can be converted into a value of
the type that you get if you replace t by Wrap t. Here is some code to
demonstrate this for binary operations:
> newtype BinOp a = BinOp (a -> a -> a)
>
> convBinOp :: (a -> a -> a) -> (Wrapped a -> Wrapped a -> Wrapped a)
> convBinOp op = let BinOp op' = conv (BinOp op) in op'
Now, you can enter
convBinOp (*) (Wrap 5) (Wrap 3)
into GHCi, and you will get
Wrap 15
as the result.
The point is, of course, that such conversions are not only possible for
binary operations but for arbitrary values and that these conversions are done
by a single generic function conv. I don’t think it would be possible to
implement conv without generalized newtype deriving.
Any thoughts?
Best wishes,
Wolfgang
More information about the Haskell-Cafe
mailing list