[Haskell-cafe] Re: Unwrapping newtypes

On Wed, 8 Sep 2010 05:51:22 -0700 (PDT), Kevin Jardine wrote:

Ah, I was missing an important piece of the puzzle.

If I write:

class ToObj a where toObj :: a -> Obj

instance ToObj Obj where toObj a = a

then

newtype Blog = Blog Obj deriving ToObj

works!

This post http://www.haskell.org/pipermail/libraries/2006-October/005950.html describes a general mini-library for this situation (having a structure-indicating newtype of an underlying type). The 'Unpack' class is a generalised version of your 'ToObj' class. Maybe it is useful to compare (or even use), especially the smart ways in which you can use this class to hide much of the wrapping/unwrapping. Regards, Arie