I think you might want -XGeneralizedNewtypeDeriving http://haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html#id659906 On 08/09/10 22:01, Kevin Jardine wrote:
I have a generic object that I want to wrap in various newtypes to better facilitate type checking.
For example,
newtype Blog = Blog Obj newtype Comment = Comment Obj newtype User = User Obj
Unlike Obj itself, whose internal structure is hidden in a library module, the newtype wrappings are purely to facilitate type checking. It is no secret that each is just a wrapper around Obj.
It is obvious how to construct the various wrapper objects. It is not so obvious how to extract the Obj they contain in a reasonably generic way however. What I want is a getObj function that works on all of them.
Of course this could work if someone using the library wrote an instance for each wrapper object:
instance GetObject Blog where getObj (Blog obj) = obj
but this is a pain in the neck to write for each newtype.
I discovered that Foldable defines a handy toList function that extracts content from generic Foldable structures.
So that I could write:
toObj :: Foldable thing => thing Obj -> Obj toObj w = head $ toList w
Slightly kludgy but it works.
Even better, recent versions of GHC will allow you to automatically derive Foldable.
Unfortunately,
newtype Blog = Blog Obj deriving Foldable
returns a kind error.
What does work is:
newtype BlogF a = Blog a deriving Foldable type Blog = BlogF Obj
After having spent close to a day on this, I am a bit baffled that such a seemingly trivial problem seems so hard to do.
I am wondering if I am missing something really, really obvious.
Any suggestions? Or is there perhaps a more Haskelly way to place type constraints on a more generic type?
Kevin _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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