Kevin Jardine
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.
Simple solution: data ObjContent = Blah data Obj = Blog { getObj :: !ObjContent } | Comment { getObj :: !ObjContent } | User { getObj :: !ObjContent } With your GetObject class this even becomes extensible: instance GetObject Obj where getObject = getObj data OtherType = OtherType ObjContent instance GetObject OtherType where getObject (OtherType obj) = obj
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.
But it's not what you are looking for. You are confusing constructor types with type kinds. Foldable expects a type of kind * -> *, which isn't quite what you want. Also I would consider this to be abuse. Also from a complexity standpoint it's nothing different from your GetObject class anyway. You still need to write the instances. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/