[Haskell-cafe] Re: Unwrapping newtypes

Ertugrul Soeylemez es at ertes.de
Wed Sep 8 21:57:22 EDT 2010

Kevin Jardine <kevinjardine at gmail.com> 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.

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.


nightmare = unsafePerformIO (getWrongWife >>= sex)

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