[Haskell-cafe] Re: Unwrapping newtypes

[Kevin Jardine]

Hi Tony,

I stared at that specific section for at least half an hour earlier
today but could not figure out how it applied in my specific case. The
only examples I have see are for deriving Num. Do you have any more
detail on how I could use that extension?

Kevin

On Sep 8, 2:05 pm, Tony Morris <tonymor... at gmail.com> wrote:
> I think you might want -XGeneralizedNewtypeDeriving
>
> http://haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html#id6...
>
> 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
> > _______________________________________________
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>
> --
> Tony Morrishttp://tmorris.net/
>
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