[Haskell-cafe] Re: Unwrapping newtypes

[Kevin Jardine]

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!

Thanks.

On Sep 8, 2:36 pm, Kevin Jardine <kevinjard... at gmail.com> wrote:
> Hi Tony and James,
>
> I'm having trouble constructing the ToObj instance.
>
> The obvious code:
>
> toObj (w o) = o
>
> fails with a syntax error.
>
> How do I unwrap the value?
>
> Kevin
>
> On Sep 8, 2:30 pm, James Andrew Cook <mo... at deepbondi.net> wrote:
>
> > On Sep 8, 2010, at 8:19 AM, Kevin Jardine wrote:
>
> > > 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'm not sure if it's what he originally had in mind, but if your Obj class has a ToObj instance (which would be reasonable), then that extension allows your other classes to derive it:
>
> > > newtype Foo = Foo Obj deriving ToObj
>
> > -- James_______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
>
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