[Haskell-cafe] Re: Unwrapping newtypes

[Kevin Jardine]

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
>
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