[Haskell-cafe] Re: Unwrapping newtypes

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 wrote:

On Sep 8, 2010, at 8:19 AM, Kevin Jardine wrote:

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Hi Tony,

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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?

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Kevin

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On Sep 8, 2:05 pm, Tony Morris wrote:

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I think you might want -XGeneralizedNewtypeDeriving

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http://haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html#id6...

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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:

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newtype Foo = Foo Obj deriving ToObj

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