[Haskell-cafe] Re: Unwrapping newtypes

Thanks Arie, I'm going to make some of my bare Obj functions accept a ToObj typeclass constraint and do the conversions inside there to avoid cluttering up the wrapped object code. Now that I know how to do this I'm going to see what more restructuring I can do to make the difference between wrapped and bare objects less visible (except of course where it should matter). Kevin On Sep 8, 3:44 pm, Arie Peterson wrote:

On Wed, 8 Sep 2010 05:51:22 -0700 (PDT), Kevin Jardine

wrote:

...

Ah, I was missing an important piece of the puzzle.

...

If I write:

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class ToObj a where     toObj :: a -> Obj

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instance ToObj Obj where     toObj a = a

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then

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newtype Blog = Blog Obj deriving ToObj

...

works!

This post http://www.haskell.org/pipermail/libraries/2006-October/005950.html describes a general mini-library for this situation (having a structure-indicating newtype of an underlying type). The 'Unpack' class is a generalised version of your 'ToObj' class. Maybe it is useful to compare (or even use), especially the smart ways in which you can use this class to hide much of the wrapping/unwrapping.

Regards,

Arie

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