[Haskell-cafe] Re: Unwrapping newtypes

Hi Ertugrul, My goal was to find a way to define all that was needed using Haskell's automatic instance deriving mechanism. Haskell can automatically derive Foldable, which is why I was looking at that. However, that requires writing two lines for each wrapper newtype to get around the kind problem. I wanted one line. Fortunately, {-# LANGUAGE GeneralizedNewtypeDeriving #-} gives me what I want now that I know how it works! I agree that the Foldable solution was a bit of a kludge. Kevin On Sep 9, 3:57 am, Ertugrul Soeylemez wrote:

Kevin Jardine wrote:

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I have a generic object that I want to wrap in various newtypes to better facilitate type checking.

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For example,

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newtype Blog = Blog Obj newtype Comment = Comment Obj newtype User = User Obj

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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.

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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.

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Of course this could work if someone using the library wrote an instance for each wrapper object:

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instance GetObject Blog where     getObj (Blog obj) = obj

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but this is a pain in the neck to write for each newtype.

Simple solution:

  data ObjContent = Blah

  data Obj     = Blog    { getObj :: !ObjContent }     | Comment { getObj :: !ObjContent }     | User    { getObj :: !ObjContent }

With your GetObject class this even becomes extensible:

  instance GetObject Obj where     getObject = getObj

  data OtherType = OtherType ObjContent

  instance GetObject OtherType where     getObject (OtherType obj) = obj

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I discovered that Foldable defines a handy toList function that extracts content from generic Foldable structures.

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So that I could write:

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toObj :: Foldable thing => thing Obj -> Obj toObj w = head $ toList w

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Slightly kludgy but it works.

But it's not what you are looking for.  You are confusing constructor types with type kinds.  Foldable expects a type of kind * -> *, which isn't quite what you want.  Also I would consider this to be abuse. Also from a complexity standpoint it's nothing different from your GetObject class anyway.  You still need to write the instances.

Greets, Ertugrul

-- nightmare = unsafePerformIO (getWrongWife >>= sex)http://ertes.de/

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