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

...

For example,

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

...

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.

...

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.

...

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

...

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

...

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

...

So that I could write:

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

...

Slightly kludgy but it works.

...

Even better, recent versions of GHC will allow you to automatically derive Foldable.

...

Unfortunately,

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newtype Blog = Blog Obj deriving Foldable

...

returns a kind error.

...

What does work is:

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newtype BlogF a = Blog a deriving Foldable type Blog = BlogF Obj

...

After having spent close to a day on this, I am a bit baffled that such a seemingly trivial problem seems so hard to do.

...

I am wondering if I am missing something really, really obvious.

...

Any suggestions? Or is there perhaps a more Haskelly way to place type constraints on a more generic type?

...

Kevin _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Tony Morrishttp://tmorris.net/

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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 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

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 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 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 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...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

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:

...

class ToObj a where     toObj :: a -> Obj

...

instance ToObj Obj where     toObj a = a

...

then

...

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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Kevin Jardine wrote:

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.

There: newtype Blog = Blog { getBlogObj :: Obj } newtype Comment = Comment { getCommentObj :: Obj } newtype User = User { getUserObj :: Obj } class GetObject a where getObject :: a -> Obj instance GetObject Blog where getObject = getBlogObject instance GetObject Comment where getObject = getCommentObject instance GetObject User where getObject = getUserObject You shouldn't abuse Foldable for this purpose, unless you really mean it.

I agree that the Foldable solution was a bit of a kludge.

And not necessary either. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/