derivable type classes

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Ozgur Akgun

23 Mar 2010 23 Mar '10

6:52 a.m.

Can a user define a derivable type class of her own? If yes, how? Cheers, -- Ozgur Akgun

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Ivan Lazar Miljenovic

23 Mar 23 Mar

7:05 a.m.

Ozgur Akgun writes:

...

Can a user define a derivable type class of her own? No.

The derivable classes are defined in the Haskell Report; GHC does allow some others to be derived with extensions however. There is one exception to this: using the GeneralizedNewtypeDeriving extension, if type Foo instantiates a typeclass Baz, then you can derive Baz for newtype Bar = Bar Foo. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

Chris Eidhof

7:20 a.m.

No. A solution for this (depending on the type class you want to derive) is Generic Programming. Using Generic Programming, you can define functions that work on the structure of the type. For example, take a look at the regular package [1]. It provides all the functionality to write your own generic functions. -chris [1]: http://hackage.haskell.org/package/regular On 23 mrt 2010, at 11:52, Ozgur Akgun wrote:

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Can a user define a derivable type class of her own? If yes, how?

Cheers,

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

Josef Svenningsson

1:44 p.m.

On Tue, Mar 23, 2010 at 11:52 AM, Ozgur Akgun wrote:

...

Can a user define a derivable type class of her own? If yes, how?

GHC has a feature which lets you define classes such that making an instance of them is as easy as deriving. It's called Generic classes. See GHC's documentation for the details: http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-classes.html Hth, Josef

Ozgur Akgun

24 Mar 24 Mar

1:16 p.m.

Thanks for the pointers again - you are of great help every time! On 23 March 2010 17:44, Josef Svenningsson wrote:

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On Tue, Mar 23, 2010 at 11:52 AM, Ozgur Akgun wrote:

...
Can a user define a derivable type class of her own? If yes, how?

GHC has a feature which lets you define classes such that making an instance of them is as easy as deriving. It's called Generic classes. See GHC's documentation for the details:

http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-classes.html

Hth,

Josef

-- Ozgur Akgun

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Chris Eidhof
Ivan Lazar Miljenovic
Josef Svenningsson
Ozgur Akgun