[Haskell-cafe] Stacking data types

[Richard Senington]

On 06/04/11 20:32, Brandon Moore wrote:
>> From: Yves Parès<limestrael at gmail.com>
>> Sent: Wed, April 6, 2011 1:57:51 PM
>>
>> Hello Café,
>>
>> I'm trying to get some modular data types.
>> The idea that came to me is that I could stack them, for instance :
>>
>> data Character a = Character { life :: Int,
>>                                charaInner :: a }
>>
>> data Gun a = Gun { firepower :: Int,
>>                    gunInner :: a }
>>
>> data Armor a = Armor { resistance :: Int,
>>                        armorInner :: a }
>>
>> Then a character with a gun and an armor can be build this way:
>>
>> chara = Character 100 $ Armor 40 $ Gun 12
>>
>> The idea now is to be able to get some part of the character:
> I don't have a better design to suggest, but I think this may end up more
> complicated than you want. In particular, it will probably be complicated to
> make a collection of characters that have e.g. an Armor but maybe any
> other stuff.
>
> If you do want to use this sort of design, check out Wouter Swiestra's
> paper "Data Type a la Cart".
>
> Brandon.
>
>
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I was struggling with this idea a few weeks ago. I think I tried the 
same types of extensions that you have, also with no luck.

I then started looking at extensible records on the Haskell wiki
http://www.haskell.org/haskellwiki/Extensible_record

This was the one I liked, but as far as I could tell it has not been 
implemented (please tell me if I was wrong about that)
http://research.microsoft.com/en-us/um/people/simonpj/Haskell/records.html

RS