[Haskell-cafe] class-instance

[Patrick Browne]

Ryan,
This is exactly what I was looking for.

Thanks,
Pat

On 20/01/2011 18:56, Ryan Ingram wrote:
> On Wed, Jan 19, 2011 at 11:56 PM, Patrick Browne <patrick.browne at dit.ie> wrote:
>> I am trying to see what how this requirement can be represented using
>> just the normal instance-implements-class relation for a comparison with
>> a specification language approach.
>>
>> If there is no simple way to do this using type classes then I am
>> obviously mis-using the technique.
> 
> Going back to your original message:
> 
> -- My intension is that the PERSON class should *specify*
> -- that a person has a constant id called p1
> -- and that a person has a name that can be found from the id.
> 
> You can do this with type level numerals.
> 
> data Z
> data S a
> 
> type Zero = Z
> type One = S Zero
> type Two = S One
> -- etc
> 
> class PersonId a where
>     name :: a -> String
> 
> instance PersonId Z where
>     name _ = "John"
> instance PersonId (S Z) where
>     name _ = "Julie"
> 
> main = putStrLn (name (undefined :: One))  -- prints "Julie"
> 
>   -- ryan


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