Re: [Haskell-cafe] class-instance

20 Jan 2011


      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  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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