Re: [Haskell-cafe] Overloading

12 Mar 2013


      If you add NoImplicitPrelude, I think you should also be able to do:

import Prelude hiding (Num)
import qualified Prelude (Num)

instance Num a => Plus a a where
    type PlusResult a a = a
    a + b = a Prelude.+ b


On Tue, Mar 12, 2013 at 2:24 PM, MigMit  wrote:
...
On Mar 13, 2013, at 12:54 AM, "Richard A. O'Keefe" 
wrote:
...
The interesting challenge here is that we should have
Date   + Period -> Date      Date   - Period -> Date
   Period + Date   -> Date      Period - Date   -> ILLEGAL
   Period + Period -> Deriod    Period - Period -> Period
   Date   + Date   -> ILLEGAL   Date   - Date   -> Date
and _also_ (remember we are trying to beat C++ here) Int +/- Int -> Int.
Well, an obvious suggestion would be to use MultiParamTypeClasses and
TypeFamilies:
{- LANGUAGE MultiParamTypeClasses, TypeFamilies -}
module Date where
import Prelude hiding (Num, (+))
data Date = Date
data Period = Period
class Plus a b where
    type PlusResult a b
    (+) :: a -> b -> PlusResult a b
instance Plus Date Period where
    type PlusResult Date Period = Date
    Date + Period = Date
instance Plus Period Date where
    type PlusResult Period Date = Date
    Period + Date = Date
instance Plus Period Period where
    type PlusResult Period Period = Period
    Period + Period = Period
But I suppose you've been thinking about Haskell98. That, I'm afraid,
doesn't seem possible.
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Re: [Haskell-cafe] Overloading

David Thomas