On Mar 13, 2013, at 12:54 AM, "Richard A. O'Keefe" <[email protected]> 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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