[Haskell-cafe] Overloading

[David Thomas]

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 <miguelimo38 at yandex.ru> wrote:

> On Mar 13, 2013, at 12:54 AM, "Richard A. O'Keefe" <ok at cs.otago.ac.nz>
> 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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