#12708: RFC: Representation polymorphic Num -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Core Libraries | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): I personally find this beautiful (and it made me think of a solution to something I promised to email you a long time ago Richard, correspondence is not my strong suit) {{{#!hs instance P.Num a => Num (a :: Type) where (+) :: a -> a -> a (+) = (+#) -- ... instance P.Show a => Show (a :: Type) where show :: (a :: Type) -> String show = P.show }}} so I believe this could easily be an “alternate” Prelude, people continue to define instances for `Prelude.Num` which makes them instances of our runtime-representation polymorphic `Num`. A great benefit of `Num` is that I can write {{{#!hs ten :: Num a => a ten = 1 + 2 + 3 + 4 }}} and later define my own type, and it was as if `ten` had been defined with it in mind: {{{#!hs data Exp = I Integer | Exp :+: Exp | ... instance Num Exp where (+) :: Exp -> Exp -> Exp (+) = (:+:) fromInteger :: Integer -> Exp fromInteger = I -- >>> ten :: Exp -- ((I 1 :+: I 2) :+: I 3) :+: I 4 }}} Trouble is `ten` must be lifted (`ten :: Prelude.Num a => a`) so we cannot pass it to `I#`, but we can write {{{#!hs reNum :: (forall a. P.Num a => a) -> (forall rep (b :: TYPE rep). Num b => b) reNum f = fromInteger (f @Integer) ten' :: forall rep (a :: TYPE rep). Num a => a ten' = reNum ten }}} which constant folds the rich structure `((I 1 :+: I 2) :+: I 3) :+: I 4` into `I 10 :: Exp` but it's fine for `I# (ten' :: Int#)` (which does work in HEAD). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12708#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler