Lauri Oksanen lassoken at gmail.com
Fri Mar 20 16:09:04 EDT 2009

```Thanks for answers. Here is some working code if somebody plays later
with similar things.

{-# OPTIONS
-XNoImplicitPrelude
-XFunctionalDependencies
-XMultiParamTypeClasses
-XFlexibleInstances
#-}
module Test (
Integer
, Double
, fromInteger
, fromRational
, (+)
) where
import Prelude (Integer, Double)
import qualified Prelude as P
import qualified GHC.Real

fromInteger :: Integer -> Integer
fromInteger = P.id

fromRational :: P.Rational -> Double
fromRational (n GHC.Real.:% d) = let
n' = P.fromInteger n :: Double
d' = P.fromInteger d :: Double
in n' P./ d'

-- Prelude types ---------

instance Semigroup Integer where plus = (P.+)
instance Semigroup Double where plus = (P.+)
instance Subset Integer Double where embed = P.fromInteger

-- Class hierarchy ---------

class Plus a b c | a b -> c where
(+) :: a -> b -> c

class Semigroup a where
plus :: a -> a -> a

class Subset a b where
embed :: a -> b

instance (Semigroup a) => (Plus a a a) where (+) = plus

-- Coercion rules ---------

instance Plus Double Integer Double where
x + j = x + (embed j :: Double)
instance Plus Integer Double Double where
j + x = (embed j :: Double) + x

Ps. I'm very interested in hearing, if somebody has ideas, how to
generalize the coercion rules to something like

instance (Semigroup a) => (Subset b a) => (Plus a b a) where
x + j = x + (embed j)

- Lauri

On Fri, Mar 20, 2009 at 3:58 PM, Lennart Augustsson
<lennart at augustsson.net> wrote:
> I think your best bet is -fno-implicit-prelude, and defining
> fromInteger = id :: Integer->Integer.
>
```