Lauri Oksanen lassoken at gmail.com
Sat Mar 21 00:07:27 EDT 2009

```True. Thanks.

- Lauri

On Sat, Mar 21, 2009 at 1:05 AM, Lennart Augustsson
<lennart at augustsson.net> wrote:
> That's a horrible definition of fromRational.  Use
> fromRational = P.fromRational.
>
> On Fri, Mar 20, 2009 at 9:09 PM, Lauri Oksanen <lassoken at gmail.com> wrote:
>> 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.
>>>
>> _______________________________________________